bluuebunny/crossref_metadata_2025 huggingface dataset:
High carbon doping efficiency of bromomethanes in gas source molecular beam epitaxial growth of GaAs
Authors: de Lyon T. J. , Buchan N. I. , Kirchner P. D. , Woodall J. M. , Scilla G. J. , Cardone F.
Published: July 2002

Carbon tetrabromide (CBr4) and bromoform (CHBr3) have been studied as carbon doping sources for GaAs grown by gas source molecular beam epitaxy (GSMBE) with elemental Ga and thermally cracked AsH3. Hole concentrations in excess of 1×1020 cm−3 have been measured by Hall effect in both CBr4- and CHBr3-doped GaAs, which agrees closely with the atomic C concentration from secondary-ion mass spectrometry, indicating complete electrical activity of the incorporated carbon. The GaAs growth rate is unaffected by the CBr4 and CHBr3 fluxes over the range of dopant flow investigated. The efficiencies of carbon incorporation from CBr4 and CHBr3 are, respectively, 750 and 25 times that of trimethylgallium (TMG), which is commonly employed as a carbon doping source in metalorganic MBE (MOMBE). The sensitivity of carbon incorporation to varying substrate temperature and V/III ratio has been observed to be significantly reduced with CBr4 and CHBr3 from that obtained under similar growth conditions with TMG in MOMBE.
Polarization rotation in asymmetric periodic loaded rib waveguides
Authors: Shani Y. , Alferness R. , Koch T. , Koren U. , Oron M. , Miller B. I. , Young M. G.
Published: July 2002

A new mechanism for polarization rotation in rib waveguides is suggested and demonstrated in InP waveguides. The polarization rotation is achieved by loading a rib waveguide in a periodic asymmetric way. Complete TE↔TM conversion, with only 2–3 dB excess loss, is obtained in a 3.7-mm-long InP loaded waveguide. Strong polarization rotation (80%), in shorter devices (0.3 mm long), is also demonstrated.
Amorphous diamond-Si semiconductor heterojunctions
Authors: Amaratunga Gehan A. J. , Segal Dan E. , McKenzie David R.
Published: July 2002

Tetrahedrally co-ordinated amorphous carbon (amorphous diamond) deposited from the plasma stream of a vacuum arc on a graphite cathode is demonstrated as having semiconductor properties. The material is shown to form a heterojunction with both p- and n-type Si. A band gap of 2.9 eV is proposed for the a-diamond, based on the observed I-V characteristics of the heterojunction diodes. The material has a measured electrical breakdown strength of 109 V/m, which is comparable to that of high quality insulators such as SiO2 and Si3N4. Gas phase doping using B2H6, PH3, and N2 has been attempted.
Photostimulable transparent NaCl:Cu single crystals for two-dimensional x-ray imaging sensors
Authors: Nanto Hidehito , Usuda Takayuki , Murayama Kazuhiko , Sokooshi Hideki , Nakamura Shouichi , Inabe Katsuyuki , Takeuchi Nozomu
Published: July 2002

Intense photostimulated luminescence (PSL) is observed, for the first time, in x-irradiated NaCl:Cu single crystals. The intensity of PSL with a peak at 353 nm is linearly increased with increasing x-ray absorbed dose over the wide range. This suggests that NaCl:Cu is useful as a material for two-dimensional x-ray imaging sensors in computed radiography utilizing PSL phenomenon. All the results obtained are consistent with the proposed emission mechanism of 353 nm PSL peak, based on the recombination of electrons released from the F centers with Cu2+ ions.
Microluminescence depth profiles and annealing effects in porous silicon
Authors: Prokes S. M. , Freitas J. A. , Searson P. C.
Published: July 2002

Cross-sectional microluminescence measurements for 1 Ω cm 70 μm porous silicon samples show a continuous decrease of the photoluminescence band as a function of sample depth. No spectral shift is observed. For samples annealed at 390 °C, in addition to spectral intensity reduction, we observe the same redshift in all luminescence spectra independent of depth. A study of this luminescence redshift as a function of annealing temperature reveals a striking similarity to results observed for optical band gap shrinking of a-Si:H as a function of hydrogen loss during annealing.
Incorporation of silicon and aluminum in low temperature molecular beam epitaxial GaAs
Authors: Manasreh M. O. , Evans K. R. , Stutz C. E. , Look D. C. , Hemsky J.
Published: July 2002

