bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of magnetic fluctuations of electromagnetic plasma waves by cross-polarization scattering
Authors: Mase A. , Bruskin L. G. , Oyama N. , Tokuzawa T. , Inutake H. , Itakura A. , Yokoi M. , Hojo H. , Ichimura M. , Tamano T.
Published: July 2002

The cross-polarization scattering (CPS) technique was applied to the GAMMA 10 tandem mirror for measurements of internal magnetic fluctuations associated with waves in the ion cyclotron range of frequency. The CPS is related to the mode-conversion effect consisting of a polarization difference of a scattered wave with regard to an incident wave. In GAMMA 10, the incident ordinary (extraordinary) wave is transferred to the extraordinary (ordinary) wave by magnetic field fluctuations near the cutoff layer of a plasma. The transferred wave penetrates through the cutoff layer and is detected. The frequency spectra and density and magnetic field fluctuations of the waves are evaluated from the CPS diagnostic and reflectometry. The ratio of the magnetic field to density fluctuation level depends on the phase velocity of the slow Alfvén wave. The CPS data are also compared with those obtained from magnetic probes.
An optoelectronic control of arc gap during formation of fullerenes and carbon nanotubes
Authors: Lange H. , Baranowski P. , Huczko A. , Byszewski P.
Published: July 2002

An automated system for controlling the interelectrode gap of the carbon arc and positioning the plasma, constructed from commercially available components, is described. The core components of the system are the linear photodetector array, analog, and digital integrated circuits, and two direct current motors. It allows for both maintaining constant arc gap and its positioning against the optical axis of the plasma diagnostic arrangement. The method of operation is demonstrated via the production of fullerenes and carbon nanotubes. This control contrivance has been used for more than one year in bench scale with very satisfactory results. In addition, the design presented here has been found to be simple to construct from readily available components and has the merit of being quite inexpensive, too (the entire cost is under $2400).
Force-balancing microforce sensor with an optical-fiber interferometer
Authors: Kato Nobuhiro , Suzuki Ippei , Kikuta Hisao , Iwata Koichi
Published: July 2002

A microforce sensor with a force feedback method for scanning force microscopy is presented. The force sensor is constructed by using an optical fiber and a microcantilever. The facet of the optical fiber is coated with a gold thin film 15 Å thick. This gold film acts not only as a partially reflected mirror but also as an electrode of the electrostatic actuator. The interaction force between a probe tip and a sample is balanced by the electrostatic force. The deflection of the cantilever is measured by an interferometer consisting of the facet of the optical fiber and the cantilever surface. We have made the force sensor and measured some force curves of a mica surface with a SiN probe tip. The force curves obtained by this sensor are quite different from the curves by a conventional measurement without a feedback system. The force resolution of the system is 10−10 N with a bandwidth of dc∼1 kHz.
Heteroepitaxy of β-SiC from methyltrichlorosilane and methyltribromosilane on Si(100) without a carbon buffer layer
Authors: Kunstmann Th. , Vepřek S.
Published: July 2002

Methyltrichlorosilane and a novel precursor methyltribromosilane have been used for the heteroepitaxial deposition of β-SiC on Si(100) at a relatively low temperature of 1150–1200 °C without any carbon buffer layer. The low deposition temperature and an improved temperature program during the annealing of the substrate for cleaning and subsequent deposition of the film allowed us to prepare β-SiC epitaxial films of high structural quality which is documented by the x-ray diffraction, Raman scattering spectroscopy and scanning electron microscopy.
Electron cyclotron resonance plasma ion source for material depositions
Authors: Delaunay M. , Touchais E.
Published: July 2002

An electron cyclotron resonance (ECR) plasma has been used in conjunction with solid metal targets for deposition of various metal and oxide compounds. A large microwave system provides dense and high-quality films at low gas pressures with a plasma generation independent on the sputtering process. The 2.45 GHz microwave power is introduced in the reactor via two separate rectangular waveguides and the plasma cross section is 20×5 cm2 in area. High plasma densities over 4×1011 cm−3 were measured by interferometry. An iron deposition rate of 2000 Å/mn has been achieved. Transparent aluminum oxide Al2O3 films with a refractive index of 1.57 were obtained under a deposition rate of 150 Å/mn. The association of several rectangular plasma chambers similar to the elementary plasma source used in this work is discussed. Direct metal ion production was also investigated in such a waveguide plasma source. Metal vaporization and ionization have been provided by energetic electrons from the ECR surface. Thus, a Zn+ ion current density of 30 mA/cm2 has been reached without support gas.
Ultrashort pulse reflectometry for electron density profile measurements
Authors: Domier C. W. , Roh Y. , Luhmann N. C.
Published: July 2002

In ultrashort-pulse reflectometry (USPR), an extremely short pulse or chirped waveform is propagated which contains a broad range of frequency components spanning the desired plasma density profile (or a significant fraction thereof). Upon reflection, each frequency component in the incident waveform reflects from a different spatial location (density layer) in the plasma, thus spreading out the reflected wave packet in time. By simultaneously collecting double-pass time delay data at many distinct frequencies, the time delay data may then be inverted to generate plasma density profiles using a single source and a single set of measurements. This article describes the basic principles of USPR and its application to magnetic fusion plasma devices, and details the UC Davis USPR system to be tested on the GAMMA-10 mirror device.
Multitip scanning bio-Kelvin probe
Authors: Baikie I. D. , Smith P. J. S. , Porterfield D. M. , Estrup P. J.
Published: July 2002

