bluuebunny/crossref_metadata_2025 huggingface dataset:
Prospects of new planar optical waveguides based on eutectic microcomposites of insulating crystals: The ZrO2(c)-CaZrO3 erbium doped system
Authors: Orera V. M. , Peña J. I. , Merino R. I. , Lázaro J. A. , Vallés J. A. , Rebolledo M. A.
Published: July 2002

A new approach to produce structured optical materials is described. The method relies on the capability of growing eutectic crystals of wide optical band gap materials by directional solidification procedures. The laser float zone technique was used to produce ordered arrays of alternate lamellae with thickness of the order of microns of erbium doped CaZrO3 and calcia stabilized zirconia single crystals. The later, having a higher refractive index, exhibited planar waveguiding effects as it has been proved experimentally. The possibility of producing waveguides from eutectic crystals promises the fabrication of hundreds of planar waveguides integrated into a crystal grown at a speed of several tenths of cm/h.
Polarization threshold switches based on ordered GaInP
Authors: Greger E. , Riel P. , Moser M. , Kippenberg T. , Kiesel P. , Döhler G. H.
Published: July 2002

We report on optoelectronic switching devices with high photoconductive gain which are sensitive to the polarization direction of the optical input. Due to the large polarization anisotropy of the absorption coefficient of ordered GaInP, an electrical output can be switched “on” and “off” by rotating the linear polarization of the input light. Depending on the external adjustment of the working point, the structure can operate as a polarization threshold switch or a polarization detector, respectively. First results exhibit a switching contrast of 25 dB and a contrast of the detector signal of 7 dB between two perpendicularly polarized incident light beams, respectively.
Low voltage performance of Pb(Zr,Ti)O3 capacitors through donor doping
Authors: Yang B. , Song T. K. , Aggarwal S. , Ramesh R.
Published: July 2002

We report low voltage (1.5–3 V) performance of ferroelectric Pb(Zr,Ti)O3 based capacitors. La substitution up to 10% was performed to systematically lower the coercive and saturation voltages of epitaxial ferroelectric capacitors grown on Si using a (Ti0.9Al0.1)N/Pt conducting barrier composite. Ferroelectric capacitors substituted with 10% La show significantly lower coercive voltage compared to capacitors with 0% and 3% La. This is attributed to a systematic decrease in the tetragonality (i.e., c/a ratio) of the ferroelectric phase. Furthermore, the samples doped with 10% La showed dramatically better retention and pulse width dependent polarization compared to the capacitors with 0% and 3% La. These capacitors show promise as storage elements in low power high density memory architectures.
Ultrasonic trapping of paramecia and estimation of their locomotive force
Authors: Saito Mitsunori , Izumida Shin-ya , Hirota Jun
Published: July 2002

Ultrasonic trapping of locomotive organisms was studied by using paramecia of ∼0.2 mm size. An ultrasonic standing wave of ∼3 MHz was generated in a sample cell to trap the paramecia. When the ultrasonic oscillation was turned on, paramecia began to swim along the nodes of the standing wave. Furthermore, when two ultrasonic waves were crossed orthogonally in a thin sample cell, paramecia were trapped at the lattice points of the nodes. The trapping efficiency increased with the ultrasonic power density. The locomotive force of the paramecia was estimated from the threshold power density for trapping.
Phonons as probes in self-organized SiGe islands
Authors: Groenen J. , Carles R. , Christiansen S. , Albrecht M. , Dorsch W. , Strunk H. P. , Wawra H. , Wagner G.
Published: July 2002

We show how optical phonons can be used as efficient probes in self-organized Si1−xGex islands grown on Si(001). Both the alloy composition and residual strain in the islands were originally determined from the phonon frequencies and Raman intensities. The experimental results are in good agreement with the strain relaxation simulated by means of the finite element method.
Extrinsic giant magnetoresistance in chromium (IV) oxide, CrO2
Authors: Sundar Manoharan S. , Elefant D. , Reiss G. , Goodenough J. B.
Published: July 2002

Polycrystalline CrO2 is shown to exhibit a giant magnetoresistance (GMR) at low temperatures. A rapid decrease in the GMR with increasing temperature is correlated with a decrease in the intergranular resistance. Single-crystal CrO2 is a half-metallic ferromagnet, as the data are interpreted to reflect two types of charge carriers, crossing a grain boundary: those that tunnel between conduction bands of adjacent grains and those that hop after residing at a localized state for a time long compared to a spin relaxation time.
The effect of Al on Ga desorption during gas source-molecular beam epitaxial growth of AlGaN
Authors: Jenny J. R. , Van Nostrand J. E. , Kaspi R.
Published: July 2002

In this letter, we report on the impact aluminum has on gallium desorption kinetics in AlGaN alloys grown by gas source-molecular beam epitaxy. Aluminum is found to preferentially incorporate into the AlGaN films over the range of fluxes and temperatures investigated [0.05⩽Ji(Ga)⩽0.5 ML/s; 0.1⩽Ji(Al)⩽0.2 ML/s; 700 °C⩽Ts⩽775 °C]. As a result, Ga is not observed to incorporate into the film until the NH3 flux exceeds that required to grow stoichiometric AlN. This preferential incorporation stems from two facts: (a) Al has an ammonia cracking efficiency ∼2.5 times greater than that of Ga, and (b) Al participates in a Al-for-Ga exchange. As a result of these factors and under NH3 limited growth conditions, the aluminum mole fraction in a layer can be controlled by changing the incident NH3 flux.
Signal strength enhancement and bandwidth tuning in moving space charge field photodetectors using alternating bias field
Authors: Wang Chen-Chia , Linke Richard A. , Nolte David D. , Melloch Michael R. , Trivedi Sudhir
Published: July 2002