The localized vibrational modes (LVMs) of silicon donor (SiGa) and aluminum isovalent (AlGa) impurities in molecular beam epitaxial GaAs layers grown at various temperatures are studied using the infrared absorption technique. It is found that the total integrated absorption of these impurities LVMs is decreased as the growth temperature decreases. This finding suggests a nonsubstitutional incorporation of Si and Al in GaAs layers grown at 200 °C. On the other hand, a subtitutional incorporation is obtained in GaAs layers grown at temperatures higher than 350 °C. A recovery of the SiGa LVMs in GaAs layers grown at 200 °C has not been achieved by thermal annealing.
Study of silicon-hydrogen bonds at an amorphous silicon/silicon nitride interface using infrared attenuated total reflection spectroscopy
Authors: Matsumoto Tomotaka , Murata Yuji , Watanabe Jun-ichi
Published: July 2002

Silicon-hydrogen bonding structures at a hydrogenated amorphous silicon (a-Si:H)/silicon nitride (SiN) interface have been investigated using Fourier transform infrared attenuated total reflection (FTIR-ATR). Depositing a SiN overlayer markedly decreased the higher hydrides, which consist of SiHn (n=2,3) bonds, on the a-Si:H surface. The low density of higher hydrides at the resulting SiN-on-a-Si:H interface may be due to plasma-enhanced extraction or a transfer of the growing surface. By contrast, at an a-Si:H-on-SiN interface, the higher hydrides density is about 8.1×1014 cm−2. We believe this large amount of hydrogen at the a-Si:H-on-SiN interface relaxes strained bonds at the interface. In both the SiN-on-a-Si:H interface and the a-Si:H-on-SiN interface, hydrogen is implanted in the underlayer during the deposition of the overlayer. Our results indicate the structure of underlayer near the interface is strongly affected by the deposition of the overlayer.
Experimental study of virtual cathode oscillator in uniform magnetic field
Authors: Kostov K. G. , Nikolov N. A. , Spassovsky I. P. , Spassov V. A.
Published: July 2002

Results from 350 kV, 7 kA virtual cathode oscillator experiments are presented. High-frequency (11–17.6 GHz) high-power (15±7 MW) microwave radiation by a virtual cathode oscillator with a guide magnetic field has been obtained. It has been found that the oscillating virtual cathode and the beam electrons trapped between the real and the virtual cathodes radiate microwaves at two distinct frequencies. The dominant source of radiation is the reflexing electrons. A nonsymmetrical microwave mode TM11 has been observed.
Ge segregation and its suppression in GaAs epilayers grown on Ge(111) substrate
Authors: Kawai T. , Yonezu H. , Yoshida H. , Pak K.
Published: July 2002

The interdiffusion of the compositional atoms was investigated at heterointerface between a GaAs epilayer and a Ge(111) substrate by secondary ion mass spectroscopy. When a thin AlAs layer is applied initially, diffusion of Ge into the GaAs epilayer was suppressed effectively. An abrupt heterointerface was successfully realized in relatively high temperature growth. The interdiffusion process at the AlAs-Ge heterointerface was clarified in high temperature growth, which was dominated by the temperature-assisted segregation of Ge atoms during the AlAs growth rather than thermal diffusion. The compositional diffusion of Al atoms into the GaAs epilayer was also observed, which was enhanced by the Ge segregation in the structure of GaAs/AlAs/Ge substrate grown at higher temperature.
Evidence for donor-gallium vacancy pairs in silicon doped GaAs grown by molecular beam epitaxy at low temperatures
Authors: McQuaid S. A. , Newman R. C. , Missous M. , O’Hagan S.
Published: July 2002

Infrared absorption due to localized vibrational modes of silicon impurities in heavily doped GaAs grown by molecular beam epitaxy at low temperatures has been measured. Almost all Si atoms were found to occupy Ga sites. Second neighbor pairs SiGa-YGa were also detected in a concentration of ∼2×1018 cm−3 and it was deduced that Y was a gallium vacancy (VGa). These results provide the first direct evidence for the presence of a high concentration of gallium vacancies in GaAs grown at low temperatures.
Optimization of the magnetic field of perpendicular ferromagnetic thin films for device applications
Authors: Van Roy W. , De Boeck J. , Borghs G.
Published: July 2002

