bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular beam epitaxy of AlAs0.16Sb0.84 and Al0.8Ga0.2As0.14Sb0.86 on InAs substrates
Authors: Lott J. A. , Dawson L. R. , Jones E. D. , Klem J. F.
Published: July 2002

Epitaxial films of AlAs0.16Sb0.84 and Al0.8Ga0.2As0.14Sb0.86 were grown lattice matched on (100) InAs substrates by molecular beam epitaxy. The material was characterized by x-ray diffraction, 4 K photoluminescence, and capacitance-voltage measurement techniques. At 300 K, background acceptor concentrations of 1.8×1015 and 1.4×1016 cm−3 were determined for the unintentionally doped AlAsSb and AlGaAsSb epitaxial layers, respectively. Compensating the AlAsSb and AlGaAsSb epitaxial layers with sulfur doping resulted in high-resistivity material with an effective donor concentration of about 1014 and 1015 cm−3, respectively.
CO2 laser drilling of copper following excimer laser pretreatment
Authors: Kinsman G. , Duley W. W.
Published: July 2002

Efficient CO2 laser drilling of copper sheet has been obtained following pretreatment with excimer laser radiation at intensities of (1–2)×106 W cm−2. The present studies show a significant reduction in the laser pulse energy, pulse duration, and gas assist parameters required to drill copper sheets with thicknesses of up to 1 mm. Optimization of laser drilling under these conditions involves a material removal regime that differs from that obtained at high laser intensity when drilling untreated surfaces.
Superconducting YBa2Cu3O6.8 films on metallic substrates using in situ laser deposition
Authors: Narumi E. , Song L. W. , Yang F. , Patel S. , Kao Y. H. , Shaw D. T.
Published: July 2002

In situ laser deposition was used to fabricate YBa2Cu3O6.8 (YBCO) films on metallic substrates. With a buffer layer, YBCO films were strongly c-axis oriented. Tc = 83 K and Jc= 4000 A/cm2 at 77 K were obtained for a 0.5 μm YBCO film with a 0.15 μm yttrium-stabilized ZrO2 buffer layer. There was negligible interdiffusion occurring at the interfaces. The n value of YBCO films ranged from 1.4 to 1.5.
Radiation hardened metal-oxide-semiconductor devices with gate dielectrics grown by rapid thermal processing in O2 with diluted NF3
Authors: Ahn J. , Lo G. Q. , Ting W. , Kwong D. L. , Kuehne John , Magee Charles W.
Published: July 2002

Radiation-hardened, fluorinated gate oxides have been obtained by rapid thermal processing of silicon in O2 with diluted NF3. Diluted NF3 is used as a source of fluorine and is introduced during the initial stage of rapid thermal processing. It is found that optimum amounts of fluorine incorporated at the Si/SiO2 interface improve resistance against x-ray radiation; however, excessive amounts of fluorine degrade the radiation hardness.
Optical absorption spectroscopy study of the role of plasma chemistry in YBa2Cu3O7 pulsed laser deposition
Authors: Sakeek H. F. , Morrow T. , Graham W. G. , Walmsley D. G.
Published: July 2002

Time-resolved optical absorption spectroscopy techniques were used to study Ba, metastable Ba+, and YO absorptions in the laser-produced plasma plume from a YBa2Cu3O7 target. Results obtained indicate an initial explosive removal of material from the target surface followed by a subsequent evaporation process. Some YO is ejected from the target in molecular form, particularly at laser fluence <6 J/cm2, whilst additional YO is formed in subsequent reactions of Y with oxygen at the plasma plume edges. The formation of metastable Ba+ (52D5/2) is also observed in the outer reactive layers of the plasma plume.
CdTe-Cd1−xMnxTe multiple quantum well structures grown by pulsed laser evaporation and epitaxy
Authors: Dubowski J. J. , Roth A. P. , Wasilewski Z. R. , Rolfe S. J.
Published: July 2002

Structural and optical properties of (001) CdTe-Cd1−xMnxTe (x=0.10) multiple quantum well structures grown by pulsed laser evaporation and epitaxy (PLEE) are investigated. The layers are grown on (001) CdZnTe wafers held at a temperature in the range of 210–230 °C. Secondary-ion mass spectroscopy in-depth profiles reveal that highly uniform structures are grown. Numerical analysis of double crystal x-ray diffraction results demonstrates high structural quality of the layers and indicates partial relaxation of the strain in these structures. Low-temperature photoluminescence exhibits excitonic recombinations in the CdTe wells whereas photoluminescence from the Cd1−xMnxTe barriers is not observed. The chemical composition of the barriers deduced from photoluminescence is in excellent agreement with the intended chemical composition set during growth.
Simple description of laterally resonant, distributed-feedback-like modes of arrays of antiguides
Authors: Zmudzinski C. A. , Botez D. , Mawst L. J.
Published: July 2002

The Bloch-function method is used to provide a simple, yet accurate treatment of the leaky modes of infinite-extent antiguided arrays. Laterally resonant modes of infinite arrays are described for the first time. Bloch-function solutions for infinite arrays are used to explain key characteristics of the modes of finite-extent antiguided arrays. It is found that the resonant modes of antiguided arrays satisfy the second-order lateral Bragg condition, and consequently are analogous to the modes of second-order distributed-feedback (DFB) lasers. In effect, the resonant-optical-waveguide (ROW) array is a second-order lateral DFB structure with the radiative component along the longitudinal device axis, and zero stopband.
Elimination of heterojunction band discontinuities by modulation doping
Authors: Schubert E. F. , Tu L. W. , Zydzik G. J. , Kopf R. F. , Benvenuti A. , Pinto M. R.
Published: July 2002