Enhancement of the photocurrent density generated by internal moving space charge electric fields in semi-insulating GaAs multiple quantum wells is observed when the sample is biased with a high speed, zero-mean square wave electric field which increases the internal space charge electric field strength. Tunability of the space charge field formation time is also demonstrated by adjusting the amplitude of the bias signal. This technique can be used to enhance the sensitivity of optical doppler frequency measurements based on moving space charge field effects in photoconductive semiconductors. Electronic tunability of the space charge field formation time also adds versatility in optical power spectrum analysis applications.
Near diffraction-limited laser emission from a polymer in a high finesse planar cavity
Authors: Schülzgen A. , Spiegelberg Ch. , Morrell M. M. , Mendes S. B. , Kippelen B. , Peyghambarian N. , Nabor M. F. , Mash E. A. , Allemand P. M.
Published: July 2002

We report near diffraction-limited laser emission from the conjugated polymer BEH:PPV in a cavity made with two dielectric mirrors providing a high finesse planar cavity. The laser has a sharp intensity threshold, a strong directionality, and a high degree of polarization. These emission characteristics are compared with those of a single polymer layer without optical feedback.
Measurement of phonon-exciton dephasing rate in GaN on sapphire by degenerate four-wave mixing
Authors: Pau S. , Kuhl J. , Scholz F. , Haerle V. , Khan M. A. , Sun C. J.
Published: July 2002

The density and temperature dependence of the exciton dephasing time of two hexagonal GaN films on sapphire is measured using degenerate four-wave mixing (DFWM). The residual 4 ps dephasing time at low temperature and density is caused by exciton-impurity scattering. We present a theory of DFWM for various amount of inhomogeneous broadening. Good agreement of the temperature dependence of the dephasing time is found between theory and experiment.
Two-step process for improved diamond deposition on titanium alloys at moderate temperature
Authors: Vandenbulcke L. , Rats D. , De Barros M. I. , Benoı̂t R. , Erre R. , Andreazza P.
Published: July 2002

A simple two-step process is reported here to deposit diamond coatings on titanium alloys at temperatures equal to or lower than 600 °C. The first step allows us to increase the carbon nucleation rate and to deposit a sacrificial layer which contains more than about 25% sp2 carbon. Its thickness is selected both to limit the interaction of titanium element with the plasmas used for diamond growth during all the second step, even when an oxygen-containing mixture is used, and to diffuse completely at the end of the process. After the first step, the formation of titanium carbide is observed by x-ray diffraction and x-ray photoelectron spectroscopy, which does not reveal any oxygen incorporation in the coating-substrate interfacial region. These results are related to the final strong diamond adherence.
Continuous wave laser radiation at 524 nm from a self-frequency-doubled laser of LaBGeO5:Nd3+
Authors: Capmany J. , Jaque D. , Garcı́a Solé J. , Kaminskii A. A.
Published: July 2002

This letter reports continuous wave laser emission at 524 nm from a self-frequency-doubled infrared laser of LaBGeO5:Nd3+ operating at a fundamental wavelength of 1048 nm. Resonant pumping in the spectral region around 800 nm for laser diode pumping is investigated and optical losses are estimated.
Simplified analysis of some complex film stacks in x-ray reflectivity
Authors: Marcus Matthew A.
Published: July 2002

X-ray reflectivity (XRR) is commonly used to measure the thickness, density, and roughness of thin films. XRR can be used on multilayered films, however the data analysis then involves a complex fit. Such fits display many local minima for stacks of more than two layers. It is shown how the modulated diffuse scattering arising from the correlated roughness effect can sometimes be used to measure the thickness of the top film in the stack, hence reducing the size of the parameter space to be searched. Data for three- and four-layer stacks will be presented.
Ballistic electron emission microscopy study of Schottky contacts on 6H- and 4H-SiC
Authors: Im H.-J. , Kaczer B. , Pelz J. P. , Choyke W. J.
Published: July 2002

We have performed ballistic-electron emission microscopy (BEEM) on (Pd,Pt)/(6H,4H)-SiC(0001) Schottky contacts. Schottky barrier heights (SBHs) determined from the BEEM data using the Bell–Kaiser model are 1.27 eV/1.34 eV (Pd/Pt) for 6H- and 1.54 eV/1.58 eV for 4H-SiC. Our measurements also give the first direct evidence of a second conduction band minimum (CBM) on 4H-SiC about 0.14 eV above the overall CBM. Spatial inhomogeneity of SBHs were examined and shown to be no larger than the fitting error due to the system noise. Additionally, enhanced ballistic transmittance was observed over a region intentionally stressed by injecting high kinetic energy (10 eV above EF) hot electrons using BEEM in the Pt/4H-SiC sample.
Short-range disorder in lanthanum-doped lead titanate ceramics probed by Raman scattering
Authors: Tavares E. C. S. , Pizani P. S. , Eiras J. A.
Published: July 2002

Raman scattering has been used to probe short-range structural information on lanthanum-doped lead titanate ceramics (LaxPb1−xTiO3) for x, ranging from 0.0 to 0.30. In highly doped samples (x>0.27), x-ray diffraction measurements indicate a cubic structure, although measurements of Raman scattering at temperatures above and below the tetragonal to cubic transition showed a residual short-range structural disorder in the cubic phase. Moreover, for these highly lanthanum-doped samples, a well-defined temperature-induced ferro–paraelectric phase transition disappears, which suggests a relationship between local disorder and relaxor behavior in this material.
Si/SiGe
quantum wells grown on vicinal Si(001) substrates: Morphology, dislocation dynamics, and transport properties
Authors: Waltereit P. , Fernández J. M. , Kaya S. , Thornton T. J.
Published: July 2002