We studied the geometrical factors controlling the magnitude of the demagnetizing field and the fringing fields of ferromagnetic thin films with perpendicular, out-of-plane magnetization. The magnetic field emerging from the patterned ferromagnetic thin film can interact with carriers in an underlying semiconductor structure and generate a Hall voltage. Different geometries for use in practical device applications have been analyzed, using a simplified model as a design tool. The optimum geometry of the ferromagnetic thin film that will give maximum magnetic-field strength in the underlying semiconductor consists of a grating-type structure with periodicity of a few 100 nm to 1 μm. These geometries are suited for realization by holographic lithography.
Microcrystal growth of GaAs on a Se-terminated GaAlAs surface for the quantum-well box structure by sequential supplies of Ga and As molecular beams
Authors: Chikyow Toyohiro , Koguchi Nobuyuki
Published: July 2002

A growth of GaAs microcrystals on a Se-terminated GaAlAs surface was demonstrated for the first time for the quantum-well box fabrication. At first, Ga molecules were supplied to the Se-terminated GaAlAs surface to form Ga droplets. The surface consisted of Ga droplets and a bared Se-terminated GaAlAs surface. In the following As molecule supply to the surface, GaAs microcrystals were observed to grow epitaxially on the surface. As for GaAs growth on an As stabilized GaAlAs surface, which was also carried out with sequential supplies of Ga and As molecules, lateral growth of GaAs was observed. From the obtained results, this newly proposed method is expected to be useful in fabricating GaAs microcrystals for the quantum-well box structure.
Laser deposition of biaxially textured yttria-stabilized zirconia buffer layers on polycrystalline metallic alloys for high critical current Y-Ba-Cu-O thin films
Authors: Reade R. P. , Berdahl P. , Russo R. E. , Garrison S. M.
Published: July 2002

Pulsed laser deposition of yttria-stabilized zirconia (YSZ) layers on polycrystalline metallic alloy substrates is used to produce an intermediate layer for YBa2Cu3O7−δ (YBCO) thin-film growth. The desired (001) YSZ texture is obtained at 1.0 mTorr oxygen pressure and 70 °C. Significant improvement in (001) texturing is demonstrated by using an ion beam to assist growth. Argon-ion-assisted growth produces layers with alignment of the in-plane crystal axes in addition to the (001)-normal texture. Highly c-axis-oriented biaxially aligned YBCO thin films can be deposited on these layers, with Tc(R=0)=92 K and Jc (77 K, B=0 T)=6×105 A/cm2 and Jc (77 K, 0.4 T)=8×104 A/cm2. With further improvement of the YSZ texture, the YBCO current-carrying capacity of films on polycrystalline metallic alloys may approach that of films on single-crystal substrates.
Optical transitions to above-barrier quasibound states in asymmetric semiconductor heterostructures
Authors: Henderson Gregory N. , West Lawrence C. , Gaylord Thomas K. , Roberts Charles W. , Glytsis Elias N. , Asom Moses T.
Published: July 2002

An asymmetric semiconductor electron wave Fabry–Perot interference filter has been designed with two above-barrier quasibound states for optical transitions. The upper state was designed to have a spatial confinement lifetime greater than three times that of the lower state (which was designed to be less than 100 fs). Such lifetime ratios and magnitudes, which are nearly impossible for below-barrier states, satisfy the criteria required for achieving population inversion. Furthermore, the transitions were designed to have large dipole matrix elements. Absorption measurements at multiple temperatures were used to demonstrate the first bound-to-quasibound transitions in an asymmetric structure. The experimental energies and dipole matrix elements are in agreement with calculated values. This type of structure could represent the basis for a new room-temperature infrared semiconductor laser.
Observation of a delocalized E′ center in buried SiO2
Authors: Vanheusden K. , Stesmans A.
Published: July 2002

The oxygen-vacancy defect (E′) generated at the surface of buried SiO2 (BOX) layers of the separation by implantation of oxygen structure has been studied by K-band electron spin resonance. Besides the usual Eγ′ signal, the Eδ′ center—a delocalized variant of the E′ center—has been identified in the BOX. No such signal, however, was observed in reoxidized (950 °C; 1.1 atm O2) BOX or conventional thermal oxide. These data lead to the proposal of an adapted model for the defect, based on the existence of small Si clusters (≳5 atoms) in the BOX serving as E′ defect precursors. This model implies that the buried oxide contains excess Si, exceedingly so near the BOX/substrate interface.
High contrast asymmetric Fabry–Perot electro-absorption modulator with zero phase change
Authors: Trezza J. A. , Pezeshki B. , Larson M. C. , Lord S. M. , Harris J. S.
Published: July 2002