Conduction- or valence-band discontinuities occurring at the junction of two heterogeneous semiconductors can be eliminated by appropriate doping of the interfacial region. We show by analytic and self-consistent calculations that simultaneous modulation doping and parabolic compositional grading result in flat band-edge potentials. The new concept is applied to distributed Bragg reflectors for vertical cavity lasers. The structures grown by molecular-beam epitaxy exhibit significantly lower resistances as compared to step-graded distributed Bragg reflectors.
Pump wavelength tuning of optical parametric oscillations and frequency mixing in KTiOAsO4
Authors: Jani Mahendra G. , Murray James T. , Petrin Roger R. , Powell Richard C. , Loiacono D. N. , Loiacono G. M.
Published: July 2002

The properties of alexandrite laser-pumped optical parametric oscillators are reported for potassium titanyl arsenate. Near infrared tuning curves and slope efficiencies were measured as functions of pump wavelength and pump power. In addition, sum frequency mixing of red and infrared wavelengths to produce green emission is also reported.
Electron mobility enhancement by confining optical phonons in GaAs/AlAs multiple quantum wells
Authors: Zhu X. Theodore , Goronkin Herbert , Maracas George N. , Droopad Ravi , Stroscio Michael A.
Published: July 2002

We report experimental evidence for an enhanced electron mobility in a multiple quantum well structure in which the optical phonon modes are confined. By inserting n ultrathin layers of AlAs into a host GaAs well, thus dividing the bulklike host well into n+1 miniwells, we observe a substantial enhancement in electron mobility for temperatures T≳100 K. We have also studied the dependence of the electron mobility on the miniwell width. The electron mobility is found to first increase with decreasing miniwell width and then decrease after reaching a maximum value at the width around 45 Å.
Optical bleaching of metastable defects in amorphous silicon
Authors: Ganguly Gautam
Published: July 2002

Sub-band-gap absorption due to metastable defects induced by intense visible illumination in hydrogenated amorphous silicon is bleached when treated with low intensity, intensely absorbed, ultraviolet light. The bleaching effect is shown to occur in the bulk and is nonthermal. It is proposed that the energy from the surface absorbed light can be transferred to the bulk through phonons generated by cooling of photo injected hot carriers.
Substrate surface step effects on microstructure of epitaxial films
Authors: Guo J. , Chang H. L. M. , Lam D. J.
Published: July 2002

Sapphire (0001) basal plane is a commonly used substrate for film depositions. Due to the threefold rotational symmetry of the (0001) substrate surface, epitaxial films deposited are expected to form at most three variants with relative orientations of 120° and 240°. However, epitaxial TiO2 (tetragonal) and VO2 (monoclinic) films deposited on sapphire (0001) substrates by the metalorganic chemical vapor deposition technique were found to have six variants of relative orientations of 60°, 120°, 180°, 240°, and 300° based on the x-ray diffraction studies. Furthermore, epitaxial MgO (cubic) films deposited on sapphire (0001) substrates by the molecular beam epitaxy technique were found to have two variants which are mirror images about the (12̄10) plane from the high resolution electron microscopy. We show that these unconventional film microstructures found experimentally are caused by the substrate surface steps.
Influence of arsenic adsorption layers on heterointerfaces in GaInAs/InP quantum well structures
Authors: Seifert W. , Liu X. , Samuelson L.
Published: July 2002

The 5 K photoluminescence of GaInAs/InP QWs a few monolayers in thickness has been used as a probe for investigating the influence of AsH3 concentration and growth temperature on the formation of additional interface layers between the GaInAs of the QW and the InP barrier. Increasing AsH3 concentrations and decreasing growth temperatures shift the QW peak energies to longer wavelengths, i.e., they increase the effective thickness of the QWs. The effect is discussed in terms of different growth regimes in MOVPE with different arsenic coverages of the GaInAs surface in analogy to (001) GaAs with (2×4)-like or c(4×4)-like arsenic-terminated surfaces. In the latter case the excess As is an effective source for As carry-over into the upper InP barrier of the QW resulting in the formation of graded InAsxP1−x interface layers.
Atom probe compositional analysis of Co-Cr sputtered magnetic thin films
Authors: Hono K. , Babu S. S. , Maeda Y. , Hasegawa N. , Sakurai T.
Published: July 2002

A Co-22 at. % magnetic thin film, sputter deposited on a heated Cu coated Si substrate, was analyzed in the planar direction of the film by atom probe field ion microscopy (APFIM). The atom probe concentration depth profile convincingly showed that the film contained two phases, one enriched in Cr up to 30–35 at. % and the other enriched in Co up to 95–93 at. %. This result indicates that Co-22 at. % Cr thin film deposited on a heated substrate consists of ferromagnetic and paramagnetic phases, each less than a few tens of nanometers in size.
Laser operation-induced migration of beryllium at mirrors of GaAs/AlGaAs laser diodes
Authors: Jakubowicz A. , Oosenbrug A. , Forster Th.
Published: July 2002

