bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical characteristics of short pulse gated microchannel plate detectors
Authors: Bell P. M. , Kilkenny J. D. , Landen O. L. , Hanks R. L. , Bradley D. K.
Published: July 2002

Gating of microchannel plates (MCPs) can produce shutter times of less than 40 ps. This is accomplished with high-fidelity electronic circuitry and short pulse (sub 80 ps) high-amplitude electronic drivers. Measurements have shown that the fidelity of the electrical circuitry is one of the limiting factors in producing shorter x-ray gating.
In situ analyzer calibration methods for heavy ion beam probes installed on stellarator-like devices
Authors: Zielinski J. J. , Aceto S. C. , Schwelberger J. G. , Connor K. A. , Lewis J. F. , Glowienka J. C. , Carnevali A.
Published: July 2002

An absolute calibration of an installed heavy ion beam probe (HIBP) energy analyzer is possible by detecting secondary ions that come from a source at a known location that is maintained at a known potential. It is also necessary that the magnetic field that exists during the calibration be the same magnetic field that exists when a plasma is present. These conditions can be met on stellarators, heliotrons, and torsatrons, which produce their magnetic configurations with external coil sets. Since no internal plasma current is required, suitable sources for producing secondary ions by interaction with the primary beam can be placed inside the vacuum vessels of these devices at known locations. Secondary ions can be produced by the interaction of the primary beam with thin films, gas box probes, electron beams, or neutral gas filling the vacuum vessel. Details of a spherical mesh probe that combines the advantages of several of these methods are given.
Development of broad beam ion sources at CSSAR
Authors: Feng Y. C. , You D. W. , Kuang Y. Z.
Published: July 2002

High-energy and intense beam current broad beam ion sources have been developed for ion implantation and dynamic recoil mixing at CSSAR. The sources can be operated over beam energy and current ranges of 3–120 keV and 5–70 mA, respectively. For sputter coating of thin films, a series of focusing beam ion sources with different structures has also been developed. The energy and current range from 1–10 keV and 100–350 mA for different applications. For some applications, low-energy (below 100 eV) ion beams are required. CSSAR has developed a 6-cm-diam broad beam ion source. The source can be operated at beam energy 10–70 eV, and the beam current 15–80 mA has been extracted. Typical structures and operational data are given for the sources mentioned above. Recently a new type of broad beam metal ion source (Electron Beam Evaporation Metal Ion Source EBE) is being studied. Ion beams of several kinds of materials such as C, W, Ta, Mo, Cr, Ti, B, Cu, etc. have been extracted from the source. Typical operation conditions and ion yields are given in this paper.
A 2.45-GHz electron cyclotron resonance multi-mA Li+ ion gun for fusion plasma diagnostics
Authors: Wutte D. , Leitner M. , Winter HP.
Published: July 2002

For the purpose of neutral-lithium beam tokamak–plasma diagnostics we have developed a compact, high current (several tens of mA/cm2) 2.45-GHz electron cyclotron resonance (ECR) Li+ plasma ion source in coaxial geometry, coupled to a helium-buffered lithium feeding system working according to the heat pipe principle. The accel–decel-type ion extraction system features either seven holes or one single aperture. First experimental results for the extractable Li+ and He+ ions in both cw and pulsed-mode operations are presented.
Dense negative ion currents from laser produced plasma
Authors: Henkelmann T. , Korschinek G. , Paul Michael
Published: July 2002

Recent studies on the Munich laser ion source were aimed at the understanding and optimization of the production mechanism of negative ions. The possibility of extremely high, pulsed negative ion currents is due to the outstanding features of the laser produced plasma: a high electron density in conjunction with a low electron temperature. The intensity of the laser beam is restricted to a relatively small range from 5×107 up to 1×109 W/cm2. More than 20 different ions have been produced from pure element targets and from a number of salts. The yields achieved are in good agreement with a model including Saha equilibrium distributions. The comparison between pure elemental targets and alloys or salt shows a strong influence of ions with a low ionization potential on the negative ion production.
A diamond anvil cell for IR microspectroscopy
Authors: Chervin J. C. , Canny B. , Besson J. M. , Pruzan Ph.
Published: July 2002

A large optical-aperture membrane diamond anvil cell designed for infrared spectroscopy is described. The cell offers definite advantages compared to existing systems. Other possibilities concerning x-ray diffraction analyses with the cells are mentioned.
Six degree-of-freedom scanning supports and manipulators based on parallel robots
Authors: Comin Fabio
Published: July 2002

The exploitation of third generation SR sources heavily relies on accurate and stable positioning and scanning of samples and optical elements. In some cases, active feedback is also necessary. Normally, these tasks are carried out by serial addition of individual components, each of them providing a well-defined excursion path. On the contrary, the exploitation of the concept of parallel robots, structures in close cinematic chain, permits us to follow any given trajectory in the six-dimensional space with a large increase in accuracy and stiffness. At ESRF, the parallel robot architecture conceived some tens of years ago for flight simulators has been adapted to both actively align and operate optical elements of considerable weight and position small samples in ultrahigh vacuum. The performance of these devices gives results far superior to the initial specification and a variety of drive mechanisms are being developed to fit the different needs of the ESRF beamlines.
Infrared TV measurement of fast ion loss on JT-60U
Authors: Tobita K. , Neyatani Y. , Kusama Y. , Takeuchi H.
Published: July 2002