We have developed a novel multitip scanning Kelvin probe which can measure changes in biological surface potential ΔVs to within 2 mV and, quasisimultaneously monitor displacement to <1 μm. The control and measurement subcomponents are PC based and incorporate a flexible user interface permitting software control of each individual tip, measurement, and scan parameters. We review the mode of operation and design features of the scanning bio-Kelvin probe including tip steering, signal processing, tip calibration, and novel tip tracking/dithering routines. This system uniquely offers both tip-to-sample spacing control (which is essential to avoid spurious changes in ΔVs due to variations in mean spacing) and a dithering routine to maintain tip orientation to the biological specimen, irrespective of the latter’s movement. These features permit long term (>48 h) “active” tracking of the displacement and biopotentials developed along and around a plant shoot in response to an environmental stimulus, e.g., differential illumination (phototropism) or changes in orientation (gravitropism).
Microtensile device for stress: Elongation tests on nonstandard specimens
Authors: Branca F. P. , Marinozzi F.
Published: July 2002

A microtensile device (MTD) especially designed to carry out test on nonstandard specimens such as thin wires, polymers, and biological samples is described. The use of a feedback control system in order to apply the load and simultaneously assure axial deformations allows for measurement with great accuracy even on specimens of only a few millimeters. Within the device, a laser interferometer gives high-resolution elongation measurements. The prototype allows for the evaluation of specimen stiffness, defined as the ratio of the applied load versus the specimen axial deformation. This measurement can be performed with an accuracy better than 1.5%, depending on the accuracy class of the load cell and its positioning in the MTD body, while the elongation measurement is affected by a systematic error of 0.1 μm/N proportional to the applied load. In order to compare the obtained results to commonly available data, some tests have been performed on specimens of well-known mechanical characteristics, such as optical fibers. A typical optical fiber Young modulus evaluation was (67±1.5) GPa. Other tests have been carried out on nonconventional (biological) specimens with the aim of demonstrating the MTD specific application field.
Measurement of elastic impedance with high spatial resolution using acoustic microscopy
Authors: Hirsekorn S. , Pangraz S. , Weides G. , Arnold W.
Published: July 2002

The magnitude of the gray levels in an image obtained by an acoustric microscope can be exploited to measure the acoustic impedance of a sample with the spatial resolution of the instrument. Theoretical considerations show the functional dependence of the maximal amplitude of specular reflection from material constants, which is confirmed by experiments. It turns out that for these amplitude values the opening angle of the lens plays a minor role. For a quantitative evaluation of the impedance, calibration by a known object material is necessary.
Vacancy generation resulting from electrical deactivation of arsenic
Authors: Lawther D. W. , Myler U. , Simpson P. J. , Rousseau P. M. , Griffin P. B. , Plummer J. D.
Published: July 2002

Electrical deactivation of arsenic in highly doped silicon has been studied using the positron-beam technique. Direct experimental evidence linking the formation of arsenic-vacancy complexes (i.e., Asn-v) to the deactivation process is reported. The average number of arsenic atoms per complex, n̄≳2, was determined by comparing the observed complex concentrations with those of the deactivated arsenic inferred from Hall-effect measurements.
Self-propagating room-temperature silicon wafer bonding in ultrahigh vacuum
Authors: Gösele U. , Stenzel H. , Martini T. , Steinkirchner J. , Conrad D. , Scheerschmidt K.
Published: July 2002

Wafer bonding of commercial 4 in. silicon wafers has been performed at room temperature under ultrahigh vacuum conditions. After local initiation of the bonding process the bonding area is self-propagating just as in the case of wafer bonding under atmospheric conditions. The room-temperature bonded wafers, without any additional heat treatment show a bonding strength typical for bulk material.
Highly coupled dielectric behavior of porous ceramics embedding a polymer
Authors: Gómez Alvarez-Arenas T. E. , Montero de Espinosa F.
Published: July 2002

We present a theoretical approach to study the dielectric properties of porous ceramics and composite materials and explain the experimentally observed notable influence of the coupling between the components of a saturated porous ceramic on the final behavior of the sample. This model is based on the assumption of a dielectric coupling between the components that modifies the expected averaged properties of the material.
Band-A emission in synthetic diamond films: A systematic investigation
Authors: Marinelli Marco , Hatta A. , Ito T. , Hiraki A. , Nishino T.
Published: July 2002

Several sets of diamond films were grown by microwave plasma enhanced chemical vapor deposition (CVD) using a CH4–H2 gas mixture. The growth conditions were systematically changed in order to obtain different film morphologies, preferential orientations, and crystal qualities. In particular, the substrate temperature Ts and the CH4 concentration in the gas mixture were varied in the ranges 850–950 °C and 0.4%–2.0%, respectively. The resulting films were characterized by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Room-temperature cathodoluminescence has been investigated in the energy range 1.55–6.20 eV (200–800 nm). A clear correlation of the 2.85 eV (435 nm) emission band, the so-called band A, with both the substrate temperature and CH4 content in the gas mixture during deposition, was observed. An explanation is given in terms of crystal defects and diamond film texturing induced by the growth process.
GaAs/AlAs/Si heterostructures for blocking dark current injection in impurity-band-conduction photodetectors
Authors: Martin B. G. , Lee H. P. , Jones E. W.
Published: July 2002

An experimental investigation was made of GaAs and GaAs/AlAs barriers on Si for blocking injected dark current in impurity-band-conduction (IBC) infrared photodetectors. Reducing dark current would improve detector performance. Experimental results indicate that barrier heights of the order of 0.29 eV were obtained. Current versus bias behavior at temperatures ≤20 K showed successful blocking of injected electron dark current for negative applied biases in the operating range for IBC detectors.
Growth of 6H and 4H silicon carbide single crystals by the modified Lely process utilizing a dual-seed crystal method
Authors: Heydemann V. D. , Schulze N. , Barrett D. L. , Pensl G.
Published: July 2002