Laser operation-induced migration of beryllium at laser mirrors was studied by electron-beam-induced current. The devices investigated were single quantum well graded index separate confinement GaAs/AlGaAs ridge geometry laser diodes. In these devices, an operation-induced displacement of the p-n junction towards the n-type cladding has been observed close to the mirrors. A similar effect was induced by electron irradiation of the mirror facets in a scanning electron microscope. These effects have been attributed to recombination-enhanced diffusion/migration of beryllium from the p-type cladding. We have measured the diffusion coefficient of beryllium and, from this value, have estimated the average mirror temperature during laser operation. This temperature was found to be in excellent agreement with recently published measured mirror temperatures.
Dry patterning of InGaN and InAlN
Authors: Pearton S. J. , Abernathy C. R. , Ren F.
Published: July 2002

Dry etch rates of InxGa1−xN and InxAl1−xN alloys are found to increase with In mole fraction in CH4/H2 microwave (2.45 GHz) discharges, and to decrease under the same conditions in Cl2/H2 mixtures. Both plasma chemistries produce smooth anisotropic etching across the entire composition range from InN to either GaN or AlN. Addition of SF6, rather than H2, to a Cl2 discharge produces faster etch rates and retains smooth morphologies. This suggests that either atomic hydrogen or fluorine is capable of effective removal of N from the III-V nitride materials. Ar+ ion milling rates for InGaAlN alloys are found to be approximately a factor of 2 lower than for more conventional III-V semiconductors like GaAs.
Oxidation resistant high conductivity copper films
Authors: Ding P. J. , Lanford W. A. , Hymes S. , Murarka S. P.
Published: July 2002

The properties of thin films of Cu-doped with different percentages of Mg were investigated. It was found that as-deposited films of Cu (2 at. % Mg) oxidize orders of magnitude more slowly than do those of pure Cu. More importantly, when Cu(2 at. % Mg) films are annealed in Ar at 400 °C for 30 min, a thin protective layer of magnesium oxide forms on the surface and completely stops further oxidation. This annealing step also reduces the resistivity of films to the value essentially the same as that of pure sputtered copper films. Films of Cu (2 at. % Mg) also adhere to SiO2 much better than do films of pure copper. Furthermore, annealing studies show that this material remains microscopically smooth and shows no diffusion into SiO2 at temperatures up to 700 °C.
Deformation induced crystallization due to instability in amorphous FeZr alloys
Authors: Trudeau M. L.
Published: July 2002

Recently, it was shown that mechanical deformation processes can completely crystallize some amorphous transition-metal–metalloid alloys. However, the mechanism is still unclear. We demonstrate here that, for the simpler amorphous Fe90Zr10 alloy, the basic thermodynamic differences between high-energy milling and rapid quenching, associated with a crystallization controlled by elemental diffusion, could be behind this structural transformation.
Effect of nitrogen on the growth of diamond films
Authors: Jin S. , Moustakas T. D.
Published: July 2002

The incorporation of nitrogen in diamond films and its effect on film growth were investigated. The nitrogen doping efficiency was found to be very low, consistent with a model of film growth involving simultaneous deposition and etching, which predicts a doping efficiency of 10−4. The growth habit was found to change from (111) to (100) with increase of nitrogen in the gas phase from N/C = 0.1% to 10%. The growth rate of the diamond films increases, and the diamond Raman peak sharpens with the amount of nitrogen, a result consistent with the model on defect-induced stabilization of diamond.
Asymmetric tilt interface induced by 60° misfit dislocation arrays in GaSb/GaAs(001)
Authors: Kang J. M. , Min Suk-Ki , Rocher A.
Published: July 2002

Transmission electron microscopy of GaSb film grown on (001) GaAs revealed regular 60° misfit dislocation arrays in abnormally large and irregularly shaped GaSb islands. Having a same orientation of the Burgers vector, the 60° dislocations induce an asymmetric tilt interface. Calculations using anisotropic elasticity show that this interface has lower elastic energy than the symmetric interface relaxed by 60° dislocations of alternating orientations. When 60° dislocations are generated at the leading edge of growing islands, a particular orientation that assists the bending of atomic planes to the free surface of islands is preferred for a partial elastic relief of the misfit strain.
Double two-dimensional electron gas structure formed by molecular beam epitaxy regrowth on an ex situ patterned n+-GaAs back gate
Authors: Evans R. J. , Grimshaw M. P. , Burroughes J. H. , Leadbeater M. L. , Tribble M. J. , Ritchie D. A. , Jones G. A. C. , Pepper M.
Published: July 2002

We have regrown two two-dimensional electron gases (2DEGs) in a wide GaAs quantum well on a large area ex situ patterned n+-GaAs back gate. The transport in these channels is controlled by this gate and a surface front gate. We present results showing the control that the patterned back gate has over the carrier concentration in the low mobility back 2 DEG and the very low leakage currents that are observed from the back gate to the source-drain channel at 1.5 K. Using four terminal resistance and magnetoresistance data the transition from two conducting channels to conduction in the low mobility back 2DEG is shown. The implications of these results for the fabrication of velocity modulated transistors are discussed.
Enhancement of diamond nucleation by ultrasonic substrate abrasion with a mixture of metal and diamond particles
Authors: Chakk Y. , Brener R. , Hoffman A.
Published: July 2002

A method of surface treatment was found to enhance diamond chemical vapor deposition nucleation on nondiamond substrates such as Si, SiO2, and Al2O3. The nucleation density obtained by ultrasonic abrasion with diamond powder alone was found to be enhanced by a few orders of magnitude using a mixed slurry consisting of diamond and metal powders. No strong nucleation enhancement was observed using a metal slurry only for surface treatment. The metal powders used were W, Ta, Mo, Nb, Ti, Al, Fe, Ni, Cu, and Si. It is concluded that the enhanced nucleation is associated with a physico-chemical modification of the substrate surface, attained through a cooperative effect of both the metal and diamond particles during the ultrasonic abrasion process. © 1995 American Institute of Physics.
Band gap of ‘‘completely disordered’’ Ga0.52In0.48P
Authors: DeLong M. C. , Mowbray D. J. , Hogg R. A. , Skolnick M. S. , Williams J. E. , Meehan K. , Kurtz Sarah R. , Olson J. M. , Schneider R. P. , Wu M. C. , Hopkinson M.
Published: July 2002