Toroidal field (TF) ripple transport, wave-particle interaction, and large magnetohydrodynamic modes can enhance fast ion losses and result in localized heat deposition on the first wall. Two-dimensional (2D) thermal measurement on the first wall provides useful information concerning these fast ion behaviors. In this article, we focus on the application of the 2D measurement with an infrared TV camera to TF ripple loss study. The content is (1) the 2D heat flux profile on the wall due to ripple loss, (2) the effects of the radial electric field, and (3) the ICRF effect on TF ripple loss. These experimental data demonstrate a great potential of infrared thermography in fast ion behavior study.
Epitaxial potassium niobate thin films prepared by metalorganic chemical vapor deposition
Authors: Nystrom M. J. , Wessels B. W. , Studebaker D. B. , Marks T. J. , Lin W. P. , Wong G. K.
Published: July 2002

Epitaxial potassium niobate thin films were deposited in situ by low pressure metalorganic chemical vapor deposition (MOCVD) at a growth temperature of 800 °C using niobium pentaethoxide and potassium tert-butoxide as volatile metalorganic precursors. Growth on single crystal (100) lanthanum aluminate substrates produced [110]-oriented potassium niobate films. The films have a smooth, featureless morphology. Atomic force microscopy of the MOCVD-derived films surface indicates a root-mean-square roughness of less than 2 nm. Second-harmonic generation of 1.064 μm incident light is observed from the potassium niobate thin films, and the effective second order nonlinear susceptibility d of the as-deposited film is as high as 13 pm/V.
Fast detection of 14 MeV neutrons on the TFTR neutron collimator
Authors: von Goeler S. , Roquemore A. L. , Johnson L. C. , Bitter M. , Diesso M. , Fredrickson E. , Long D. , Strachan J.
Published: July 2002

Current mode operation of the NE451 ZnS scintillation detectors of the TFTR neutron collimator has enabled us to record the development of radial neutron emission profiles with much faster speed and higher accuracy than in the pulse counting mode. During high power deuterium–tritium (DT) operation, the intrinsic shot noise on the detector traces was so low that we could observe sawtooth instabilities and disruptions with good precision and, in addition, were able to identify precursor magnetohydrodynamic (MHD) activity and fishbone instabilities. These results demonstrate that in future tritium burning machines like ITER or TPX, the neutron collimator should be designed not only as a monitor of radial fusion power profiles but also as a wave detector for MHD activity.
A magnetic manipulator for studying local rheology and micromechanical properties of biological systems
Authors: Amblard François , Yurke Bernard , Pargellis Andrew , Leibler Stanislas
Published: July 2002

A magnetic micromanipulator capable of generating two-dimensional translational and rotational motions on a microscope stage is described. With 3 μm-diam paramagnetic beads, forces in the piconewton range and torques on the order of 10−14 N m are obtained and can be modulated in time at moderate frequencies (<5 Hz). Typical magnetic fields between 0.1 and 0.2 T, and gradients between 10 and 20 T m−1 are created by four independent feedback-controlled electromagnets. Video microscopy and computerized image analysis are used to locate the beads on each image with a resolution of 0.1 pixel (20 nm). The device is primarily designed to study, at a microscopic scale, the local mechanical properties of biological polymers such as actin in solution, and of cell cytoplasm. Possible applications include the in situ manipulation of intracellular organelles.
Imaging ellipsometry revisited: Developments for visualization of thin transparent layers on silicon substrates
Authors: Jin Gang , Jansson Roger , Arwin Hans
Published: July 2002

Imaging ellipsometry is presented as a technique for quantification and visualization of the lateral thickness distribution of thin (0–30 nm) transparent layers on solid substrates. The main advantage of imaging ellipsometry is that every point on a surface is measured at the same time with a high lateral resolution. The method is based on the use of combined null and off-null ellipsometry at an incident angle close to the pseudo-Brewster angle of a high index substrate such as silicon. In the present experimental setup, a xenon lamp, a collimator, and a wavelength-selective filter provide an expanded collimated probe beam with a diameter of 25 mm. Other major components in the system are a polarizer, a compensator, and an analyzer. In this way, a 15×30 mm2 image of a sample surface can be focused onto a charge-coupled-device video camera and transferred to a computer for further evaluation by image processing. Thickness measurements are performed for calibration purposes with ordinary null ellipsometry. The imaging ellipsometer has an accuracy of better than 0.5 nm at a lateral resolution of 5 μm in the present configuration, but improvements of at least a factor of 5 can be foreseen. Several aspects of the ellipsometric imaging system are illustrated in selected applications including continuous protein thickness distributions, stepped silicon dioxide thickness distributions, and visualization of protein patterning of surfaces. The latter can be used in a biosensor system as illustrated here by antigen–antibody binding studies.
Design of an omega filter for a 200 kV electron microscope
Authors: Tsuno K. , Munro E.
Published: July 2002