The influence of polarity on the SiC crystal growth has been demonstrated using a dual-seed technique to grow on both the C- and Si-face seed simultaneously. For the investigated range of growth conditions, 4H-SiC crystals were grown on the C-face of 6H-SiC seed crystals with on-axis orientation, when the growth rate exceeded 1.2 g/h. We were never able to grow 4H-SiC on the Si- face of 6H- or 4H-SiC seed crystals under these growth conditions. The incorporation of nitrogen is shown to be 2–3 times higher in crystals grown on the C-face than on the Si-face, and is independent of both polytype and 8° off-axis orientation. The addition of more than 15% silicon powder to the SiC sublimation source resulted in polycrystalline growth.
Schottky barriers and contact resistances on p-type GaN
Authors: Mori T. , Kozawa T. , Ohwaki T. , Taga Y. , Nagai S. , Yamasaki S. , Asami S. , Shibata N. , Koike M.
Published: July 2002

We measured the Schottky barrier heights and specific contact resistivities of four different metals on p-type GaN. The Schottky barrier heights of Pt, Ni, Au, and Ti were obtained from the current-voltage characteristics to be 0.50, 0.50, 0.57, and 0.65 eV, respectively. The specific contact resistivities were 0.013, 0.015, 0.026, and 0.035 Ω⋅cm2, respectively. Our experimental results proved that the Schottky barrier heights and specific contact resistivities decrease with increase in metal work function as expected theoretically.
Phase locked fluxon-antifluxon states in stacked Josephson junctions
Authors: Carapella G. , Costabile G. , Petraglia A. , Pedersen N. F. , Mygind J.
Published: July 2002

Measurements were made on a two-stack long Josephson junction with very similar parameters and electrical access to the thin middle electrode. Mutually phase-locked fluxon-antifluxon states were observed. The observed propagation velocity is in agreement with the theoretical prediction. The I-V curves recorded using the bias current in one of the junctions as fixed parameter show that these states continue to also exist when the other junction is oppositely biased. This is found in numerical simulations based on the experimental junction parameters as well. We propose a model in which waves in the junctions coexist with fluxons.
Suppression of leakage currents and their effect on the electrical performance of AlGaN/GaN modulation doped field-effect transistors
Authors: Fan Zhifang , Mohammad S. N. , Aktas Ö. , Botchkarev A. E. , Salvador A. , Morkoç Hadis
Published: July 2002

Electrical characteristics of high performance of AlGaN/GaN modulation doped field-effect transistors (MODFETs) grown by reactive molecular beam epitaxy method are studied experimentally. The maximum measured drain-source current is 490 mA/mm, which saturates at a relatively low drain-source voltage VDS. The transconductance increases with decreasing gate length reaching a value of 186 mS/mm in devices with a gate length of LG=2 μm. Breakdown voltages of about 90 V have also been exhibited by devices with gate length LG=2 μm and drain-gate separation LDG=1 μm. To our knowledge, these are the best values obtained so far from GaN MODFETs which we attribute to the suppression of leakage currents and improved materials structure of the devices.
Optical determination of the direct bandgap energy of lead iodide crystals
Authors: Ferreira da Silva A. , Veissid N. , An C. Y. , Pepe I. , Barros de Oliveira N. , Batista da Silva A. V.
Published: July 2002

The optical bandgap energy of lead iodide PbI2, grown by Bridgman’s method, is obtained by photoacoustic spectroscopy. Due to its potential applications, as a room temperature, semiconductor material detector, which may be used as a photocell, or as a x- and γ-ray radiation detector, the physical properties of PbI2 have attracted much attention. We computed, by different methods the bandgap energy. We found the energy in a range 2.301±0.038⩽〈Eg〉⩽2.359±0.037 eV.
Microhardness study of amorphous hydrogenated boron carbide deposited on a cathode substrate by plasma deposition
Authors: Lin Shu-Han , Feldman Bernard J. , Li Dong
Published: July 2002

We have grown amorphous hydrogenated boron carbide thin films on a cathode substrate by rf plasma decomposition of diborane and methane. The chemical composition, infrared absorption, optical absorption, microhardness, and adhesion of these thin films were measured. As a function of increasing diborane concentration in the feedstock, we observe increasing boron and decreasing hydrogen concentrations, increasing infrared absorption at 1300 cm−1 due to boron icosahedra, increasing optical band gaps, dramatically increased microhardness, and increased adhesion to the underlying substrates of these thin films. These results provide evidence that the presence of boron icosahedra increases microhardness, adhesion, and optical band gaps.
Nanocrystallites in tetrahedral amorphous carbon films
Authors: Ravi S. , Silva P. , Xu Shi , Tay B. X. , Tan H. S. , Milne W. I.
Published: July 2002

The microstructure of filtered cathodic vacuum arc deposited tetrahedral amorphous carbon films is studied as a function of ion energy. An optimum energy window in the density and C–C  sp3 content at an ion energy of ∼90 eV observed in this study. It is shown that the density of the amorphous carbon films are closely related to the sp3 content. The observation of nanocrystals embedded in the amorphous carbon matrix is reported. Most of the crystals observed by transmission electron microscopy can be indexed to graphite, but some of the crystals can be indexed to cubic diamond. The chemical composition of the crystals is analyzed using electron energy loss spectroscopy (EELS). The only discernible EELS edge is that of C at an energy of 285 eV.
Epitaxial growth of ultrathin Si caps on Si(100):B surface studied by scanning tunneling microscopy
Authors: Zhang Z. , Kulakov M. A. , Bullemer B. , Eisele I. , Zotov A. V.
Published: July 2002