Compositionally graded, strain relaxed Si0.72Ge0.28 buffers were grown on vicinal Si(001) substrates by gas source molecular beam epitaxy. Misfit dislocations are shown to run along intersections of the {111} glide planes with the (11n) interface. X-ray diffraction studies demonstrate a relative tilt of the epilayer to the substrate in a direction which depends on the interplay between substrate orientation related preferential dislocation nucleation rates and surface contamination induced heterogeneous nucleation. Atomic force microscopy (AFM) images reveal an anisotropy in surface roughness on the μm scale related to reduced growth rates on vicinal surfaces. Transport properties at 0.4 K in two dimensional electron gases grown on these relaxed SiGe buffers show anisotropic scattering times similar to interface roughness scattering which can be correlated to terrace configurations in the nm range determined by AFM.
Pressure dependence of the negative bias voltage for stabilization of cubic boron nitride thin films deposited by sputtering
Authors: Otaño-Rivera Wilfredo , Pilione Lawrence J. , Messier Russell
Published: July 2002

Thin films of boron nitride were deposited by unbalanced magnetron sputtering at pressures between 0.065 and 1.32 Pa in order to study the effects of pressure and negative substrate bias on the energy of the bombarding plasma ions and subsequent stabilization of the cubic phase. It was found that the threshold bias voltage for nucleation of films with a high percentage of the cubic phase increases with the product of the pressure and sheath thickness. This trend is explained in terms of the changes in the average energy of the particles bombarding the growing film produced by pressure-dependent charge-exchange collisions in the plasma/substrate sheath. This energy modification process has predictable consequences in complex deposition processes.
Investigation of intrinsic channel characteristics of hydrogenated amorphous silicon thin-film transistors by gated-four-probe structure
Authors: Chiang Chun-sung , Chen Chun-ying , Kanicki Jerzy , Takechi Kazushige
Published: July 2002

We use a new hydrogenated amorphous silicon (a-Si:H) device structure, the gated-four-probe a-Si:H thin-film transistor (TFT), to investigate the intrinsic channel characteristics of inverted-staggered a-Si:H TFTs without the influence of source/drain series resistances. The experimental results have shown that, for the conventional a-Si:H TFT structure, the field-effect mobility, threshold voltage, and field-effect channel conductance activation energy have a strong dependence on a-Si:H thickness and TFT channel length. On the other hand, for the gated-four-probe a-Si:H TFT structure, these values are a-Si:H thickness and TFT channel length independent, clearly indicating that this new a-Si:H TFT structure can be effectively used to measure the channel intrinsic properties of a-Si:H TFTs.
Electronic and transformation properties of a metastable defect introduced in epitaxially grown boron-doped p-type Si by alpha particle irradiation
Authors: Mamor M. , Auret F. D. , Goodman S. A. , Meyer W. E. , Myburg G.
Published: July 2002

Titanium (Ti) Schottky barrier diodes on epitaxially grown boron-doped p-type Si films with a free carrier density of 6–8×1016 cm−3 were irradiated with alpha particles at room temperature using an americium-241 (Am-241) radio nuclide. We report the electronic and transformation characteristics of an α-particle irradiation-induced defect Hα2 in epitaxially grown p-Si with metastable properties. The energy level and apparent capture cross section, as determined by deep-level transient spectroscopy, are Ev+0.43 eV and 1.4×10−15 cm2, respectively. This defect can be removed and re-introduced using a conventional bias-on/off cooling technique.
Electric field control of cathodoluminescence from phosphors excited at low electron energies
Authors: Seager C. H.
Published: July 2002

It is demonstrated that electric fields applied across phosphor layers strongly influence the cathodoluminescence emitted from these materials at low electron beam energies. The magnitude of these effects suggest that the primary mechanism may be alteration of the relative capture rates of electrons and holes at surface or near-surface defect sites associated with nonradiative energy losses. The sign of these effects in the two materials investigated suggests that the rate limiting step at low electric fields is the capture of electrons. Strong evidence for field-induced polarization and trapping of the beam-created electron-hole pair population is also presented.
Fabrication of patterned electroluminescent polymers that emit in geometries with feature sizes into the submicron range
Authors: Rogers John A. , Bao Zhenan , Dhar Lisa
Published: July 2002

This letter describes the fabrication of structured polymer light-emitting devices (LEDs) that produce patterns of light with feature sizes as small as ∼0.8 μm. Solvent-assisted micromolding of a film of a precursor to poly(p-phenylene vinylene) (PPV), produces variations in its thickness that replicate the relief on the mold. Thermal conversion of this precursor produces a film of PPV with the same surface relief. LEDs formed with the structured PPV emit preferentially in the thin regions of the film. Devices fabricated in this manner may be important for constructing plastic visual displays, and could eventually lead to new subwavelength sources of light suitable for applications in near field optics.
Floating low-temperature radio-frequency plasma oxidation of polycrystalline silicon-germanium
Authors: Fan Zhineng , Zhao Gang , Chu Paul K. , Jin Zhonghe , Kwok Hoi S. , Wong Man
Published: July 2002