An omega (Ω) filter has been designed to satisfy the following four constraints: (1) the dispersion at 200 kV is larger than 1 μm/eV, (2) the principal second-order aberration coefficients Aγγγ, Bγδδ are less than 1000 mm, (3) all of the inclination angles of the entrance and the exit faces of the sector magnets are within 0–45° (positive), and (4) the drift length between the first and the second magnet (or between the third and the fourth magnet) is less than the radius of the largest magnet. The latter two are practical requirements and are important for the experimental adjustment of the optical axis. The optimization of filter dimensions was made using a numerical electron trajectory method as follows: (1) The optical axis was found by calculating the central electron trajectories using a table of the fringing field distribution and a constant field strength inside the magnet. (2) The stigmatic focus condition was found by changing the inclination angles of the faces of the magnets. (3) The 18 secondary aberration coefficients [H. Rose, Optik 51, 15 (1978)] were numerically calculated. A three-dimensional trajectory calculation was made for the selected filter shape. A filter with a 1.3 μm/eV dispersion at 200 kV has been designed.
A new method for eliminating self-absorption effect from dispersed emission spectra
Authors: Yamasaki Katsuyoshi , Kimura Tomoko , Masuya Hiroshi , Tokue Ikuo
Published: July 2002

A new apparatus has been constructed to eliminate the self-absorption effect from dispersed emission spectra. Droplets of solution smaller than 5 μm in diameter were generated by ultrasonic vibration from bulk solution, and irradiated with a laser to observe dispersed emission spectra. Effective suppression of self-absorption was demonstrated using ethanol droplets containing rhodamine 640 dye. A slight increase in the ambient temperature—less than 10°—gave rise to a release of solvent molecules from droplets, and consequently, the size of the droplets drastically decreased. Emission spectra over a wide range of concentrations were observed by the method of solvent release. The change of the ratio between monomer/excimer fluorescence from pyrene was clearly shown using droplets generated from solution with single concentration. Effective concentrations and the sizes of droplets at different temperatures were determined from comparison to the results obtained by droplets with different concentrations. The concentration of droplets warmed up to 305 K and was 29 times that of droplets at 297 K.
Performance of an energy-compensated three-dimensional atom probe
Authors: Cerezo A. , Godfrey T. J. , Sijbrandij S. J. , Smith G. D. W. , Warren P. J.
Published: July 2002

A wide acceptance angle first-order reflectron lens has been incorporated into a three-dimensional atom probe (3DAP) to provide improved mass resolution. This new 3DAP instrument is capable of resolving isotopes in the mass spectrum, with resolutions better than m/Δm=500 full width at half maximum and 250 full width at 10% maximum. However, use of a reflectron for energy compensation within an imaging system means that improvements in mass resolution result in degradation of the spatial resolution. This article addresses the detailed design of the energy compensated 3DAP, and the minimization and compensation of chromatic aberrations in the imaging performance of the instrument. Some applications of the new instrument are included to illustrate its capabilities in the atomic-scale analysis of engineering alloys.
Isotope effect and cesium dependence of negative ion production in volume H− and D− ion sources
Authors: Fukumasa Osamu , Yoshino Kyogo
Published: July 2002

Isotope effect of H− and D− productions with and without cesium vapor injection is studied theoretically as a function of plasma parameters. Model calculation in a tandem volume source is performed by solving a set of particle balance equations for steady-state hydrogen and deuterium discharge plasmas. In a pure volume case, as a whole, H− production is higher than D− production for various plasma conditions. This isotope effect is caused by mainly atomic collision processes, i.e., collisional cooling of vibrational molecules and collisional destruction of negative ions. Therefore, density of atoms plays an important role for deciding negative ion density. On the other hand, in a cesium case, there is no remarkable isotope effect on H−/D− production. Considering H−/D− surface production caused by both atoms and positive ions, H−/D− production is enhanced by a large factor and then isotope effect observed in pure volume case seems to be almost masked.
A two-thermocouple probe technique for estimating thermocouple time constants in flows with combustion: In situ parameter identification of a first-order lag system
Authors: Tagawa M. , Shimoji T. , Ohta Y.
Published: July 2002

A two-thermocouple probe, composed of two fine-wire thermocouples of unequal diameters, is a novel technique for estimating thermocouple time constants without any dynamic calibration of the thermocouple response. This technique is most suitable for measuring fluctuating temperatures in turbulent combustion. In the present study, the reliability and applicability of this technique are appraised in a turbulent wake of a heated cylinder (without combustion). A fine-wire resistance thermometer (cold wire) of fast response is simultaneously used to provide a reference temperature. A quantitative and detailed comparison between the cold-wire measurement and the compensated thermocouple ones shows that a previous estimation scheme gives thermocouple time constants smaller than appropriate values, unless the noise in the thermocouple signals is negligible and/or the spatial resolution of the two-thermocouple probe is sufficiently high. The scheme has been improved so as to maximize the correlation coefficient between the two compensated-thermocouple outputs. The improved scheme offers better compensation of the thermocouple response. The present approach is generally applicable to in situ parameter identification of a first-order lag system.
Measurements of gas preionization for plasma radiation sources
Authors: Moosman B. , Weber B. V. , Stephanakis S. J. , Commisso R. J. , Fisher A.
Published: July 2002

Azimuthally symmetric ultraviolet (UV) preionization of the outer periphery of a gas puff z pinch, prior to current initiation, may reduce the growth of magnetically driven Rayleigh–Taylor instabilities or other nonuniformities affecting the final implosion stage of the pinch, leading to an improvement in K-shell x-ray yield. We report on measurements of a flashover UV photoionization scheme, capable of ionizing the periphery of argon gas puffs to 1%–10% of the initial gas density. Measurements are made with a two-color, (1064 and 532 nm), high sensitivity (∼10−5λ), Mach–Zehnder type interferometer. Two methods of measuring preionization are investigated. The first uses a single laser wavelength, 1064 nm, to probe a chord of the cylindrical gas puff. The gas density is measured first without preionization and then with the UV flashover discharge. The difference in phase shift determines the free electron density. The second technique uses both wavelengths to simultaneously probe the same line of sight, determining neutral and free electron densities using only one gas puff event. Gas preionization may introduce refractive species that complicate the conversion of phase shift to density. However, single-color interferometry with the IR and green beams indicates that it is sufficient to consider only two species, neural argon and free electrons for times of interest.
A nonstationary microwave spectrometer
Authors: Vaks V. L. , Khodos V. V. , Spivak E. V.
Published: July 2002