Epitaxial growth of extremely thin Si films (few monolayers) is studied by scanning tunneling microscopy. Using specific boron-induced surface features as an indication of the presence of boron on the surface, boron segregation to the surface is tested both in solid phase epitaxy and molecular beam epitaxy regimes. The results show that under appropriate conditions there is no indication on boron segregation even on atomic scale.
Soluble and processable regioregular poly(3-hexylthiophene) for thin film field-effect transistor applications with high mobility
Authors: Bao Zhenan , Dodabalapur Ananth , Lovinger Andrew J.
Published: July 2002

The electrical characteristics of field-effect transistors using solution cast regioregular poly(3-hexylthiophene) are discussed. We demonstrate that both high field-effect mobilities (ca. 0.045 cm2/V s in the accumulation mode and 0.01 cm2/V s in the depletion mode), and relatively high on/off current ratios (greater than 103) can be achieved. We find that the film quality and field-effect mobility are strongly dependent on the choice of solvents. In addition, treating a film with ammonia or heating to 100 °C under N2 can increase the on/off ratio without decreasing the mobility.
Mapping the potential of graphite nanotubes with electron holography
Authors: Lin Xiwei , Dravid Vinayak P.
Published: July 2002

The technique of transmission electron holography has been employed to investigate the nanoscale features of graphitic nanotubes (buckytubes) and their particulate derivatives. Reconstructed phase images of holograms offer a quantitative means to calculate the mean inner potential (MIP), thickness profiles, and three-dimensional morphology for multi-/single shell buckytubes, their ‘‘tips’’ and other enclosed structures. It is demonstrated that electron holography is also sensitive to the defect nature of termination and bending in buckytubes and their particulate derivatives. Phase sensitivity of about 0.1 rad is demonstrated for single-layer buckytube, which corresponds to change in phase due to two monolayers of carbon.
8 W continuous wave front-facet power from broad-waveguide Al-free 980 nm diode lasers
Authors: Mawst L. J. , Bhattacharya A. , Lopez J. , Botez D. , Garbuzov D. Z. , DeMarco L. , Connolly J. C. , Jansen M. , Fang F. , Nabiev R. F.
Published: July 2002

Al-free 980 nm InGaAs/InGaAsP/InGaP laser structures grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) have been optimized for high cw output power by incorporating a broad waveguide design. Increasing the optical-confinement layer total thickness from 0.2 to 1.0 μm decreases the internal loss fivefold to 1.0–1.5 cm−1, and doubles the transverse spot size to 0.6 μm (full width half-maximum). Consequently, 4-mm long, 100-μm-aperture devices emit up to 8.1 W front-facet cw power. cw power conversion efficiencies as high as 59% are obtained from 0.5-mm long devices. Catastrophic-optical-mirror-damage (COMD) power-density levels reach 15.0–15.5 MW/cm2, and are found similar to those for InGaAs/AlGaAs facet-coated diode lasers.
X-ray photoelectron spectroscopy of carbon nitride films deposited by graphite laser ablation in a nitrogen postdischarge
Authors: Tabbal Malek , Mérel Philippe , Moisa Simona , Chaker Mohamed , Ricard André , Moisan Michel
Published: July 2002

Carbon nitride thin films have been deposited on silicon substrates, using a newly developed surface wave discharge/pulsed laser deposition system. Nitrogen incorporation in the films is examined by x-ray photoelectron spectroscopy (XPS). It shows that interaction between the laser ablated carbon species and nitrogen atoms from the surface-wave N2 plasma enhances the incorporation of N in the carbon nitride layers, for example, up to 19% at a deposition pressure of 2 mTorr. Increasing the deposition temperature decreases nitrogen incorporation and changes the local chemical environment of nitrogen atoms.
Solitary-pulse regimes of solid-state laser additively mode locked by a cascading nonlinearity
Authors: Mel’nikov I. V. , Shipulin A. V.
Published: July 2002

The steady-state additive mode locking of a laser incorporating phase-mismatched frequency converter in an external cavity is analyzed. The model includes a main cavity with a slowly bleaching lasing transition and a fiber with χ(2) grating, in the form of Fabry-Pérot interferometer. The dependence of the parameters of the spectral-limited pulse on the laser control parameter is investigated.
Raman spectroscopy investigation of size effects in cubic boron nitride
Authors: Werninghaus T. , Hahn J. , Richter F. , Zahn D. R. T.
Published: July 2002

Raman spectra were taken of cubic boron nitride (c-BN) crystals with diameters between 100 nm and 1 mm. The Raman line shape of the optical phonons was found to become increasingly asymmetric towards lower frequency shifts, broader and weaker with decreasing crystal diameters. The results can be explained in terms of a spatial correlation model. The corresponding correlation lengths lie in the nanometer range, i.e. several orders of magnitude below the actual crystallite sizes as determined by electron microscopy, thus revealing a high defect density. An additional examined typical c-BN film on a Si(100) substrate exhibits even weaker and broader structures consistent with an even higher defect density.
Evidence of strain and lattice distortion in lead sulfide nanocrystallites
Authors: Qadri S. B. , Yang J. P. , Skelton E. F. , Ratna B. R.
Published: July 2002