By analyzing the exciton line shapes of quantum wells, we can determine the wavelengths and biases at which they exhibit large absorption changes and zero refractive index changes relative to zero bias. To use this effect, we placed GaAs/AlGaAs quantum wells in the intrinsic region of a reverse-biased p-i-n diode asymmetric Fabry–Perot modulator. We then developed a sample that could be post-growth processed to optimize all relevant parameters. We used computer simulation to determine the exact post-growth correction required and produced reflection modulators with 90% reflection change and zero relative phase change.
Degradation of II-VI based blue-green light emitters
Authors: Guha S. , DePuydt J. M. , Haase M. A. , Qiu J. , Cheng H.
Published: July 2002

We have carried out the first detailed structural studies of degradation in II-VI blue-green light emitters. Electroluminescence and transmission electron microscopy studies carried out on light emitting diodes fabricated from quantum well laser structures and electroluminescence studies on stripe laser structures show that degradation occurs by the formation and propagation of crystal defects. The studies indicate that room temperature cw lasing in such structures is possibly prevented by the rapid formation of such defects at the high current densities required for lasing.
Green up-conversion laser emission in Er-doped crystals at room temperature
Authors: Brede R. , Heumann E. , Koetke J. , Danger T. , Huber G. , Chai B.
Published: July 2002

We report room-temperature pulsed up-conversion laser oscillation in Er-doped LiYF4 and KYF4 at 551 and 562 nm, respectively. In both crystals laser oscillation is observed on the 4S3/2-4I15/2 ground state transition. Excitation was provided by a tunable flashlamp-pumped Ti:sapphire laser in the spectral region around 810 nm. Additional pumping with a continuous wave krypton ion laser at 647 nm was beneficial to both lasers. Laser action has also been observed in Er-doped Y3Al5O12 on the same transition.
Dissociative adsorption of Si2H6 on silicon at hyperthermal energies: The influence of surface structure
Authors: Engstrom J. R. , Xia L.-Q. , Furjanic M. J. , Hansen D. A.
Published: July 2002

The reactions of Si2H6 with the (100) and (111) surfaces of silicon have been investigated employing supersonic molecular beam scattering techniques. Incident translational energy has been found to influence strongly the probability of dissociative adsorption (SR) on both surfaces. The reaction on the Si(111) surface is distinct from that observed on Si(100) concerning the dependence of SR on the substrate temperature and the incident angle. In particular, the reaction on the (111) surface shows sensitivity to the surface phase transformation (7×7)→(1×1) that occurs above 825 °C. Both the dangling bond density and the effective corrugation of the surface are important in determining the reaction probability.
Microstructure of porous silicon and its correlation with photoluminescence
Authors: Takasuka E. , Kamei K.
Published: July 2002

Microstructure of porous silicon (PS) was observed through high-resolution transmission electron microscope to study a relationship between the microstructure and a photoluminescence (PL) from PS. Three PS samples were made through anodization with different current densities. The samples had different PL spectra and were supposed to have different microstructures. Threadlike structures of Si and Si crystallites in the PS layer were observed in all the samples. The threadlike structure has the same density in all samples and has very little structural change, in spite of a significant change of PL spectra. The density of the Si crystallites varies for the samples and has a strong correlation with the PL intensity. No particular change of crystallites shape was observed. The size of the crystallites ranges from a few nanometers to a few tens of nanometers. Although no positive evidence to a quantum confinement is indicated, the Si crystallites are responsible for a light emission from PS.
Charge exchange recombination spectroscopy as a plasma diagnostic tool (invited)
Authors: Fonck Raymond J.
Published: July 2002

Intensity and line profile measurements of the spectra of light hydrogenic ions which are excited by charge exchange reactions with fast neutral atoms are being widely used as diagnostics for fusion plasma research. This technique, which is referred to as charge exchange recombination spectroscopy, allows measurements of the densities of fully stripped impurity ions and particle transport coefficients with only minor uncertainties arising from atomic processes. The excitation of long-wavelength transitions in light ions such as He+, C5+, and O7+ allows relatively easy measurements of ion velocity distributions to determine ion temperatures and plasma rotation velocities. Among its advantages for such measurements are the facts that fiber-optic coupling between a remote spectrometer and the immediate reactor environment is possible in many cases, the measurement is localized by the intersection region of a neutral beamline and viewing sightline, and intrinsic ions can be used so that injection of potentially perturbing impurities can be avoided. A particularly challenging application of this technique lies in the diagnosis of alpha particles expected to be produced in the present generation of Q≊1 tokamak experiments.
Measurements of mildly relativistic electron distribution functions during lower hybrid heating and current drive (invited)
Authors: Hutchinson I. H. , Kato K. , Texter S. C.
Published: July 2002