A submillimeter spectrometer for measurements of low concentrations in gas mixtures is developed and created. The radiation source is a 260–380 GHz backward wave oscillator (BWO). The BWO frequency is stabilized by means of a reference microwave channel, incorporating a cell with a gas under test the spectral line of which is used as frequency discriminator. Fast frequency scanning in the vicinity of the spectral line is used to reduce the effects of acoustic noise and vibrations and to upgrade time resolution. The background’s signals due to interference in the measuring cell of the spectrometer are suppressed by using a original nonstationary modulation method. Theoretical analysis of that method has been carried out. A description of the design and analysis of possible applications for the spectrometer, as well as the results of some experimental studies supported by the spectrometer are provided.
A method for the growth and cooling of strain-free single crystals of cryogenic solids
Authors: Gregoryanz E. , Clouter M. J. , Kiefte H.
Published: July 2002

A technique has been developed for growing and cooling single crystals of cryogenic solids along the solid-vapor saturation line while maintaining a condition of negligible mechanical strain. The crystal volumes were ∼50 mm3 and optical access was provided for application of Brillouin and Raman spectroscopies. Single-crystal studies of CH4 and CD4 have been completed over the temperature range from the triple point (∼90 K) down to approximately 15 K. In the case of CD4 three distinct solid phases were involved.
Thermal tuning of a fiber-optic interferometer for maximum sensitivity
Authors: Bruland K. J. , Garbini J. L. , Dougherty W. M. , Chao S. H. , Jensen S. E. , Sidles J. A.
Published: July 2002

We describe a fiber-optic interferometer that employs wavelength changes to achieve maximum sensitivity. Wavelength changes are induced by adjusting the operating temperature of the laser, eliminating the need for an actuator to vary the spacing between the sensing fiber and the object to be monitored. The instrument and techniques described are suitable for cryogenic, high vacuum applications such as magnetic resonance force microscopy, where space is limited and micromanipulation can be challenging. The noise floor of 1.6×10−3 nm/Hz is adequate for monitoring subangstrom displacement of force microscope cantilevers.
An efficient 8 T extraction vector magnetometer with sample rotation for routine operation
Authors: Dufeu D. , Eyraud E. , Lethuillier P.
Published: July 2002

We describe here a fully automated system for magnetic anisotropy study operating in the field range ±8 T, between 1.8 and 300 K. The three components of the magnetization are simultaneously measured, using the flux extraction method, through three sets of orthogonal pickup coils. The sample is fixed on a rod which enables its vertical movement for the magnetization measurement and its rotation around a horizontal axis, perpendicular to the magnetic field, with a precision of about 1°. The sensitivity of the measurements is 10−7 A m2 along the vertical axis and 5×10−7 A m2 along the horizontal axes.
Contact-free determination of ultralow resistances of micron-sized wires
Authors: Rossmy T. , Skwirblies S. , Kötzler J.
Published: July 2002

We apply a limiting case of recently developed general inversion schemes between complex magnetic susceptibilities and electrical resistances ρ to the case of copper whiskers and demonstrate the feasibility and high accuracy of this contactless method to measure ρ. The present data reveal strong effects of ideal surface scattering and Bloch–Grüneisen phonon scattering on ρ in these samples with very low residual bulk resistivity, ρb(0)=0.27 nΩ cm.
Microwave plasma source for high current ion beam neutralization
Authors: Korzec D. , Müller A. , Engemann J.
Published: July 2002

A new microwave based plasma source for high current ion beam neutralization is presented. It consists of a tapered WR340 waveguide with a directly coupled 2.45 GHz magnetron and a quartz tube of 40 mm diameter inside the waveguide confined by two graphite electrodes. The upper electrode houses the gas feed, whereas the lower electrode serves as the plasma outlet. Both electrodes can be biased independently. Tuning is done via a magnetically controlled stab inserted into the waveguide. The plasma was characterized by use of the Langmuir probe. The electron concentration of 5×1011 cm−3 at 250 W was measured. Typical electron temperature is below 2.5 eV even for pressure as low as 2×10−3 Pa. The extractable electron current is dependent on microwave power and gas flow. A limitation is caused by the ion saturation current to the negatively biased electrodes. The voltage on the electrodes allows an easy control of electron current. Maximum electron current achieved so far is 400 mA for an argon flow of 5 sccm and microwave power of 300 W.
Effective piezoelectric activity of zinc oxide films grown by radio-frequency planar magnetron sputtering
Authors: Wacogne B. , Roe M. P. , Pattinson T. J. , Pannell C. N.
Published: July 2002