X-ray diffraction studies on nanometer sized lead sulfide particles reveal the presence of a compressive strain. A number of samples with particle sizes ranging from 2 to 16 nm were synthesized using the three dimensional periodic, bicontinuous cubic phase as a matrix. Samples of the larger size particles could be indexed to an fcc lattice. As the particle size decreased below 6 nm, a tetragonal distortion of the cubic lattice was observed, accompanied by a decrease in the unit cell volume.
Growth and ultraviolet application of Li2B4O7 crystals: Generation of the fourth and fifth harmonics of Nd:Y3Al5O12 lasers
Authors: Komatsu R. , Sugawara T. , Sassa K. , Sarukura N. , Liu Z. , Izumida S. , Segawa Y. , Uda S. , Fukuda T. , Yamanouchi K.
Published: July 2002

A 2 in. diam single crystal of lithium tetraborate (Li2B4O7) was successfully grown by the Czochralski method. The crystal was free from macrodefects and had a dislocation density as low as 100/cm2. It had an excellent homogeneity of the refractive index and a wide transparency down to 170 nm. The optical damage threshold was 40 GW/cm2. Second-harmonic generation and sum frequency generation were investigated in association with the generation of the fourth and fifth harmonics of a Q-switched Nd:YAG laser. The conversion efficiency of the second-harmonic generation from the green (532 nm) light was 20%.
On the kinetics of growth of highly defective GaN epilayers and the origin of the deep trap responsible for yellow-band luminescence
Authors: Liu H. , Kim J. G. , Ludwig M. H. , Park R. M.
Published: July 2002

The kinetics of growth of GaN/(0001) sapphire heteroepitaxial films have been examined in the relatively low substrate temperature range, 560–640 °C, using the reflection high energy electron diffraction (RHEED) specular reflection intensity monitoring technique. In particular, an alternate element exposure method of growth was employed in which Ga and N atoms were supplied separately (rather than simultaneously, as in conventional molecular beam epitaxy) to the substrate with the inclusion of a time delay between successive Ga flux and N flux exposures. We interpret the observed time dependent recovery of the RHEED specular reflection intensity during the time delay phases to be associated with Ga–N surface molecule migration on Ga-terminated surfaces and the activation energy for this migration process was determined to be 1.45±0.25 eV.
A superconductive magnetoresistive memory element using controlled exchange interaction
Authors: Oh Sangjun , Youm D. , Beasley M. R.
Published: July 2002

A memory device that can be switched between the normal state and superconducting state by an external magnetic field is proposed. The device consists of a superconducting/double magnetic (SM1M2) trilayer and is switched in a manner analogous to giant magnetoresistive memory devices. Using Usadel equations it is shown that the superconducting transition temperature of the device changes when the magnetic configurations of magnetizations of the two lower layers are switched between parallel and antiparallel. Appropriate design parameters are discussed and the materials issues analyzed.
Mechanism of organization of three-dimensional islands in SiGe/Si multilayers
Authors: Mateeva E. , Sutter P. , Bean J. C. , Lagally M. G.
Published: July 2002

The organization of coherent three-dimensional islands during the growth of SiGe/Si multilayers on Si(100) was investigated with cross-sectional transmission electron microscopy. Merging of islands of different initial size is found to be the dominant mechanism leading to a uniform size distribution. Upon overgrowth with Si, we observe a change of the shape of the islands from the {105}-faceted “hut” to a boxlike shape bounded on top by a (100) facet.
Degradation of nonencapsulated polymer-based light-emitting diodes: Noise and morphology
Authors: Savvate’ev Vadim N. , Yakimov Aharon V. , Davidov Dan , Pogreb Roman M. , Neumann Ronny , Avny Yair
Published: July 2002

We show that the degradation of a nonencapsulated polymer-based light-emitting diode (LED) is accompanied by the appearance of strong fluctuations,—that is noise both in the radiance and in the film resistance. We demonstrate a correlation between the morphological changes which occur during the degradation process and the noise, suggesting that the sampling of noise during LED operation can be used as a very efficient tool to predict the approaching failure of LEDs in real-life applications. The morphological changes in LED degradation are essentially a two-stage process. First, there is formation of “bubbles” at the metal–polymer interface due to delamination of the polymer film from the metal surface. Second, carbonized areas in the form of “black spots” are formed. Accumulation of carbonized areas leads to short and/or open circuits and final LED failure.
High rate and selective etching of GaN, AlGaN, and AlN using an inductively coupled plasma
Authors: Smith S. A. , Wolden C. A. , Bremser M. D. , Hanser A. D. , Davis R. F. , Lampert W. V.
Published: July 2002

The etching behavior of gallium nitride (GaN), aluminum gallium nitride (AlxGa1−xN), and aluminum nitride (AlN) has been systematically examined in an inductively coupled plasma (ICP) using Cl2 and Ar as the reagents. Etch rates were strongly influenced by ICP power and dc bias, while relatively insensitive to pressure, flow rate, and gas composition. Maximum etch rates of 9800 Å/min for GaN, 9060 Å/min for Al0.28Ga0.72N, and 7490 Å/min for AlN were attained. The etch profiles were highly anisotropic over the range of conditions studied. The dc bias had to exceed certain voltages before significant etch rates were obtained. These values were <−20 V for GaN, −40 V for Al0.28Ga0.72N, and >−50 V for AlN. As such, increasing selectivity for GaN over Al0.28Ga0.72N and AlN was achieved at dc biases below −40 V. At −20 V, the GaN etch rates were 38 times greater than AlN and a factor of 10 greater than Al0.28Ga0.72N. These results demonstrate the importance of ion bombardment in the etching of these materials.
Boron-doped amorphous diamondlike carbon as a new p-type window material in amorphous silicon p-i-n solar cells
Authors: Lee Chang Hyun , Lim Koeng Su
Published: July 2002