The phenomenon of ordering in Ga0.52In0.48P is well known to reduce the optical band gap; the amount of band gap reduction is often used to measure the degree of ordering. For such measurements to be meaningful, the band gap of the random (‘‘completely disordered’’) binary alloy must be known. Values of this fundamental material parameter appearing in the literature vary by up to 40 meV, while the largest band gap reduction reported to date is only about 120 meV, i.e., within a factor of 3 of the uncertainty in one endpoint. We report here a low temperature band gap of 2.010±0.007 eV for material lattice matched to GaAs as deduced from a broad spectrum of samples believed for different reasons to contain minimal ordering. The corresponding value at 295 K is 1.910±0.008 eV.
Contact mode atomic force microscopy imaging of nanometer-sized particles
Authors: Junno T. , Anand S. , Deppert K. , Montelius L. , Samuelson L.
Published: July 2002

A general problem in scanning probe microscopy (SPM) of small particles is that the probe itself moves and removes the particles as it tries to image them. This fact has strongly limited the use of SPM in the field of nm-sized particles, and high-resolution electron microscopy has been the only alternative for detailed imaging. Here, we take advantage of a moderate sintering at 350 °C to effectively stabilize nanometer-sized metallic particles on a semiconductor surface, allowing characterization by SPM. Using this technique, we have been able to successfully image aerosol deposited Ag particles of sizes around 35 nm by contact mode atomic force microscopy (AFM). This new sample preparation technique for SPM overcomes the problems of probe-induced particle movement during scanning and is proposed as a method to ‘‘freeze’’ artificial nanostructures produced by manipulation of particles with the AFM probe.
Transferred-electron induced current instabilities in heterojunction bipolar transistors
Authors: Posse V. A. , Jalali B. , Levi A. F. J.
Published: July 2002

Current driven instabilities in the collector of III–V heterojunction bipolar transistors (HBT) are investigated. Numerical simulations indicate that in a modified AlGaAs/GaAs HBT the collector current shows oscillatory behavior due to the transferred-electron (Gunn–Hilsum) effect. Influence of the Kirk effect as well as conditions for oscillation are discussed.
Compact acoustic levitation device for studies in fluid dynamics and material science in the laboratory and microgravity
Authors: Trinh E. H.
Published: July 2002

An ultrasonic levitation device operable in both ordinary ground-based as well as in potential space-borne laboratories is described together with its various applications in the fields of fluid dynamics, material science, and light scattering. Some of the phenomena which can be studied by this instrument include surface waves on freely suspended liquids, the variations of the surface tension with temperature and contamination, the deep undercooling of materials with the temperature variations of their density and viscosity, and finally some of the optical diffraction properties of transparent substances.
Measurement of roughness exponent for scale-invariant rough surfaces using angle resolved light scattering
Authors: Fang K. , Adame R. , Yang H.-N. , Wang G.-C. , Lu T.-M.
Published: July 2002

It is shown that, by modifying the conventional light scattering geometry, it is possible to characterize the root-mean-square roughness w in a scale-invariant rough surface for w values more than one order of magnitude larger than those previously reported. Measurement of w on the order of the wavelength of light has been demonstrated. A consequence of this development is that one can combine the diffraction theory and the measurement to determine the surface roughness exponent α, which was not possible under conventional light scattering conditions.
High field strength wedged-pole hybrid undulator
Authors: Quimby D. C. , Pindroh A. L.
Published: July 2002

The wedged-pole concept is introduced as a means for enhancing the field strength and field quality of the rare-earth permanent magnet hybrid undulator. The wedged-pole configuration results in higher on-axis field strength since it causes the magnet surface which faces the gap to be driven to the full magnet coercivity, Hc. Pole tip saturation is avoided by angular wedging of the poles and magnets to provide larger pole tip area without sacrificing magnet volume. The advantages of the wedged-pole concept are investigated for an example case with a gap to wavelength ratio of 0.22 and SmCo5 magnets. It is found that the field strength in the fundamental can be improved by as much as 15%. Alternatively, the field quality can be enhanced by operation of the poles far from saturation without loss of field strength. An additional benefit is reduction of the harmonic content in the field distribution.
1-mm wave ESR spectrometer
Authors: Lynch W. Bryan , Earle Keith A. , Freed Jack H.
Published: July 2002

An ESR spectrometer operating at 250-GHz frequency (1.22-mm wavelength) and 9-T magnetic field is described. The utilization of far-infrared (FIR) technology greatly simplifies its design and performance. Good frequency and field stability are achieved by unique designs which also conveniently permit the magnetic field to be swept. The potential utility of FIR–ESR is illustrated with examples of spectra from polycrystalline and liquid samples. In the latter case the increased spectral sensitivity to motional dynamics is stressed. Several ways in which the FIR–ESR spectrometer may be improved are also discussed.
Method for extracting volume produced negative ions
Authors: Bacal M. , Bruneteau J. , Devynck P.
Published: July 2002