We present a study of the effective piezoelectric activity of thin ZnO films produced by radio-frequency (rf) planar magnetron sputtering. The energetic plasma particles which bombard the substrate in the above deposition system increase the substrate temperature, thus causing a gradual variation in film structure during the beginning of the film growth. As a result, a precursor layer is formed which consists of small randomly oriented crystallites, exhibiting poor piezoelectric activity. Hence, the film thickness responsible for piezoelectric activity is generally less than the physical thickness of the film and is adjacent to a layer having different acoustic impedance. This leads to an increase in the resonant frequency of the film. For example, a film designed to have a half-wave resonance at 288 MHz, was found to be resonant at 332 MHz. The poorly structured initial layer meant in this typical case that only 87% of this film volume exhibited piezoelectric activity. Investigations based on the substrate temperature, the optical losses, scanning electron microscope imaging, and rf electrical behavior are presented in this letter.
Frequency dependent hole diffusion in InGaAs double heterostructures
Authors: Hollenhorst James N. , Hasnain Ghulam
Published: July 2002

Two-dimensional diffusion of holes is studied in n-type InGaAs heterostructures by frequency dependent measurements of the photoresponse in the periphery of a mesa diode. An analytical theory is presented that gives the spatial and frequency dependent photoresponse. Measurements agree well with theory and establish a hole diffusion length of 60 μm, a hole recombination lifetime of 3 μs, and a hole mobility of 480 cm2/V s for our material. An upper limit is also established for the recombination velocity at the InP or InGaAsP heterointerfaces.
Ballistic transport in one-dimensional constrictions formed in deep two-dimensional electron gases
Authors: Thomas K. J. , Simmons M. Y. , Nicholls J. T. , Mace D. R. , Pepper M. , Ritchie D. A.
Published: July 2002

We show that small structures can be defined in high mobility two-dimensional electron gases formed at a depth of 2770 Å below the surface in GaAs/Al0.33Ga0.67As heterostructures. The differential conductance of one-dimensional constrictions defined by split gates in such deep electron gases showed more than 20 quantised plateaus. The absence of resonant structures on the plateaus demonstrates the absence of potential fluctuations in the constrictions. By applying a dc source-drain bias we have measured the energy spacings of the first 18 subbands, and the effect of a small perpendicular magnetic field on the energy spacings has been investigated.
Transport-magnetism correlations in the ferromagnetic oxide La0.7Ca0.3MnO3
Authors: Hundley M. F. , Hawley M. , Heffner R. H. , Jia Q. X. , Neumeier J. J. , Tesmer J. , Thompson J. D. , Wu X. D.
Published: July 2002

We present results of temperature and magnetic field dependent resistivity ρ(H,T) and bulk magnetization M(H,T) measurements on post-annealed La0.7Ca0.3MnO3 thin films that were grown via pulsed-laser deposition. Both the resistivity and the anomalously large negative magnetoresistance peak near the ferromagnetic ordering temperature (Tc=250 K), with Δρ/ρ0=−85% at 50 kOe. A clear correlation is found between ρ and M that is described by the phenomenological expression ρ(H,T)∝exp[−M(H,T)/M0]. This correlation reflects the important interplay between transport and magnetism in this system, and suggests that the transport below Tc involves polaron hopping.
Epitaxial deposition and properties of Bi2Sr2CaCu2O8+δ/Bi2Sr2YCu2O8+δ/Bi2Sr2CaCu2 O+δ trilayers
Authors: Cucolo A. M. , Di Leo R. , Romano P. , Bacca E. , Gomez M. E. , Lopera W. , Prieto P. , Heiras J.
Published: July 2002

We have deposited Bi2Sr2CaCu2O8+δ/Bi2Sr2YCu2O8+δ/Bi2Sr2CaCu2 O+δ (2212/22Y2/2212) heterostructures by an in situ dc sputtering technique at high oxygen pressures on (001) SrTiO3 substrates. The formation of highly c-axis oriented trilayers with sharp interfaces is demonstrated by x-ray diffraction and transmission electron microscope (TEM) analysis. Both the top and the bottom 2212 layers are superconducting below 87 K. Tunneling phenomena on junctions fabricated from these trilayers are observed. The conductance versus voltage curves at low temperatures exhibit a change of slope indicative of a gap structure at about 30 mV, a zero-bias peak, as well as linear background at high voltages.
Reciprocity based transfer function analysis in magnetic force microscopy
Authors: Wright C. D. , Hill E. W.
Published: July 2002

The transfer function of the imaging process in magnetic force microscopy is calculated theoretically using a reciprocal force based approach. The reciprocal force is that exerted on the sample by the tip, and according to Newton’s third law, is simply equal but opposite to the force exerted on the tip by the sample. The method allows the role of the tip in the imaging process to be represented in a particularly simple fashion, and the system transfer function is shown to depend on the Fourier transform of the field distribution produced by the microscope tip at the surface of that tip, an exponential spacing loss term reflecting the separation of the tip and the sample and finally a loss term dependent on the sample thickness.
Role of ion energy in determination of the sp3 fraction of ion beam deposited carbon films
Authors: Grossman E. , Lempert G. D. , Kulik J. , Marton D. , Rabalais J. W. , Lifshitz Y.
Published: July 2002