A boron-doped hydrogenated amorphous diamondlike carbon (a-DLC:H) was prepared using a mercury-sensitized photochemical vapor deposition (photo-CVD) method. The source gases were B2H6 and C2H4. By increasing the boron doping ratio (B2H6/C2H4) from 0 to 12 000 ppm, the dark conductivity increased from ∼10−9 to ∼10−7 S/cm. A boron-doped a-DLC:H with an energy band gap of 3.8 eV and a dark conductivity of 1.3×10−8 S/cm was obtained at a doping ratio of 3600 ppm. By using this film, amorphous silicon (a-Si) solar cells with a novel p-a-DLC:H/p-a-SiC double p-layer structure were fabricated using the photo-CVD method and the cell photovoltaic characteristics were investigated as a function of a-DLC:H layer thickness. The open circuit voltage increased from 0.766 V for the conventional cell with a 40-Å-thick p-a-SiC to 0.865 V for the cell with a p-a-DLC:H (15 Å)/p-a-SiC (40 Å) double p-layer structure. The thin (<15 Å) p-a-DLC:H layer proved to be an excellent hole emitter as a wide band gap window layer.
Cutoff anomalies in light emitted from the tunneling junction of a scanning tunneling microscope in air
Authors: Pechou R. , Coratger R. , Ajustron F. , Beauvillain J.
Published: July 2002

Light emission from noble metal tunneling junctions of a scanning tunneling microscope has been detected in air. A spectroscopic study of the photons emitted from Au/Au and PtIr/Au junctions has been performed as a function of the applied bias voltage. A higher-energy emission band, which grows in as a function of bias voltage, has been observed in the recorded spectra. The high energy cutoffs of the recorded spectra do not follow the quantum law hνcutoff=eVsample.
Mechanical relaxation and “intramolecular plasticity” in carbon nanotubes
Authors: Yakobson B. I.
Published: July 2002

The question of how carbon nanotubes (CNT)—believed to be the strongest filaments—relax under tension has been addressed. A dislocation theory applied to a two-dimensional nanocrystal such as the CNT describes the main routes of mechanical relaxation in this molecular structure: a brittle cleavage or, at high temperatures, a plastic flow. Both start with diatomic rotation, which “unlocks” the pristine wall of CNT by creating a dislocation dipole with the pentagon–heptagon cores. Under high stress, the dislocations depart from each other along helical paths, leaving behind a nanotube of smaller diameter, well-defined new symmetry, and changed electrical properties.
Gradient force: The mechanism for surface relief grating formation in azobenzene functionalized polymers
Authors: Kumar Jayant , Li Lian , Jiang Xin Li , Kim Dong-Yu , Lee Taek Seung , Tripathy Sukant
Published: July 2002

A model for the formation of holographic surface relief gratings in azobenzene functionalized polymers is presented. Forces leading to migration of polymer chains upon exposure to light in the absorption band of the azo chromophore are attributed to dipoles interacting with the gradient of the electric field present in the polymer material. Efficient trans–cis cycling in the azobenzenes allows cooperative movement of the chromophores under the influence of gradient forces.
Effect of roughness slope on exchange biasing in NiO spin valves
Authors: Hwang D. G. , Lee S. S. , Park C. M.
Published: July 2002

Exchange biasing field Hex and coercive field Hc of the exchange-biased NiO spin valves deposited over differently etched glass substrates and glass Si3N4 buffer layers were measured in order to investigate the effect of roughness at the NiO/NiFe interfaces. The magnetoresistive (MR) ratio, Hex, and Hc were not influenced by etching time, even though the rms roughness Rrms increased from 4.7 to 33 Å. However, the MR ratio, Hex, and Hc increased with Si3N4 buffer thickness, even when the Rrms had almost the same values. To explain this ambiguous dependence of Rrms, we consider an effect of the average slope of roughness instead of Rrms in an atomic force microscope image. The steep slope of roughness played an important role in Hex and Hc in NiO spin valves due to an increase in magnetostatic energy and the decrease in antiferromagnetic domain size.
A scaling law of the second-order hyperpolarizability in armchair nanotube
Authors: Xie Rui-Hua , Rao Qin
Published: July 2002

The static second-order hyperpolarizabilities γ of armchair nanotubes C60+i×10 are studied. Their magnitudes obey a scaling law given by γ/γ60=(1+i/6)3.15, where γ60 is the static γ magnitude of C60. Based on this law, as an example, we predict that the static γ magnitude of armchair nanotube with a length of 40 Å generally seen in experiment is about 3.5×10−29 esu, which is an appropriate value needed in photonic devices, and implies that armchair nanotube is an ideal candidate among all third-order materials for photonic applications.
Polyquinoline/bismaleimide composites as high-temperature-resistant materials
Authors: Nalwa Hari Singh , Suzuki Masahiro , Takahashi Akio , Kageyama Akira
Published: July 2002

Polyquinoline/bismaleimide composites containing bismaleimide contents between 5 and 50 wt % have been investigated as high-temperature-resistant materials. The effect of bismaleimide loading on thermal, mechanical, and dielectric properties of polyquinoline has been reported. The dielectric constant of composite thin films was found to vary between 2.90 and 3.10 with the loss tangent ranging between 0.004 and 0.007, depending upon the incorporated bismaleimide content. The thermal, mechanical, and dielectric properties of polyquinoline/bismaleimide composite thin films are discussed.
Viscosity of eutectic Pd78Cu6Si16 measured by the oscillating drop technique in microgravity
Authors: Egry I. , Lohöfer G. , Seyhan I. , Schneider S. , Feuerbacher B.
Published: July 2002