The extraction of H− ions from a volume source is greatly improved by the presence of a weak magnetic field parallel to the plasma electrode that magnetically insulates this electrode from the bulk plasma. When biased positive, the plasma electrode depletes the electron population in the neighboring region. To maintain plasma neutrality, negative ions from the main plasma replace the electrons in this region. Thus a high fraction of negative ions builds up in front of the plasma electrode. The implications of these phenomena for negative ion beam formation are discussed.
Highly sensitive automated setup for measuring surface photovoltage spectra
Authors: Germanova K. , Nikolov L. , Hardalov Ch.
Published: July 2002

We present an automated experimental setup for dc measurement of surface photovoltage (SPV) spectra in wide spectral and/or temperature ranges. A Pt boss, sealed on a bimorphic piezoelement, has been used as a small area vibrating electrode and a programmable digital-to-analog convertor (DAC) as a source of compensation. In addition, a combination of automatic data acquisition and statistical analysis has been applied, thus ensuring reliability and stability of SPV measurements. Moreover, the automated setup provides a high sensitivity and objectivity of SPV investigations.
Measurement of fuel ion temperatures in ICF implosions using current-mode neutron time-of-flight detectors
Authors: Russotto M. A. , Kremens R. L.
Published: July 2002

In order to obtain useful fuel ion temperature measurements in inertial confinement fusion experiments using neutron time-of-flight techniques, detailed analysis of the data must be performed. We will present a data analysis technique to measure fuel ion temperature accurately, and a Monte Carlo model which uses known detector and target physics to simulate the current pulse produced by the detector. These simulations are compared with data taken for DT fuel implosions on the OMEGA laser system. Based on this model we will determine the neutron yields necessary to accurately assess the fuel ion temperature.
Noncontact temperature measurement. I. Interpolation based techniques
Authors: Khan Mansoor A. , Allemand Charly , Eagar Thomas W.
Published: July 2002

Various ratio pyrometry techniques (two, three, and four color) are analyzed and shown to possess hitherto unknown sources of error under certain conditions. These conditions are shown to arise quite frequently. The ratio method is shown to be better suited to the shorter wavelengths. It has been shown, for various emissivities, that the predicted temperature has an asymptote at some wavelength. Ratio pyrometry methods are shown to be very sensitive to measurement noise and this sensitivity grows quickly with the number of terms in the ratio. The effect of a reference temperature in the system is examined and it is shown that under certain conditions this reference temperature can be used to make accurate predictions regarding the temperature elsewhere in the system even if the emissivity changes.
Amorphous transparent electroconductor 2CdO⋅GeO2: Conversion of amorphous insulating cadmium germanate by ion implantation
Authors: Hosono Hideo , Kikuchi Naoto , Ueda Naoyuki , Kawazoe Hiroshi , Shimidzu Ken-ichi
Published: July 2002

Amorphous 2CdO⋅GeO2 thin films (optical band gap determined by a Tauc plot; 3.4 eV) prepared by rf sputtering were implanted with H+ or Li+ ions to a dose of 2×1016 cm−2. Direct current conductivities in the specimens at 300 K were increased from 3×10−9 S cm−1 to ∼10 S cm−1 after the implantation. The conductivities in these implanted specimens remained almost constant down to 77 K. No deleterious coloring was perceived after implantation. The Hall mobilities (conduction type; n) in the implanted specimens at ∼300 K were ∼5 cm2 V−1 s−1, which are larger by several orders of magnitude than those in existing amorphous semiconductors. Such a large mobility indicates that the electronic state near the bottom of the conduction band of this amorphous material is extended.
An extension to the dynamic plane source technique for measuring thermal conductivity, thermal diffusivity, and specific heat of dielectric solids
Authors: Karawacki Ernest , Suleiman Bashir M. , ul-Haq Izhar , Nhi Bui-Thi
Published: July 2002

The recently developed dynamic plane source (DPS) technique for simultaneous determination of the thermal properties of fast thermally conducting materials with thermal conductivities between 200 and 2 W/mK has now been extended for studying relatively slow conducting materials with thermal conductivities equal or below 2 W/mK. The method is self-checking since the thermal conductivity, thermal diffusivity specific heat, and effusivity of the material are obtained independently from each other. The theory of the technique and the experimental arrangement are given in detail. The data evaluation procedure is simple and makes it possible to reveal the distortions due to the nonideal experimental conditions. The extension to the DPS technique has been implemented at room temperature to study the samples of cordierite-based ceramic Cecorite 130P (thermal conductivity equal to 1.48 W/mK), rubber (0.403 W/mK), and polycarbonate (0.245 W/mK). The accuracy of the method is within ±5%.
A segmented disk electrode to produce and control parallel and transverse particle drifts in a cylindrical plasma
Authors: Carroll J. J. , Koepke M. E. , Amatucci W. E. , Sheridan T. E. , Alport M. J.
Published: July 2002

A segmented disk electrode is described that produces and controls both relative drift parallel to the magnetic field and localized flow perpendicular to the magnetic field in a Q machine plasma. Measurements of the electron drift velocity vd with respect to the ions, and the electric field E for many bias-voltage combinations are used to demonstrate the controllability of the ratio vE/vd, where vE=E×B/B2, in a narrow channel between the regions that magnetically map to the segments. Radial electric fields of magnitudes ranging from 0 to ±3 V/cm localized within a current channel with parallel electron drift velocities up to half the electron thermal velocity have been produced with the segmented disk electrode.
A cryostatic setup for the low-temperature measurement of thermal diffusivity with the photothermal method
Authors: Bertolotti M. , Liakhou G. , Li Voti R. , Paoloni S. , Sibilia C. , Sparvieri N.
Published: July 2002