Nonthermal electron distribution functions, arising during lower hybrid heating and current drive, have been measured on Alcator C tokamak using vertical electron cyclotron emission and also bremsstrahlung x-ray emission. The methods used for deconvolving the distribution from the radiation measurements are summarized and examples are given of results from different types of plasma, illustrating the present capabilities of the techniques. The cyclotron emission results are consistent with the x-ray analysis but provide additional information, particularly at lower energies (30–100 keV). The x rays are less ambiguous at high energy and give good forward/backward asymmetry diagnosis. The quality of information appears to be already adequate for a quantitative comparison with theory, but further experimental improvements seem possible.
Variable-temperature scanning tunneling microscope
Authors: Lyding J. W. , Skala S. , Hubacek J. S. , Brockenbrough R. , Gammie G.
Published: July 2002

A thermally compensated tube scanner scanning tunneling microscope (STM) has been constructed and successfully tested. This design utilizes two concentric piezoelectric tubes, one for scanning and one for thermal compensation and inertial sample translation (over several mm), as well as fine adjustment of sample position while in tunneling range. This design eliminates the need for mechanical components such as springs, levers, gears, or stepper motors that are known to result in considerable vibration sensitivity, thermal drift, and low-resonance frequencies. This new design demonstrates continuously variable-temperature operation as well as atomic resolution without vibration isolation for the first time in a STM. Thermal drift of less than 1 Å/h and less than 10 Å/K have been determined. Also, the lowest mechanical resonance frequency of 21 kHz makes this new design suitable for high-speed applications such as video rate scanning.
Apparatus for adsorption studies
Authors: Rocca M. , Valbusa U. , Gussoni A. , Maloberti G. , Racca L.
Published: July 2002

In this paper a UHV apparatus for studying adsorption of gases on surfaces will be described. The system is specifically designed for measuring the sticking coefficient of a molecule on the surface and its dependence on energy and angle of incidence. The method is based on the use of an electron energy loss spectrometer which detects the amount of the adsorbed species as a function of the exposure of the surface to a supersonic nozzle beam which deposits the molecular species at different energies and angles of incidence. Measurements of the sticking probability of O2 on Ag(001) are presented showing the capability of the method.
A combined molecular-beam epitaxy and scanning tunneling microscopy system
Authors: Orr B. G. , Snyder C. W. , Johnson M.
Published: July 2002

A combined molecular-beam epitaxy and scanning tunneling microscopy system has been constructed. The design has been optimized for the study of III-V semiconductors with the goal of examining the surface with both in situ scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). Using this system, it is possible to quench the growth and produce real-space images of the surface as it appeared during deposition. Measurements obtained with both RHEED and STM are presented.
Lateral band gap modulation by controlled elastic relaxation of strained multiquantum well structures on InP
Authors: Leclercq J. L. , Viktorovitch P. , Letartre X. , Nuban M. F. , Gendry M. , Benyattou T. , Guillot G. , Fierling G. , Priester C.
Published: July 2002

We describe a method of lateral band gap modulation by creating steep lateral strain gradients in multiquantum (MQW) structures on InP. The strain modulation is obtained by controlled elastic relaxation of micromachined free-standing strained MQW stacks. Large lateral energy modulation is achieved, suitable for lateral optical mode confinement and lateral quantum confinement of excitons. It is pointed out that this technology can be applied in monolithic integration of semiconductor optical devices.
Plasma removal by box type electrode from electron beam generated uranium vapor
Authors: Nishio Ryoji , Suzuki Kazumichi
Published: July 2002