During the STS-83 Spacelab mission, a sample of Pd78Cu6Si16 was processed in the electromagnetic levitation facility TEMPUS. Surface oscillations of the levitated liquid droplet were excited, and frequency and damping of the oscillations were observed. Under microgravity conditions, the damping constant is simply related to the viscosity. This method was successfully applied. The experiments were performed in a temperature range of 400 K, including the eutectic temperature Te=1033 K. At this temperature, our data agree well with those of S. K. Lee, K. H. Tsang, and H. W. Kui [J. Appl. Phys. 70, 4842 (1991)].
Data storage with 0.7 nm recording marks on a crystalline organic thin film by a scanning tunneling microscope
Authors: Ma L. P. , Yang W. J. , Xue Z. Q. , Pang S. J.
Published: July 2002

Ultrahigh density data storage on a novel organic thin film by scanning tunneling microscope (STM) under ambient conditions is demonstrated. The material, N-(3-nitrobenzylidene)p-phenylenediamine (NBPDA), is used for preparing thin film by vacuum evaporation method. Crystalline NBPDA films with electrical bistability are obtained by this method. Recording experiment on the films is made by applying voltage pulses between the STM tip and substrate. The recorded marks are 0.7 nm in size, corresponding to a storage density of 1014 bit/cm2. Current–voltage characteristic measurement shows that the resistance of the unrecorded region of the NBPDA films is much higher than that of the recorded region. The mechanism of recording is discussed.
Carbon nanotube tipped atomic force microscopy for measurement of <100 nm etch morphology on semiconductors
Authors: Nagy G. , Levy M. , Scarmozzino R. , Osgood R. M. , Dai H. , Smalley R. E. , Michaels C. A. , Flynn G. W. , McLane G. F.
Published: July 2002

The use of carbon nanotubes as tips in atomic force microscopy for a systematic study of dry etching pattern transfer in GaAs is described. The GaAs samples are patterned via electron beam lithography and then etched using magnetron reactive ion or chemically assisted ion beam processing. The technique allows diagnosis, in air, of etched features with scale sizes of <100 nm.
Aluminum based cathode structure for enhanced electron injection in electroluminescent organic devices
Authors: Jabbour G. E. , Kippelen B. , Armstrong N. R. , Peyghambarian N.
Published: July 2002

Cathodes made with Al–LiF or Al–CsF composites are found to greatly enhance the performance of organic light-emitting devices (OLEDs). With a composite cathode, devices based on an organic bilayer structure have shown lower operating voltage, higher efficiency, and better forward light output than devices with LiF/Al, Mg, or Al cathode. Unlike devices with an Al and Li alloy cathode, OLEDs with a composite cathode can be made with good reproducibility.
High-performance, reliable, 730-nm-emitting Al-free active region diode lasers
Authors: Al-Muhanna A. , Wade J. K. , Earles T. , Lopez J. , Mawst L. J.
Published: July 2002

Compressively strained InGaAsP quantum well (QW) active (λ=732 nm) diode lasers achieve 2.9 W continuous wave (cw) power from facet-coated (4%/95%) 100-μm-wide apertures, with reliable operation demonstrated at 0.5 W cw power. A broad waveguide structure is used to obtain a large transverse spot size (d/Γ=0.433 μm), resulting in a low internal loss (αi∼2 cm−1) and narrow transverse far-field beam width (θ1/2=38°). Record-high characteristic temperatures for the threshold current and the differential quantum efficiency (T0=115 K and T1=285 K) are obtained by growing on misoriented substrates.
Diagnostics of “colossal” magnetoresistance manganite films by Raman spectroscopy
Authors: Podobedov V. B. , Romero D. B. , Weber A. , Rice J. P. , Schreekala R. , Rajeswari M. , Ramesh R. , Venkatesan T. , Drew H. D.
Published: July 2002

Polarized Raman scattering by phonons is used to characterize thin films prepared by laser ablation of La1−xCaxMnO3 targets. It was found that, in the temperature range from 6 to 300 K, phonon spectra of La0.7Ca0.3MnO3 films exhibit observable differences from those in bulk materials (microcrystalline ceramics and single crystals). A significant difference was found in the spectra of “as-grown” films compared to those annealed in oxygen at 800 °C. The observed Raman peaks and their linewidths exhibit an irregular temperature dependence near Tc. A correlation of Raman data with magnetization of the sample was also found.
Ohmic contacts formed by electrodeposition and physical vapor deposition on p-GaN
Authors: DeLucca J. M. , Venugopalan H. S. , Mohney S. E. , Karlicek R. F.
Published: July 2002

Electrodeposited Pt and sputtered Ni/Pt contacts to p-GaN (p=4.6×1017 cm−3) are reported and compared to sputtered Ni, Pt, and Ni/Au contacts and electron beam and thermally evaporated Ni contacts. Sequential rapid thermal annealing was employed with samples receiving an initial five minute heat treatment of 400 °C followed by 1 min anneals at 500, 600, and 700 °C, all under flowing N2. Plots of current versus voltage for all contacts showed nonlinearity through the origin as deposited and for all annealing conditions. Extracted values of specific contact resistance are thus determined using the measured resistance for a given value of applied current. The lowest contact resistivity was reproducibly provided by the electrodeposited Pt contacts. After a 1 min anneal at 600 °C, a contact resistivity of 1.50×10−2 Ω cm 2 was obtained using the circular transmission line method at a measurement current of 10 mA. Sputtered Ni/Pt contacts provided a contact resistivity of 1.81×10−2 Ω cm2 at 10 mA after a 1 min anneal at 600 °C, while all other metallizations yielded contact resistivities from 3–4×10−2 Ω cm 2. Possible reasons for the lower contact resistivity of the electrodeposited contacts are discussed.
An application of the apertureless scanning near-field optical microscopy: Imaging a GaAlAs laser diode in operation
Authors: Bachelot R. , Wurtz G. , Royer P.
Published: July 2002