Low temperature oxide formation is an important process in the fabrication of thin-film transistors (TFT) used in active-matrix liquid crystal displays. However, low temperature oxide is prone to have defects at the SiO2/polycrystalline–SiGe interfaces. We have recently developed a novel rf (radio frequency) plasma oxidation method for polycrystalline SiGe (poly-SiGe) materials. The poly-SiGe wafers are oxidized in an oxygen rf plasma with the samples electrically floating. That is, the sample voltage is the same as the sheath potential of the floating wall and is always negative with respect to the bulk of the plasma since electrons have higher mobility than ions. The slightly negative potential on the wafers attracts low energy oxygen ions from the plasma and the resulting damage on the wafers is thus lower than that induced by the more commonly used and energetic electron cyclotron resonance (ECR) source. No deliberate heating is applied during oxidation since the samples are heated spontaneously by the plasma, but the temperature is measured to be below 100 °C throughout the entire process. The oxidation rate is comparable to that of ECR plasma oxidation. Depth profiles are acquired by Auger electron spectroscopy and the interfaces are examined by x-ray photoelectron spectroscopy. The n-channel metal oxide silicon device fabricated on the as-grown gate oxide shows good electrical characteristics. The process is thus compatible with inexpensive large-area, low-temperature fabrication of TFTs on glass substrates.
Growth of GaN(0001) thin films on Si(001) by pulsed reactive crossed-beam laser ablation using liquid Ga and N2
Authors: Willmott P. R. , Antoni F.
Published: July 2002

Wurtzitic GaN films were grown by reactive crossed-beam pulsed laser deposition at 248 nm (KrF) using a liquid Ga target and a synchronous N2 pulse on atomically flat, initially two domain 2×1 reconstructed Si(001) substrates. The films were (0001) single phase for substrate temperatures between 200 and 700 °C, and also grew in a twinned epitaxial manner with the crystallites oriented parallel to the [110] and [1̄10] in-plane directions of the Si(001) substrate between 550 and 700 °C. Above 700 °C, no GaN could be detected and only Ga liquid films were produced. The films were subsequently investigated ex situ by x-ray diffraction, reflection high-energy electron diffraction, and photoluminescence.
Impact ionization in AlxGa1−xAs/GaAs superlattices
Authors: Pearsall T. P.
Published: July 2002

The role of band discontinuities on electron ionization rates is examined for the case of GaAs/AlGaAs superlattice avalanche photodiode structures. Our results show that the energy discontinuity experienced by a hot electron (E>0.4 eV above the Fermi level) is small or even negative relative to the conduction-band energy discontinuity experienced by an electron at the band edge of the Γ valley in GaAs/AlGaAs heterostructures. This finding does not support the measurement of an enhanced electron ionization rate due to transport through a superlattice composed of this materials system.
Control of antenna coupling in high-density plasma production by m=0 helicon waves
Authors: Sakawa Y. , Takino T. , Shoji T.
Published: July 2002

Antenna coupling has been controlled in high-density plasma production by m=0 helicon waves using multiple-loop antennas and varying the phase and the distance between antennas. When two single-loop antennas are connected in series with the phase between the two being zero, threshold power for the density jump Pth is reduced by a factor of 2–3 compared with that of the one-turn loop antenna. This reduction in Pth is caused by the increase in antenna coupling at the lower plasma density. The observed differences in antenna coupling among different antennas have been examined by using a two-dimensional wave code.
Electric fatigue in sol–gel prepared Pb(Zr,Sn,Ti)NbO3 thin films
Authors: Jang Jae Hyuk , Yoon Ki Hyun , Shin Hyun Jung
Published: July 2002

The fatigue properties of the sol–gel derived Pb0.99[(Zr0.6Sn0.4)1−XTiX]0.98Nb0.02O3 (40/100X/2) thin films deposited on platinized silicon substrates were investigated as a function of Ti content (0.03⩽X⩽0.15). With an increase of Ti content X, the antiferroelectric phase was transformed to the ferroelectric phase and the degradation of polarization under the repetitive switching was increased. This behavior should be closely related to the fact that the antiferroelectric phase contains mainly 180° domains, which have smaller internal stresses during switching than those of 90° domains. The difference of nanoscale domain structure between the antiferroelectric and ferroelectric thin films was confirmed by scanning force microscopy.
Electrical and optical properties of CdGeAs2 single crystals irradiated with fast electrons
Authors: Zwieback I. , Perlov D. , Maffetone J. P. , Ruderman W.
Published: July 2002

The effects of 10 MeV fast electron irradiation on conductivity type, resistivity, and optical absorption of CdGeAs2 single crystals were studied. Irradiation of as-grown p-type crystals with a fluence of 1×1017 cm−2 caused compensation of the semiconductor and a marked improvement in the optical transmission. A level of absorption below 0.1 cm−1 at 5⩽λ⩽11 μm was registered. Increase in the fluence to 2×1017 cm−2 led to conversion to n-type, a drop in the resistivity and deterioration of the transmission. Analysis of the absorption in p-type crystals showed that transitions from the V2 and V3 split-off bands to the V1 valence band represent the major absorption mechanism. Splitting energies ΔE2−1=0.15 eV and ΔE3−1=0.35 eV have been calculated. Absorption in n-type crystals increased as λ2.4 indicating absorption by free electrons and their scattering by optical phonons. The effect of irradiation on the optical absorption of CdGeAs2 is assigned to the radiation-induced changes in the compensation level.
Quantum tailoring of optical transitions in InxGa1−xAs/AlAs strained quantum wells
Authors: Jancu Jean Marc , Pellegrini Vittorio , Colombelli Raffaele , Beltram Fabio , Mueller Bernhard , Sorba Lucia , Franciosi Alfonso
Published: July 2002

The optical properties of n-doped InxGa1−xAs/AlAs pseudomorphic quantum wells grown on GaAs(001) are investigated as a function of layer thickness and indium concentration. The nature of interband and intersubband transitions is clarified using an improved tight-binding model and a combination of low-temperature photoluminescence spectroscopy and intersubband absorption studies. A type I→type II crossover is found to occur in very narrow wells. Moreover, for appropriate quantum-well parameters, tailoring of the intersubband transition wavelength down to the 1.2 μm range appears feasible while retaining the type I character of the band alignment. These results provide a valid framework for the implementation of InxGa1−xAs/AlAs heterostructures for ultrafast optical communication applications.
Electromigration threshold in damascene versus plasma-etched interconnects
Authors: Proost Joris , Maex Karen , Delaey Luc
Published: July 2002