The role of ion energy over the range 5 eV≤E≤20 keV in the production of the dense diamondlike sp3-bonded phase of carbon films deposited from ion beams has been investigated. Films with a significant sp3 component (≳40%), as determined by electron energy loss spectroscopy (EELS), can be formed over the wide energy region 30 eV≤E≤10 keV at room temperature. The sp3 fraction is completely suppressed only for E≤10 eV or E≥20 keV. For both cases, this suppression is associated with a sharp increase of the surface roughness, as determined by atomic force microscopy (AFM). The different nature of the mechanisms responsible for the suppression of sp3 bonding in both the low and high energy regions is discussed.
Tailoring the dielectric properties of HfO2–Ta2O5 nanolaminates
Authors: Kukli Kaupo , Ihanus Jarkko , Ritala Mikko , Leskela Markku
Published: July 2002

Dielectric thin films applicable, for instance, as insulating layers in electroluminescent display devices have been studied. In order to improve dielectric characteristics HfO2–Ta2O5 nanolaminates were prepared by atomic layer epitaxy at 325 °C. The nanolaminates were evaluated in capacitance and current–voltage measurements. By optimizing the layer thicknesses in the nanolaminate structures the dielectric properties, especially leakage current densities, could be tailored remarkably. The best nanolaminates showed charge storage factors improved up to 8 times when compared with those of the single oxide films. The presence of nanosize crystallites of monoclinic and metastable tetragonal HfO2 was observed by x-ray diffraction analysis.
Piezoresistive effect in wurtzite n-type GaN
Authors: Bykhovski A. D. , Kaminski V. V. , Shur M. S. , Chen Q. C. , Khan M. A.
Published: July 2002

We report on the measurements of the piezoresistive effect in the n-type wurtzite GaN films. The 3–5 μm thick GaN layers were deposited slightly off axis over basal plane sapphire substrates. The static and dynamic gauge factor (GF) of these structures was measured at room temperature for both longitudinal and transverse configurations. The dynamic effect is related to a strong piezoeffect in GaN. The maximum dynamic GF observed was ∼130 (approximately four times larger than for SiC).
Generation of high energy 10 fs pulses by a new pulse compression technique
Authors: Nisoli M. , De Silvestri S. , Svelto O.
Published: July 2002

An optical compression technique which is particularly suitable for ultrashort pulses of high energy is presented. Spectral broadening is achieved by pulse propagation along a hollow-core fused silica waveguide filled with noble gases at high pressure. Pulse compression is then obtained in a prisms dispersive delay line. Experiments performed with pulses of 140 fs duration and 660 μJ energy from a Ti:sapphire laser demonstrate the generation of compressed pulses of 10 fs duration and 240 μJ energy.
All laser-assisted heteroepitaxial growth of Si0.8Ge0.2 on Si(100): Pulsed laser deposition and laser induced melting solidification
Authors: Serna R. , Blasco A. , Missana T. , Solís J. , Afonso C. N. , Rodríguez A. , Rodríguez T. , da Silva M. F.
Published: July 2002

The growth of heteroepitaxial Si0.8Ge0.2 films on Si(100) by a novel all laser-assisted technique using only ArF excimer laser radiation is demonstrated. Amorphous 30 nm thick films are grown by pulsed laser deposition from alternating pure Si and Ge targets on clean Si substrates. Melting and rapid solidification is then induced by pulsed irradiation (0.54 J/cm2), promoting epitaxial growth.
Evolution of the surface cross-hatch pattern in InxGa1−xAs/GaAs layers grown by metal-organic chemical vapor deposition
Authors: Yoon Meeyoung , Lee Bun , Baek Jong-Hyeob , Park Hyo-Hoon , Lee El-Hang , Lee Jeong Yong
Published: July 2002

The evolution of the cross-hatch pattern (CHP) in InxGa1−xAs/GaAs heterostructures has been studied. It is found that stress is concentrated at the valleys of the CHP from the results of crack formation at the CHP valleys in the thick GaAs cap layer grown on an InGaAs layer. Residual strain in the InGaAs/GaAs epitaxial layer showing a CHP is confined along the valleys of the CHP with a nonuniform distribution throughout the epitaxial layer. The skeleton of the CHP is formed at the beginning of the rapid strain relaxation period and the depth of the CHP valleys increases after most of the strain has been released. We propose that the development of the CHP in the later stage of the growth takes place by the growth suppression at the CHP valleys due to the high level of stress concentration.
Er doping of AlN during growth by metalorganic molecular beam epitaxy
Authors: MacKenzie J. D. , Abernathy C. R. , Pearton S. J. , Hömmerich U. , Wu X. , Schwartz R. N. , Wilson R. G. , Zavada J. M.
Published: July 2002

The doping of AlN during growth by metalorganic molecular beam epitaxy with an Er effusion source has resulted in AlN:Er films exhibiting strong room-temperature 1.54 μm photoluminescence (PL). The luminescence detected in the AlN:Er grown during this study was orders of magnitude greater in intensity than that from ion-implanted samples and represents the first demonstration of strong emission from rare-earth doped, epitaxial group III nitrides. Secondary ion mass spectroscopy was used to verify a dynamic range for this doping technique of 3×1017–2×1021 Er cm−3 with varying effusion cell temperature. The effects of growth temperature on Er incorporation and segregation behavior were also determined. PL studies, including room-temperature and thermal quenching experiments, were conducted.
Nondestructive evaluation of the oxidation stresses through thermal barrier coatings using Cr3+ piezospectroscopy
Authors: Christensen R. J. , Lipkin D. M. , Clarke D. R. , Murphy K.
Published: July 2002