A cryostatic setup is described to perform photothermal deflection measurements from room temperature to 77 K. The setup uses gaseous nitrogen as a medium where the photodeflection is produced. The ability of the system to work is demonstrated presenting some measurements of thermal diffusivity of high-temperature superconductor samples and of yttrium-iron garnets with variable aluminum content.
Spectra of third-order optical nonlinear susceptibilities of epitaxial chloro-indium-phthalocyanines
Authors: Tajalli H. , Jiang J. P. , Murray J. T. , Armstrong N. R. , Schmidt A. , Chandross M. , Mazumdar S. , Peyghambarian N.
Published: July 2002

We have measured the third harmonic generation spectra of epitaxial chloro-indium phthalocyanine (ClInPc) thin films. The χ(3) spectrum in the wavelength range of 970–1650 nm reveals resonance features of ClInPc. Theoretical analysis of the third-order nonlinear optical channels in phthalocyanine allows the assignment of the observed resonances to three- and two-photon resonances of χ(3).
An inversion technique to obtain full poloidal velocity profiles in a tokamak plasma
Authors: Bell Ronald E.
Published: July 2002

An inversion technique has been developed to calculate local poloidal rotation velocities of impurity ions from line-integrated measurements of an extended emission source in a tokamak plasma. This technique can recover radial spatial resolution that is lost due to the curvature of the field lines. With charge exchange emission from neutral beams, it can be used to obtain poloidal velocity profiles across the entire minor radius of the plasma without requiring restricted neutral beam height to localize the emission. The technique consists of two Abel-like matrix inversions to obtain the emissivity and the velocity-weighted emissivity from which the velocity is obtained. Use of the neutral beams breaks the assumed symmetry, but knowledge of the beam deposition and geometry allow analogous matrices to be generated. The inversion technique is demonstrated with a simulation of a poloidal rotation diagnostic for the TFTR tokamak with tangential neutral beams with a height of 80 cm. Two opposing vertical views are required to handle effects of the charge exchange cross section, which can cause apparent velocities greatly in excess of expected poloidal velocities. The simulated errors due to the inversion process are about 1.5 km/s near the plasma center.
A precision piezodriven micropositioner mechanism with large travel range
Authors: Chang S. H. , Du B. C.
Published: July 2002

A micropositioning stage with large travel range has been designed and built. The stage combines a piezoelectric driving element, flexure pivoted multiple Scott–Russell linkage, and a parallel guiding spring. Quality engineering techniques are used to optimize the configuration of the device in order to achieve the maximum displacement gain and the minimum angular deviation. A simple open-loop compensator is applied to reduce the hysteresis of the dynamic response of the stage. The experiment shows that the stage achieved a vacuum-compatible device with a travel of greater than 100 μm, a resolution of 0.04 μm, and an angular deviation of less than 31.1 μrad. The first natural frequency of the stage is 80 Hz and the settling time is approximately 50 ms. Compared with the uncontrolled condition, the controlled hysteresis is reduced significantly.
Optimal conditions for imaging in scanning tunneling microscopy: Theory
Authors: Anguiano E. , Oliva A. I. , Aguilar M.
Published: July 2002

The scanning tunneling microscopy (STM) feedback system is analyzed to look for the optimal conditions for measurement. The typical feedback circuit normally used in STM and the parameters involved on it are studied, and their relative importance into the loop are discussed. The analysis of the role of each parameter demonstrated the importance of a detailed knowledge of the instrument to assure that the images obtained are reliable. We obtained equations that involve the main parameters of the loop, and yield the optimal conditions for imaging taking into account stability, signal amplitude, and phase shift. Combining the stability conditions with the imaging conditions obtained in this work, we found values for the feedback parameters to perform optimal STM measurements. Moreover, with this work, we highlight the importance to include in further publications the value of the main parameters used to obtain STM images. The conclusion of our work is that in some cases (in particular atomic resolution and fractal analysis) STM users should pay more attention to the setting of the instrument.
Evolution of the tokamak ISTTOK plasma density and electron temperature radial profiles determined by heavy ion beam probing
Authors: Malaquias A. , Nedzelskii I. S. , Varandas C. A. F. , Cabral J. A. C.
Published: July 2002

This article presents the temporal variation of the ISTTOK plasma density and electron temperature radial profiles determined by a heavy ion beam diagnostic. Pulsed Xe+ and Hg+ primary beams were injected into the tokamak in two similar discharges and Xe2+ and Hg3+ ions were collected by a multiple cell array detector. The ne and Te profiles were determined from the detected currents using an algorithm that takes into account the beam attenuations as well as the ratio between the effective cross sections of the ionization processes Xe+→Xe2+ and Hg+→Hg3+. These profiles were obtained every 1.5 ms along the central 6 cm of the plasma column diameter. The values of the average plasma density and central electron temperature are in good agreement with those achieved by other diagnostics.
A 32-channel time-resolved instrument for medical optical tomography
Authors: Schmidt Florian E. W. , Fry Martin E. , Hillman Elizabeth M. C. , Hebden Jeremy C. , Delpy David T.
Published: July 2002