Electromigration threshold has been measured for unpassivated damascene Al(Cu) interconnects by drift experiments on Blech-type test structures, and results are compared to equivalent plasma-etched lines. When using a damascene implementation, the critical product of current density and stripe length increases significantly. As a result, the incubation period, needed for Cu depletion beyond a critical length, is increased over a wide temperature range (155–230 °C). This is shown to be an intrinsic effect, directly controlling the relative electromigration performance of both metallization schemes.
Evidence for self-mode-locking in p-Ge laser emission
Authors: Muravjov A. V. , Strijbos R. C. , Fredricksen C. J. , Weidner H. , Trimble W. , Withers S. H. , Pavlov S. G. , Shastin V. N. , Peale R. E.
Published: July 2002

Investigations of the dynamics of the far-infrared p-Ge laser emission reveal strong periodic soliton-like intensity spikes with less than 100 ps duration. We interpret these spikes as self-mode-locking of p-Ge laser modes. The effect becomes more pronounced when a GaAs/AlGaAs/InGaAs quantum well structure on a semi-insulating GaAs substrate is inserted into the laser cavity.
Fowler–Nordheim tunneling of holes through thermally grown SiO2 on p+ 6H–SiC
Authors: Waters Richard , Van Zeghbroeck Bart
Published: July 2002

Fowler–Nordheim tunneling of holes through thermally grown silicon dioxide on 6H–silicon carbide is reported. Oxides of 5.2, 10, and 14.2 nm thickness were grown on the p+ face of a p+n SiC junction. The p+n junction served to separate the electron and hole tunneling currents. Hole tunneling was found to be the dominant current mechanism through the oxide. Fowler–Nordheim analysis, using a parabolic E–K relationship, was performed to extract a barrier height–effective mass product, ΦB3/2(mox/m0)1/2, for electrons and holes of 2.88%±4.9% and 2.38%±3.8% (V3/2) respectively. An estimate for the effective mass of holes within the oxide was made using both the parabolic and Franz dispersion relations.
Internal photoemission measurement of Schottky barrier height for Ni on AlGaN/GaN heterostructure
Authors: Yu L. S. , Xing Q. J. , Qiao D. , Lau S. S. , Boutros K. S. , Redwing J. M.
Published: July 2002

The internal photoemission method was used to measure the Schottky barrier height of Ni on AlGaN/GaN heterostructures. A barrier height of 1.31 eV was found for the Ni/Al0.15Ga0.85N/GaN heterojunction structure, as compared to a barrier height of 1.28 and 1.02 eV for the Ni/Al0.15Ga0.85N and Ni/GaN Schottky diodes, respectively.
Holographic nanopatterning of the organic semiconductor poly(p-phenylene vinylene)
Authors: Gigli G. , Rinaldi R. , Turco C. , Visconti P. , Cingolani R. , Cacialli F.
Published: July 2002

We report a flexible method for the patterning of organic semiconductors in the submicrometer range, which we have successfully applied to thin films of poly(p-phenylene vinylene) (PPV) prepared on a variety of substrates, such as quartz, indium–tin oxide coated glass, or inorganic dielectric mirrors. The method is based on holographic lithography performed by a corner cube interferometer of our own design and construction, followed by Ar-ion etching.
Technique for integration of vertical cavity lasers and resonant photodetectors
Authors: Sjölund O. , Louderback D. A. , Hegblom E. R. , Ko J. , Coldren L. A.
Published: July 2002

We demonstrate a design that allows fabrication of substrate input/output resonant-cavity photodetectors and vertical cavity lasers (VCLs) on the same substrate without regrowth. By selectively oxidizing a few layers in the bottom mirror the as-grown 80% reflectivity mirror, used as the input mirror for the detector, is converted to a 99.3% reflectivity mirror allowing fabrication of VCLs from the same epitaxial material. Since these two reflectivities are uncorrelated, the detectors and VCLs can be individually designed. Despite the change in refractive index from ∼3 to ∼1.6 in the oxidized layers, the structure can be designed to have nearly the same resonance wavelength for both the detectors and VCLs. Using this design strategy, we have successfully fabricated high-performance resonant photodetectors and VCLs from the same epitaxial material. The photodetectors have an absorption of 56% and an optical bandwidth of 5.9 nm, in good agreement with theory. Small diameter, single-mode VCLs have threshold currents as low as 180 μA with 33% slope efficiencies while multimode devices have slope efficiencies exceeding 60% with less than 500 μA threshold currents.
Magnetic dissipation microscopy in ambient conditions
Authors: Proksch Roger , Babcock Ken , Cleveland Jason
Published: July 2002

We have quantified sub-picowatt power losses in very soft (yttrium iron garnets) and relatively hard (metal evaporated tape) ferromagnetic materials associated with dissipative micromagnetic processes during magnetic force microscope imaging. We had a thermally limited power resolution of 2×10−15 W in a 1 kHz bandwidth operating in air. In the epitaxial garnet film, peak dissipation (3×10−13 W) was spatially correlated with domain wall motion induced by the localized field from the tip. In metal-evaporated recording tape imaged with a cantilever coated with 50 nm of CoCr, the dissipation was observed in extremely localized regions of the sample (<10 nm). Absence of dissipation features when using a different tip suggests the dissipation originated in the tip rather than the sample. This technique shows promise for mapping micromagnetic structure and dissipative processes, quantitatively evaluating magnetic force microscope (MFM) tip performance, and for detecting perturbations in MFM images.
Isomagnetic reversal in sintered NdFeB
Authors: Crew D. C. , McCormick P. G. , Street R.
Published: July 2002