The stresses in the aluminum oxide formed during high-temperature oxidation of a bond-coated superalloy are shown to be measurable through zirconia thermal barrier coatings. The basis for the measurements is the piezospectroscopic shift in the R-line fluorescence (photoluminescence) from Cr3+ impurities incorporated into the growing aluminum oxide scale. Measurements through the thermal barrier coating are feasible because (partially stabilized) zirconia coatings have some transparency at both the excitation and at fluorescence frequencies.
Observation of ultrafast all-optical modulation based on intersubband transition in n-doped quantum wells by using free electron laser
Authors: Suzuki T. , Mitsuyu T. , Nishi K. , Ohyama H. , Tomimasu T. , Noda S. , Asano T. , Sasaki A.
Published: July 2002

A picosecond-range all-optical modulation based on the nonlinear interaction between interband- and intersubband-resonant light in GaAs/AlGaAs multiple quantum wells was demonstrated. A free electron laser (FEL) with a very short pulse width (<10 ps) was used for the intersubband excitation. The dependence of the modulation depth on the wavelength of the intersubband-resonant light was investigated by utilizing the wide tunability of the FEL. The result is in good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by FT-IR spectroscopy, which indicates that the observed modulation is indeed based on the intersubband transition induced by the FEL.
Temperature dependent segregation of metals at Si–SiO2 interfaces during oxygen ion bombardment
Authors: Williams J. S. , Short K. T. , White A. E.
Published: July 2002

Rutherford backscattering and channeling techniques have been used to study temperature-dependent segregation of Cu, Au, and Pd at Si–SiO2 interfaces during high dose 15 keV O+ bombardment of Si. Results illustrate a strong tendency for metals to be retained in an amorphous Si layer adjacent to the continuous SiO2 layer formed by O+ bombardment of Si. The magnitude of segregation depends on the bombardment temperature. We explain our temperature-dependent data in terms of a thermodynamic model whereby the segregation is driven by a large solubility difference between metals in amorphous Si and SiO2. Segregation is enhanced when the metal diffusivity in amorphous Si is high.
Analysis of laser absorption on a rough metal surface
Authors: Ang L. K. , Lau Y. Y. , Gilgenbach R. M. , Spindler H. L.
Published: July 2002

We have developed a simple model to estimate the cumulative absorption coefficient of an ultraviolet laser pulse impinging on a pure metal, including the effects of surface roughness whose scale is much larger than the laser wavelength λ. The multiple reflections from the rough surface may increase the absorption coefficient over a pristine, flat surface by an order of magnitude. Thus, as much as 16% (at room temperature) of the power of a 248 nm KrF excimer laser pulse may be absorbed by an aluminum target. A comparison with experimental data is given.
Lateral stiffness: A new nanomechanical measurement for the determination of shear strengths with friction force microscopy
Authors: Carpick R. W. , Ogletree D. F. , Salmeron M.
Published: July 2002

We present a technique to measure the lateral stiffness of the nanometer-sized contact formed between a friction force microscope tip and a sample surface. Since the lateral stiffness of an elastic contact is proportional to the contact radius, this measurement can be used to study the relationship between friction, load, and contact area. As an example, we measure the lateral stiffness of the contact between a silicon nitride tip and muscovite mica in a humid atmosphere (55% relative humidity) as a function of load. Comparison with friction measurements confirms that friction is proportional to contact area and allows determination of the shear strength.
Magnetic domain formation in lithographically defined antidot Permalloy arrays
Authors: Cowburn R. P. , Adeyeye A. O. , Bland J. A. C.
Published: July 2002

The magnetic switching of a continuous Permalloy film is artificially modified by lithographically structuring an antidot array. The antidots introduce a spatially variant shape anisotropy field that competes with the intrinsic uniaxial anisotropy of the continuous film. This results in a well-defined periodic domain structure at remanence as observed by scanning Kerr microscopy, and which could be significant for high-density data storage. A micromagnetic calculation explains the structure and indicates optimum anisotropy conditions for data storage applications.
Band gaps of GaPN and GaAsN alloys
Authors: Bellaiche L. , Wei S.-H. , Zunger Alex
Published: July 2002

The importance of atomic relaxations, chemical disorder, and epitaxial constraints on the band gap of random, anion-mixed nitride alloys GaPN and GaAsN have been investigated, via pseudopotentials calculation. It has been demonstrated that simple approximations such as the virtual crystal approximation, or the use of high-symmetry ordered structure to mimic a random alloy, or the neglect of atomic displacements, are inadequate. It is found that a fully relaxed, large supercell calculation reproduces well the experimental band gaps of GaPN and GaAsN films.
High speed, low noise ultraviolet photodetectors based on GaN p-i-n and AlGaN(p)-GaN(i)-GaN(n)structures
Authors: Xu G. Y. , Salvador A. , Kim W. , Fan Z. , Lu C. , Tang H. , Morkoç H. , Smith G. , Estes M. , Goldenberg B. , Yang W. , Krishnankutty S.
Published: July 2002