A prototype multichannel time-resolved medical optical tomography system is presented, and various instrumental aspects and performance issues are discussed. The instrument has been designed primarily as a continuous bedside monitor for obtaining functional images of premature infants’ brains that are at an increased risk of injury due to dysfunction in cerebral oxygenation or hemodynamics. Separate maps of the internal absorption and scattering properties can be reconstructed from purely temporal measurements of photons transmitted diffusely through the tissue, and without recourse to reference or baseline measurements. The instrument employs 32 source fibers that sequentially deliver near-infrared pulsed laser radiation of picosecond duration. Transit time measurements of very high temporal resolution and stability are made between these sources and 32 detector optodes that are located on the surface. The effectiveness of this instrument is demonstrated by successfully imaging a tissue-equivalent phantom.
Efficient Peltier refrigeration by a pair of normal metal/insulator/superconductor junctions
Authors: Leivo M. M. , Pekola J. P. , Averin D. V.
Published: July 2002

We suggest and demonstrate in experiment that two normal metal/insulator/superconductor (NIS) tunnel junctions combined in series to form a symmetric SINIS structure can operate as an efficient Peltier refrigerator. Specifically, it is shown that the SINIS structure with normal-state junction resistances 1.0 and 1.1 kΩ is capable of reaching a temperature of about 100 mK starting from 300 mK. We estimate the corresponding cooling power to be 1.5 pW per total junction area of 0.8 μm2 at T=300 mK.
Formation of ultralow friction surface films on boron carbide
Authors: Erdemir A. , Bindal C. , Fenske G. R.
Published: July 2002

In this letter, we describe the formation and ultralow friction mechanisms of a surface film on boron carbide (B4C). This film results from sequential reactions between B4C and oxygen and between the resulting boron oxide (B2O3) and moisture; it can afford friction coefficients of 0.03 to 0.05 to sliding steel surfaces. At temperatures above 600 °C, B4C undergoes oxidation and forms a layer of boron oxide (B2O3) in the upper surface. During cooling to room temperature, the B2O3 reacts with moisture in the air to form a secondary film, boric acid (H3BO3). The sliding friction coefficient of 440C steel balls against this film is 0.04, compared to 0.7 against the bare B4C surfaces. Mechanistically, we propose that the ultralow friction behavior of the heat-treated B4C surface is due mainly to the layered-crystal structure of the H3BO3 film that forms on the sliding surface.
Thermal stability of amorphous carbon films grown by pulsed laser deposition
Authors: Friedmann T. A. , McCarty K. F. , Barbour J. C. , Siegal M. P. , Dibble Dean C.
Published: July 2002

The thermal stability in vacuum of amorphous tetrahedrally coordinated carbon (a-tC) films grown on Si has been assessed by in situ Raman spectroscopy. Films were grown in vacuum on room-temperature substrates using laser fluences of 12, 22, and 45 J/cm2 and in a background gas of either hydrogen or nitrogen using a laser fluence of 45 J/cm2. The films grown in vacuum at high fluence (≳20J/cm2) show little change in the a-tC Raman spectra with temperature up to 800 °C. Above this temperature the films convert to glassy carbon (nanocrystalline graphite). Samples grown in vacuum at lower fluence or in a background gas (H2 or N2) at high fluence are not nearly as stable. For all samples, the Raman signal from the Si substrate (observed through the a-tC film) decreases in intensity with annealing temperature indicating that the transparency of the a-tC films is decreasing with temperature. These changes in transparency begin at much lower temperatures (∼200 °C) than the changes in the a-tC Raman band shape and indicate that subtle changes are occurring in the a-tC films at lower temperatures.
New amorphous semiconductor: 2CdO⋅PbOx
Authors: Hosono Hideo , Yamashita Yasuhisa , Ueda Naoyuki , Kawazoe Hiroshi , Shimidzu Ken-ichi
Published: July 2002

A new amorphous semiconductor, 2CdO⋅PbOx (band gap: 1.58 eV), was found. Thin films of this material were prepared by rf sputtering of a Cd2PbO4 target in O2–Ar. The dc conductivity of the resulting amorphous thin films was ∼180 S cm−1 at 300 K and remained almost constant down to ∼4 K. The concentration of carrier electrons and the Hall mobility in the as-deposited state were 1×1020 cm−3 and 9 cm2 V−1 s−1, respectively. When the as-deposited specimens were heated to 250 °C, which is far below the crystallization (to Cd2PbO4) temperature (460 °C), the conductivity and the carrier concentration at 300 K became approximately twice as high. The thermal O2-desorption measurements demonstrated that carrier electrons are generated via the formation of oxygen vacancies at the initial stage (<250 °C) of thermal desorption of O2 from the amorphous structure. The effective mass of carrier electrons was estimated as 0.57m0.
Fabrication of electron injecting Mg:Ag alloy electrodes for organic light-emitting diodes with radio frequency magnetron sputter deposition
Authors: Suzuki Hiroyuki
Published: July 2002

Electron injecting electrodes made of magnesium and silver alloys (Mg:Ag) with a wide range of Ag concentration from 0 to 26.6 at. % were successfully fabricated for organic light-emitting diodes (LEDs) by using the radio frequency sputter deposition technique. The LED characteristics such as intensity, turn-on voltage, and durability of electroluminescence were strongly dependent on the Mg:Ag composition. Both the work function and the structural order of these Mg:Ag electrodes played a crucial role in determining the LED characteristics.
Effects of vacuum annealing on the optical properties of porous silicon
Authors: Balagurov L. A. , Yarkin D. G. , Petrova E. A. , Orlov A. F. , Karyagin S. N.
Published: July 2002