Measurements of reversible and irreversible magnetization have been used to examine the reversal process on the initial magnetization curve in sintered NdFeB. It is found that the reversal process involves equal and opposite changes of irreversible and reversible magnetization, described here as isomagnetic reversal, resulting in no net change in magnetization during reversal. This result is consistent with the observation that magnetic viscosity is negligible on the initial magnetization curve.
Microstructure and magnetic properties of Sm2(Fe, Si)17Cx/α-Fe nanocomposite magnets prepared under high pressure
Authors: Zhang X. Y. , Zhang J. W. , Wang W. K. , Yu W. , Zhao J. H. , Xu Y. F.
Published: July 2002

We present microstructure and magnetic properties of Sm2(Fe, Si)17Cx/α-Fe nanocomposite magnets prepared under a pressure of 4 GPa at a temperature of 923 K. A high-pressure experiment was carried out in a belt-type pressure apparatus. Analyses of x-ray diffraction and transmission electron microscopy show that the nanocomposite magnets have a grain size of 6–8 nm, which provides a strong exchange coupling between hard and soft magnetic phases. As a result, the magnets have, compared with nanocomposite magnets prepared under normal pressure, a significant increase in both coercivity, from 132 to 500 kA/m, and remanent magnetization, from 0.68 to 0.83Ms.
Optically induced surface relief phenomena in azobenzene polymers
Authors: Holme N. C. R. , Nikolova L. , Hvilsted S. , Rasmussen P. H. , Berg R. H. , Ramanujam P. S.
Published: July 2002

Azobenzene polymers and oligomers show intriguing surface relief features when irradiated with polarized laser light. We show through atomic force microscopic investigation of side-chain azobenzene polymers after irradiation through an amplitude mask that large peaks or trenches result depending on the architecture of the polymer. Extensive mass transport over long distances has been observed, paving the way for easy replication of nanostructures. We also show that it is possible to store microscopic images as topographic features in the polymers just through polarized light irradiation.
Modeling of the temperature dependence of the field-effect mobility in thin film devices of conjugated oligomers
Authors: Schön J. H. , Batlogg B.
Published: July 2002

A simple model is proposed for charge carrier transport across grain boundaries with an acceptor-like trap level. Potential wells between the grains are formed due to negatively charged grain boundaries. Based on this model, a variety of temperature dependencies of the charge carrier mobility can be described. Using realistic parameters, this model reproduces very well the measured temperature dependencies of the field-effect mobility in polycrystalline pentacene and oligothiophene thin film devices. Therefore, it seems to be difficult to investigate the intrinsic material properties of organic semiconductors using only polycrystalline field-effect devices, since they may be masked by the effects of traps and grain boundaries.
Electric-field-induced insulator–metal transitions in thin films of charge-ordered rare-earth manganates
Authors: Ponnambalam V. , Parashar Sachin , Raju A. R. , Rao C. N. R.
Published: July 2002

Thin films of charge-ordered Nd0.5Ca0.5MnO3, Y0.5Ca0.5MnO3, and Nd0.5Sr0.5MnO3 have been prepared on Si(100) and LaAlO3(100) substrates by the nebulized spray pyrolysis of organometallic precursors. Small electric fields induce insulator–metal transitions in the films with the resistivity decreasing with increasing current. The current–voltage characteristics are non-ohmic and show some hysteresis. The current-induced negative differential resistance found in these manganate films could have potential technological applications.
Optical microscopy of electronic and structural properties of epitaxial laterally overgrown GaN
Authors: Kaschner A. , Hoffmann A. , Thomsen C. , Bertram F. , Riemann T. , Christen J. , Hiramatsu K. , Shibata T. , Sawaki N.
Published: July 2002

Local strain relaxation as well as inhomogeneous impurity incorporation in epitaxial laterally overgrown GaN (ELOG) structures is microscopically characterized using spectrally resolved scanning cathodoluminescence (CL) and micro-Raman spectroscopy. We correlate the different CL emission spectra with results of spatially resolved Raman-scattering experiments sensing the local strain and free-carrier concentration.
Segregation coefficient of beryllium in traveling-solvent-grown BeyZn1−ySe1−xTex bulk crystals using tellurium solution
Authors: Maruyama Kenji , Suto Ken , Nishizawa Jun-ichi
Published: July 2002

Quaternary bulk single crystals of BeyZn1−ySe1−xTex have been grown using the traveling-solvent method with a tellurium solution at a peak temperature of 1050 °C. BeyZn1−ySe1−xTex mixed crystals contain beryllium from 0.02 to 5.0 at. % and tellurium from 1.8 to 5.5 at. % (0.03<x<0.11, 0.0004<y<0.10). The best full width of half maximum of the x-ray rocking curve shows 33 arcsec and the best etch pit density is 3.0×105/cm2. The beryllium segregation coefficient was evaluated from the beryllium composition profiles. We obtained a beryllium segregation coefficient of k=1.65±0.45. Compositionally uniform BeZnSeTe bulk crystals matching the ZnSe in the lattice constant can be grown. Large parts of the BeZnSeTe ingot are anticipated to match the ZnSe lattice, because the beryllium segregation coefficient is close to unity.
Aligned island formation using an array of step bands and holes on Si(111)
Authors: Homma Yoshikazu , Finnie Paul , Ogino Toshio
Published: July 2002