We have investigated the spectral response of front-surface-illuminated GaN and AlGaN/GaN p-i-n ultraviolet photodetectors prepared by reactive molecular beam epitaxy on sapphire substrates. GaN homojunction p-i-n photodiodes exhibited a peaked response near the band edge. This enhanced response was absent in the AlGaN/GaN heterojunction p-i-n detectors. We analyzed the effect of p-layer thickness of the GaN p-i-n diodes on the magnitude of the peak photoresponse. The AlGaN/GaN photodiodes had a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than 3 orders of magnitude for wavelengths longer than 390 nm. A reverse bias of −10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The root-mean-square noise current in a 1 Hz bandwidth is ∼1.0 pA, corresponding to a noise-equivalent-power of ∼8.3 pW. We measured extremely fast decay times of 12 ns for the AlGaN/GaN and 29 ns for the GaN photodiodes.
Strain effects in Nb3Al multifilamentary conductors prepared by phase transformation from bcc supersaturated-solid solution
Authors: Takeuchi T. , Iijima Y. , Inoue K. , Wada H. , ten Haken B. , ten Kate H. H. J. , Fukuda K. , Iwaki G. , Sakai S. , Moriai H.
Published: July 2002

Strain effects on critical current densities have been examined for conductors containing nearly stoichiometric Nb3Al filaments with fine grains. The Nb3Al phase in these multifilamentary conductors are prepared by phase transformation from supersaturated Nb(Al) bcc solid solution and show high-field critical current densities much larger than those for conventionally prepared Nb3Al conductors, where the Nb3Al phase is known to be off-stoichiometric. The degradation of critical current densities with −0.7% intrinsic strain is ca. 20% at 12 T, comparable with those for conventional Nb3Al conductors of high strain tolerance.
Negative differential conductance in strained Si point contacts and wires
Authors: Koester S. J. , Ismail K. , Lee K. Y. , Chu J. O.
Published: July 2002

The observation of negative differential conductance (NDC) in simple nanostructure geometries fabricated on high-mobility Si/SiGe strained-layer heterostructures is reported. The NDC is observed in the drain characteristic of etch-defined “point contacts” with lithographic width and length of 0.22, and 0.12 μm, respectively. Current peak-to-valley ratios as large as 2.0 are observed at T=1.3 K. The NDC is also observed in “wire” geometries as long as 19 μm, and can persist to temperatures as high as 83 K, with a minimum in the differential conductance observable at 103 K. The NDC in long wires is accompanied by the formation of a high-field domain at the drain end of the wire. The effect is only observed in laterally constricted geometries, but is not a result of quantum confinement or impurity-related trapping. We suggest that the NDC and the attendant domain formation are caused by phonon emission by hot electrons within the constricted geometry.
Improved luminescence properties of pulsed laser deposited Eu:Y2O3thin films on diamond coated silicon substrates
Authors: Cho K. G. , Kumar D. , Lee D. G. , Jones S. L. , Holloway P. H. , Singh R. K.
Published: July 2002

Europium activated yttrium oxide (Eu:Y2O3) phosphor films have been grown in situ on (100) bare and diamond-coated silicon substrates using a pulsed laser deposition technique. Diamond-coated silicon substrates were prepared using hot filament chemical vapor deposition of diamond onto silicon. Photoluminescence brightness from Eu:Y2O3 films grown at 700 °C on diamond-coated silicon substrates was about twice that of films on bare silicon, and reached 80% of the brightness of powders. The higher brightness from Eu:Y2O3 film on diamond-coated silicon substrates is attributed to reduced internal reflections from the Eu:Y2O3 film surface, which results from the roughness of the diamond layer.
Lateral photocurrent spreading in single quantum well infrared photodetectors
Authors: Ershov M.
Published: July 2002

Lateral physical effects in single quantum well infrared photodetectors (SQWIPs) under nonuniform illumination over the detector area are considered. These effects are due mainly to the in-plane transport of the photoinduced charge in the QW. The length of the lateral photocurrent spreading is determined by the in-plane conductivity of the carriers in the QW and characteristic time of the QW recharging, and can be as large as 101–104 μm. Closed-form analytical expressions for SQWIP responsivity for modulated infrared signal and modulation transfer function are obtained. Possible techniques to suppress lateral photocurrent spreading are discussed.
The effect of doping and growth stoichiometry on the core structure of a threading edge dislocation in GaN
Authors: Wright A. F. , Grossner Ulrike
Published: July 2002

Density-functional-theory calculations have been performed to study the effect of doping and growth stoichiometry on the core structure of a threading edge dislocation in GaN. Four candidate structures were examined and their formation energies were found to depend strongly on Fermi level and growth stoichiometry. A structure having gallium vacancies at the dislocation core is predicted to be most stable in n-type material grown under nitrogen-rich conditions, while a structure without vacancies is most stable in p-type material grown under these conditions. In material grown under gallium-rich conditions, a structure having nitrogen vacancies at the dislocation core is predicted to be most stable in p-type material, whereas a variety of core structures should be present in n-type material. Edge dislocations are predicted to behave as electron traps in n-type material and may act as hole traps in p-type material depending on the growth conditions.
Charge storage in Co nanoclusters embedded in SiO2 by scanning force microscopy
Authors: Schaadt D. M. , Yu E. T. , Sankar S. , Berkowitz A. E.
Published: July 2002

Scanning force microscopy was used to study localized charge deposition and subsequent transport in Co nanoclusters embedded in SiO2 deposited on an n-type Si substrate. Co nanoclusters were charged by applying a bias voltage pulse between tip and sample, and electrostatic force microscopy was used to image charged areas, to determine quantitatively the amount of stored charge, and to characterize the discharging process. Charge was deposited controllably and reproducibly within areas ∼20–50 nm in radius, and an exponential decay in the peak charge density was observed. Longer decay times were measured for positive than for negative charge; this is interpreted as a consequence of the Coulomb-blockade energy associated with single-electron charging of the Co nanoclusters.