The imaging of a commercial GaAlAs semiconductor laser emitting at 0.78 μm has been performed by apertureless scanning near-field optical microscopy using a sharp metallic tip. This tip was used as an optical and force probe simultaneously. In stimulated emission, we observed the single transverse laser mode whose dimensions are in good agreement with theoretical results. In spontaneous emission, an optical contrast linked to the known laser layers structure was observed.
A simple, reliable technique for making electrical contact to multiwalled carbon nanotubes
Authors: de Pablo P. J. , Graugnard E. , Walsh B. , Andres R. P. , Datta S. , Reifenberger R.
Published: July 2002

A simple method of making reliable electrical contact to multiwalled carbon nanotubes is described. With these contacts, current in the mA range can be routinely passed through individual multiwalled nanotubes without adverse consequences, thus allowing their resistance to be measured using a common multimeter. The contacts are robust enough to withstand temperature excursions between room temperature and 77 K. I(V) data from different multiwalled nanotubes are presented and analyzed.
Lateral epitaxial overgrowth of GaN films on sapphire and silicon substrates
Authors: Kung P. , Walker D. , Hamilton M. , Diaz J. , Razeghi M.
Published: July 2002

We report the lateral epitaxial overgrowth of GaN films on (00.1) Al2O3 and (111) Si substrates by metalorganic chemical vapor deposition. The lateral epitaxial overgrowth on Si substrates was possible after achieving quasimonocrystalline GaN template films on (111) Si substrates. X-ray diffraction, photoluminescence, scanning electron microscopy, and atomic force microscopy were used to assess the quality of the lateral epitaxial overgrown films. Lateral growth rates more than five times as high as vertical growth rates were achieved for both lateral epitaxial overgrowths of GaN on sapphire and silicon substrates.
Optical poling of azo-dye-doped thin films using an ultrashort pulse laser
Authors: Kitaoka Kenji , Si Jinhai , Mitsuyu Tsuneo , Hirao Kazuyuki
Published: July 2002

DR1(4-[N-ethyl-N-(2-hydroxyethyl)]amino-4′-nitro-azobenzene)-doped poly(methyl methacrylate) and DR1-doped silica gel thin films were optically polarized by the coherent superposition of the 1500 nm fundamental and the 750 nm second-harmonic light of a 130 fs pulse laser. These films were successfully polarized and saturated in 2 min, despite the transparent seed beams’ wavelength for the dye. The mechanism of the optical poling likely occurred in such a way that a multiphoton absorption and a photoisomerization of DR1 induced a polarized structure in the films.
A (23×213) surface phase in the 6H–SiC(0001) surface studied by scanning tunneling microscopy
Authors: Naitoh M. , Takami J. , Nishigaki S. , Toyama N.
Published: July 2002

The structure of Si-rich 6H–SiC(0001) surfaces has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction. We observed a surface phase with (−2  4 26) periodicity (designated as (23×213) for convenience), coexisting with the known stable (3×3) phase, in a surface obtained by annealing the (3×3) surface at 800 °C. A structural model containing eight Si adatoms per unit cell on the Si adlayer is proposed, which is consistent with the present STM images and with the extension of the (3×3) model structure.
Soft breakdown of gate oxides in metal–SiO2–Si capacitors under stress with hot electrons
Authors: Lombardo S. , La Magna A. , Gerardi C. , Alessandri M. , Crupi F.
Published: July 2002

We have investigated the intrinsic dielectric breakdown of gate oxide layers with thickness of 12 and 7 nm in n+ polycrystalline Si–SiO2–Si metal/oxide/semiconductor (MOS) capacitors after stress with constant current either under Fowler-Nordheim or under hot electron injection. Occurrence of soft breakdown without thermal damage in the MOS structure is demonstrated even in a 12 nm oxide under particular stress conditions. In general, it is found that the type of stress determines the breakdown mode (soft or hard).
Long-wavelength lasing from multiply stacked InAs/InGaAs quantum dots on GaAs substrates
Authors: Zhukov A. E. , Kovsh A. R. , Maleev N. A. , Mikhrin S. S. , Ustinov V. M. , Tsatsul’nikov A. F. , Maximov M. V. , Volovik B. V. , Bedarev D. A. , Shernyakov Yu. M. , Kop’ev P. S. , Alferov Zh. I. , Ledentsov N. N. , Bimberg D.
Published: July 2002

An InAs quantum dot (QD) array covered by a thin InGaAs layer was used as the active region of diode lasers grown by molecular beam epitaxy on GaAs substrates. The wavelength of the ground-state transition in such heterostructures is in the 1.3 μm range. In the laser based on the single layer of QDs, lasing proceeds via the excited states due to insufficient gain of the ground level. Stacking of three QD planes prevents gain saturation and results in a low threshold (85 A/cm2 in broad-area 1.9-mm-long stripe) long-wavelength (1.25 μm) lasing at room temperature via the QD ground state with relatively high differential efficiency (>50%).
Tuning exchange bias
Authors: Miltényi P. , Gierlings M. , Bamming M. , May U. , Güntherodt G. , Nogués J. , Gruyters M. , Leighton C. , Schuller Ivan K.
Published: July 2002

The exchange bias shift of the hysteresis loop, HE, in antiferromagnetic/ferromagnetic layer systems can be easily controlled (within certain limits) by cooling in zero field from different magnetization states above the antiferromagnetic Néel temperature, TN. This indicates that for moderate cooling fields, HE is determined by the magnetization state of the ferromagnet at TN, and not by the strength of the cooling field.