We investigated the control of island formation using a patterned Si(111) surface with a periodic array of holes. During annealing, Au and Ga form islands at fixed locations on the array of step bands surrounding the holes. These materials or their silicides are in liquid phase at elevated temperatures. They diffuse along the step bands and accumulate at particular points on the periodic step bands. The size of the islands can be precisely designed by controlling the size of the pattern.
Onset of blistering in hydrogen-implanted silicon
Authors: Huang L.-J. , Tong Q.-Y. , Chao Y.-L. , Lee T.-H. , Martini T. , Gösele U.
Published: July 2002

The onset of surface blistering in hydrogen-implanted single crystalline silicon was studied. A combination of atomic force microscopy and optical measurements shows that hydrogen-containing platelets grow laterally below silicon surface until they suddenly pop up as surface blisters due to the internal hydrogen pressure after a critical size has been reached. Experimentally and theoretically, the critical size of the onset blisters was found to increase with increasing implantation depth or top layer thickness.
Short-wavelength photoluminescence from silicon and nitrogen coimplanted SiO2 films
Authors: Zhao J. , Mao D. S. , Lin Z. X. , Ding X. Z. , Jiang B. Y. , Yu Y. H. , Liu X. H. , Yang G. Q.
Published: July 2002

Intense short-wavelength photoluminescence (PL) observed at room temperature from thermal SiO2 films implanted with Si and N is reported. A flat Si profile was first created. N ions were subsequently implanted into the same depth region as the implanted Si ions. Two PL bands peaking at ∼330 and ∼430 nm were observed from the samples at room temperature with and without annealing. It is found that the PL has a strong dependence on the stabilized N in the Si- and N-coimplanted SiO2 films. The PL may originate from a complex of Si, N, and O.
Photoluminescence from laser assisted debonded epitaxial GaN and ZnO films
Authors: Tavernier P. R. , Verghese P. M. , Clarke D. R.
Published: July 2002

Photoluminescence from epitaxial GaN and ZnO films is employed as a diagnostic of the laser assisted debonding process used to separate the films from their sapphire substrates. The observed shifts in the photoluminescence spectra, recorded after separation, at 77 K are consistent with the removal of the thermal expansion mismatch strain associated with the films being cooled from their deposition temperature and indicate that after separation the films are essentially stress free.
In situ observation of epitaxial microcrystals in thermally grown SiO2 on Si(100)
Authors: Awaji N. , Sugita Y. , Horii Y. , Takahashi I.
Published: July 2002

We succeeded with an in situ observation of the epitaxial microcrystal produced in thermally grown oxide on Si(100) by applying the grazing incidence diffraction of a synchrotron x ray. Oxidation was at 900 °C for 8 h in an environment of oxygen diluted with nitrogen, producing thermal oxides that were 122 Å thick. During oxidation, we observed the diffraction peak corresponding to the epitaxial microcrystal, even in 23-Å-thick oxide. The peak intensity increased as the oxidation proceeded but decreased by about 2/3 after cooling at the end of the oxidation. The lattice constant of the c axis of the microcrystal changed from 10.4 Å for 23-Å-thick oxide to 11.7 Å for oxides thicker than 45 Å.
Experimental investigation of the nonreciprocal phase shift of a transverse electric mode in a magneto-optic rib waveguide
Authors: Fehndrich M. , Josef A. , Wilkens L. , Kleine–Börger J. , Bahlmann N. , Lohmeyer M. , Hertel P. , Dötsch H.
Published: July 2002

Magnetic garnet films of composition (Nd,Lu,Bi)3(Fe,Al,Ga)5O12 are grown by liquid-phase epitaxy on [111] oriented substrates of gadolinium gallium garnet. They have positive uniaxial anisoptropy and support lattices of parallel stripe domains. A monomode optical rib waveguide is prepared by photolithography and ion-beam etching parallel to the (112̄) direction, which is the preferred direction of the stripe domains. A magnetic domain pattern can be created such that a domain wall is located at the rib center while the adjacent walls stick to the flanks of the rib. At a wavelength of 1.3 μm a nonreciprocal phase shift of the guided fundamental transverse electric mode is observed.
Abrupt PbTiO3/SrTiO3 superlattices grown by reactive molecular beam epitaxy
Authors: Jiang J. C. , Pan X. Q. , Tian W. , Theis C. D. , Schlom D. G.
Published: July 2002

PbTiO 3 /SrTiO 3 superlattices were grown on (001) SrTiO3 substrates by reactive molecular beam epitaxy (MBE). Sharp superlattice reflections were observed by x-ray diffraction. High-resolution transmission electron microscopy of a [(PbTiO3)10/(SrTiO3)10]15 superlattice revealed that the PbTiO3/SrTiO3 interface structure is atomically sharp. The superlattice interfaces are fully coherent; no misfit dislocations or other crystal defects were observed in the superlattice by transmission electron microscopy. Selected area electron diffraction patterns indicated that the PbTiO3 layers are oriented with the c axis parallel to the growth direction. The dimensional control and interface abruptness achieved in this model system indicate that MBE is a viable method for constructing oxide multilayers on a scale where enhanced dielectric effects are expected.
Magnetic properties and magnetic-domain structures of nanocrystalline Sm2Fe14.5Cu0.5Ga2Cy
Authors: Zhang Shao-ying , Sun Zhi-gang , Zhang Hong-wei , Han Bao-shan , Shen Bao-gen , de Boer F. R. , Buschow K. H. J.
Published: July 2002