The effects of vacuum annealing on the optical absorption spectra in the visible and infrared ranges, photoluminescence intensity, and concentration of paramagnetic centers in free-standing porous silicon films were investigated in a temperature range of 100–600 °C. It was found that heat-induced hydrogen desorption decreased the porous silicon band gap, which suggests that band-gap energy depends on hydrogen coverage of nanoparticles. The annealing also leads to increasing concentration of defects that were identified as silicon dangling bonds. The energy distribution of the dangling-bond states was estimated from the absorption spectrum.
Novel all optical light modulation based on complex refractive index changes of organic dye-doped polymer film upon photoexcitation
Authors: Nagamura Toshihiko , Hamada Takahiro
Published: July 2002

A novel all-optical light modulation has been proposed based on photoinduced changes of an imaginary part of complex refractive index in guided wave geometry. Very fast and repeated modulation of a reading beam (reflected intensity) was demonstrated at the incident angle corresponding to a sharp guided wave resonance using phthalocyanine-doped poly(vinyl alcohol) film as a photoresponsive layer and a nanosecond pulsed laser as a writing beam. Such responses were mainly due to the formation and decay of excited triplet phthalocyanine. The rise and fall time was markedly improved as compared with previous systems based on photothermal changes of the refractive index.
Bulk glass formation in the Pd–Ni–P system
Authors: He Y. , Schwarz R. B. , Archuleta J. I.
Published: July 2002

Bulk amorphous Pd–Ni–P rods with diameters ranging from 10 to 25 mm were prepared by a fluxing technique over a wide composition range. For most bulk glassy alloys studied, the difference between the glass transition temperature and the crystallization temperature, Tx−Tg, is larger than 90 K. Of all the alloy compositions examined, Pd40Ni40P20 has the highest glass formability, and 300-g bulk amorphous cylinders, 25 mm in diameter and 50 mm in length, were easily and repeatedly formed. This size, however, is not an upper limit. The elastic properties of these bulk amorphous alloys were determined by a resonant ultrasound spectroscopy technique.
Quantitative magnetic field measurements with the magnetic force microscope
Authors: Proksch Roger , Skidmore George D. , Dahlberg E. Dan , Foss Sheryl , Schmidt J. J. , Merton Chris , Walsh Brian , Dugas Matt
Published: July 2002

We have developed a technique to quantitatively image the magnetic field above a magnetic specimen using a modified magnetic force microscope (MFM). The technique depends on the nonlinear response of a magnetically soft MFM tip to the sample field and to an externally applied field, similar in principle to fluxgate magnetometry. We demonstrate the technique with high resolution, quantitative images of the magnetic field above a sample of longitudinal recording media. The magnetic field resolution is on the order of 1 Oe, with sub-100 nm spatial resolution comparable to standard MFM techniques.
Submicron processing of InAs based quantum wells: A new, highly selective wet etchant for AlSb
Authors: Morpurgo A. F. , van Wees B. J. , Klapwijk T. M. , Borghs G.
Published: July 2002

We describe a processing technology for patterning InAs/AlSb heterostructures far in the submicron regime. The processing is based on a new, highly selective wet etchant for AlSb. We discuss the electrical characterization of narrow ballistic channels (down to ≈140 nm width) realized with present technology, and demonstrate that the processing preserves the high mobility of the material.
Electronic and optical properties of (001) Si/ZnS heterostructures
Authors: Lew Yan Voon L. C. , Ram-Mohan L. R. , Soref R. A.
Published: July 2002

A theoretical characterization of (001)-grown (Si2)m/(ZnS)2m multiple quantum wells is presented. Confined states in the Γ1c well have strong X1c and light-hole admixtures, and there are two types of hole states in the valence band. Strong intersubband transitions can be exploited in the infrared for both n- and p-doped devices.
A simple-cubic photonic lattice in silicon
Authors: Wada Mitsuo , Doi Yoshiyuki , Inoue Kuon , Haus Joseph W. , Yuan Zhenyu
Published: July 2002

A three-dimensional, quasi-simple-cubic, air-hole photonic crystal was fabricated in silicon. The lattice constant is 530 μm and the air-filling factor is 0.84, which corresponds to the simplest structure proposed showing a direct photonic band gap. The observed transmittance in the Γ−Z direction with five periods along each direction agrees very well with the calculated transmittance. Theoretical calculations indicate that a similar lattice with 8×8×8 periods, which can be fabricated by state-of-the-art techniques, should suffice to observe band gap effects.
Triple-junction amorphous silicon alloy solar cell with 14.6% initial and 13.0% stable conversion efficiencies
Authors: Yang J. , Banerjee A. , Guha S.
Published: July 2002

We have achieved 14.6% initial and 13.0% stable conversion efficiencies using an amorphous silicon-based alloy in a spectrum-splitting, triple-junction structure. These efficiencies have been confirmed independently by the National Renewable Energy Laboratory. Key factors leading to this major advance include improvements made in the low band-gap amorphous silicon–germanium alloy cell, the pn tunnel junction between the component cells, and the top conducting oxide.
In situ monitoring of molecular beam epitaxy using specularly scattered ion beam current oscillations
Authors: Labanda J. G. C. , Barnett S. A.
Published: July 2002

Specular scattering of 3 keV He ions was observed for incidence angles of 2°–6° from GaAs(001). During molecular beam epitaxy, the scattered ion current dropped rapidly upon opening the Ga shutter, showed damped oscillations, and then increased gradually upon closing the shutter. The oscillation periods corresponded to monolayer growth times. Oscillation amplitudes decreased with increasing substrate temperature, indicating a transition to step-flow growth. The oscillations were not a diffraction effect, allowing a quantitative interpretation based on scattering by adatoms and step edges.