The magnetic properties and magnetic-domain structures of nanocrystalline Sm2Fe14.5Cu0.5Ga2Cy (y=0–2.0) alloys prepared by melt spinning and subsequent heat treatment were studied. The coercivity of optimum annealing ribbons first increases significantly with increasing carbon content for y⩽1.0. It does not change much for 1<y⩽2. The maximum coercivity μ0Hc is 2.6 T at room temperature for ribbons with y=1.5–2.0 after annealing in a Sm atmosphere at 1047 K for 30 min. The domain structure was studied by magnetic-force microscope. The as-spun amorphous Sm2Fe14.5Cu0.5Ga2C1.5 ribbons exhibit very weak magnetic contrast. The magnetic-domain length scale is 300–700 nm and much larger than the grain size in the Sm2Fe14.5Cu0.5Ga2C1.5 ribbons, which may be caused by interactions between the grains. In the Sm2Fe14.5Cu0.5Ga2 ribbons, the domain-length scale is 100–200 nm and the magnetic-tip–sample interaction strength is weaker than in the Sm2Fe14.5Cu0.5Ga2C1.5 ribbons. These results have been correlated with the magnetic properties and microstructure.
High-speed, optically controlled surface-normal optical switch based on diffusive conduction
Authors: Yairi M. B. , Coldren C. W. , Miller D. A. B. , Harris J. S.
Published: July 2002

We report a surface-normal optically controlled optoelectronic modulator made from a reversed biased p-i (multiple quantum well)-n GaAs/AlGaAs structure with ultrathin barriers (5 Å) whose recovery time is based on diffusive conduction. Modulation of reflectivity from 0.3 to 0.6 and back again in about 50 ps was demonstrated using a 750 fJ control pulse at 855 nm. We also demonstrated modulated changes in power greater than the control pulse power—a type of signal gain—by a factor of 1.8-to-1. Strong changes in reflectivity combined with low required control power make this device potentially useful for high-speed switching arrays in such applications as time division demultiplexing.
Significant improvement of device durability in organic light-emitting diodes by doping both hole transport and emitter layers with rubrene molecules
Authors: Sakamoto Gosuke , Adachi Chihaya , Koyama Toshiki , Taniguchi Yoshio , Merritt Charles D. , Murata Hideyuki , Kafafi Zakya H.
Published: July 2002

We have developed highly durable organic light-emitting diodes. The basic structure of the diodes is anode/hole injection layer/hole transport layer+dopant/emitter layer+dopant/cathode. Both the hole transport and the emitter layers were doped with the highly fluorescent rubrene molecules. With the doping of both layers, 85% of the initial luminance was successively maintained even after 1000 h of continuous operation under constant current driving. Doping of only one of these layers, either the hole transport layer or emitter layer, on the other hand, resulted in shorter lifetime. We mention the possible mechanisms of the doping that enhance the device duration.
Effect of tolerance factor and local distortion on magnetic properties of the perovskite manganites
Authors: Zhou J. P. , McDevitt J. T. , Zhou J. S. , Yin H. Q. , Goodenough J. B. , Gim Y. , Jia Q. X.
Published: July 2002

The substitutions of rare earths for La on the magnetic properties of the perovskites La0.7−xRxA0.3MnO3 (0<x<0.7), R=Pr or Gd and A=Ca, Sr, or Ba have shown that small substitution of Pr slightly increases the coercive field Hc and magnetization M, and strongly improves the magnetoresistance (MR) while lowering Tc. On the other hand, the substitution of Gd lowers Hc, M and Tc, however, increases MR of the system La0.7−xGdxSr0.3MnO3 at temperature <Tc. Large differences in the A-site ionic radii rA of the AMnO3 perovskites proved detrimental. The optimal composition has been discussed for the half-metallic ferromagnet of a spin-switch device based on the manganese oxides.
Dielectric properties of RbTiOAsO4 single crystals
Authors: Yang Yusin , Yoon Choon Sup
Published: July 2002

Using the impedance analysis, we obtain the dielectric constants of RbTiOAsO4 (RTA) single crystals for the three principle crystallographic axes in the frequency range of 1 kHz–6 MHz and in the temperature range of 303–1123 K. The conductivity of RTA along the polar c axis is five orders-of-magnitude smaller than that of KTiOPO4 (KTP) in the temperature range of 500–970 K. The conductivities of RTA along the three principle axes have the same order of magnitude, 10−5 Ω−1 m−1 at 720 K, which is contrasted with the high anisotropy shown in KTP.
Chemomagnetic fields produced by solid combustion reactions
Authors: Nersesyan M. D. , Claycomb J. R. , Ming Q. , Miller J. H. , Richardson J. T. , Luss Dan
Published: July 2002

We report here the observation of chemomagnetism, generation of a magnetic field by rapid high-temperature solid reactions producing various oxides. The low-level transient magnetic fields were measured with a high-Tc superconducting quantum interference device magnetometer during the synthesis of both nonferromagnetic and ferromagnetic (ferrite) compounds. The magnetic field was most likely produced by chemoionization processes generated by the moving high-temperature reaction zone. The permanent magnetic field formed by the synthesis of ferromagnetic materials depended on the difference between the combustion temperature and the Curie temperature of the product.
Two-dimensional metallodielectric photonic crystal with a large band gap
Authors: Jin Chongjun , Cheng Bingying , Man Baoyuan , Zhang Daozhong , Ban Shouzheng , Sun Bo , Li Lieming , Zhang Xiangdong , Zhang Zhaoqing
Published: July 2002

Three kinds of square metallodielectric photonic crystals have been fabricated. In these crystals, a very small metal cylinder is inserted into the center of each unit cell, while the dielectric cylinders are located at the corners of each unit cell. The transmissions of these photonic crystals have been measured in the microwave region. Compared with dielectric photonic crystals, the first photonic band gap is enlarged by a factor of 2. The experimental results agree well with the simulation obtained by the multiscattering approach. This method may be valuable for the design of photonic crystals with a large band gap.