When an electron beam produces a uranium vapor in an electron beam evaporator, a uranium plasma is formed on the evaporating surface which expands with the vapor. To clarify the effect of the polarity of the applied voltage to the electrodes and also the effect of electrode areas when the plasma is removed by surrounding electrodes, the relationship between the plasma reaction to the applied voltage and the amount of plasma removed was investigated by using a box-type electrode with inner fins which involves the plasma. When a positive bias, compared to the plasma potential, was applied to extract electrons from the plasma, the plasma was effectively removed while increasing the bias potential. The plasma became more diffusive due to increases in electron temperature, attributable increases in plasma sheath instabilities. However, when the vapor density increased, the instability resulted in ionization of the vapor to form an additional plasma. Therefore, under this condition, it became difficult to remove the plasma by increasing the bias potential. However, when a negative bias potential was applied to extract ions, the instability did not arise, because an ion sheath was formed in front of the hot electrode. However, a high absolute potential was required to remove the plasma. A simple plasma removal model showed that Si /Se=470 was an optimum electrode area ratio when removing a uranium plasma, where Si is the ion removal electrode area and Se is the electron removal electrode area; this effect was confirmed experimentally.
Microwave interferometer with a differential quadrature phase detection
Authors: Vergamota S. , Cupido L. , Manso M. E. , Eusébio F. , Silva A. , Varela P. , Cabral J. A. C. , Serra F. , Varandas C. A. F.
Published: July 2002

A single channel microwave interferometer has been developed for a tokamak with an improved detection that uses an heterodyne configuration and is based on a differential quadrature phase concept. With the new detection very sensitive and accurate direct phase measurements are obtained due to the elimination of the spurious variations of the detected signals resulting from the nonideal behavior of the output mixers. As a result, the line integrated density along the vertical central chord of the vacuum chamber can be determined during the whole plasma discharge. Fast density variations occurring both at the formation phase of the plasma and during sawtooth cycles could also be measured.
Vacuum compatible, variable cross-section magnetic coil diagnostic used in digital feedback control of plasma position in TEXT-Upgrade
Authors: Foster M. S. , Craig J. L. , Wootton A. J. , Phillips P. E. , Uglum J. , Solano E. R. , Brower D. L. , Jiang Y. , McCool S. C. , Lierzer J. , Castle George G.
Published: July 2002

A magnetic pickup coil diagnostic set is used to measure the position of the plasma column in the Texas Experimental Tokamak Upgrade (TEXT-U) project. The output from this coil set is used in a digital feedback system to control the plasma position. To provide a fast time response for the feedback system, one complete coil set is located on the interior of the vacuum vessel. Another set with a slower time response is located on the outside of the vessel. To simplify and speed up data processing, the coils are constructed so that the X and Y coordinates of the plasma current centroid are each determined using the signals from only two separate coils. For each coordinate one coil is used to measure a tangential (relative to the coil surface) magnetic field component, while the second coil measures a normal field component. Due to physical constraints, the coils are not continuous around the vacuum vessel. The presence of gaps in the coils causes pickup of the external current flowing in the divertor coil windings during TEXT-U diverted discharges. This pickup has been successfully nulled out by adding a divertor current Rogowski coil to the X position coil circuit. The data indicate that these coils, along with the digital feedback system, are useful tools for flexible position control over a wide range of TEXT-U plasma parameters.
Measurement of thermal diffusivities through processing of infrared images
Authors: Philippi I. , Batsale J. C. , Maillet D. , Degiovanni A.
Published: July 2002

The measurement of the thermal diffusivity of a thin layer in the direction of its plane is usually a difficult operation. The standard ‘‘flash technique’’ is very appropriate for diffusivity measurement in the direction of the thickness of the sample but adaptations of this method to in-plane measurements remain very sensitive to the position and form of heat excitation and temperature sensors. The new procedure proposed here consists of applying any geometrically nonuniform heat impulse on the front face of the sample and recording the entire transient temperature image on the rear face thanks to an infrared camera. The influence of axial diffusion can be avoided for periods much longer than the axial diffusion characteristic time. Integral transforms on the radial space variables (Fourier transform) are very suitable for treating the temperature field and to estimate radial diffusivity. The main advantage of this method is to avoid any experimental precaution (no knowledge of the geometrical form of the excitation - replacement of the sensor positioning by an image calibration). Furthermore, the considerable number of data produced by the camera is processed using a statistical approach. The validation of the method is made on a homogeneous sample by comparison between the in-plane direction measurements (obtained with the present procedure) and the thickness direction measurements (obtained by the classical flash technique).
Performance characteristics of scintillators for use in an electronic neutron imaging system for neutron radiography
Authors: Brenizer J. S. , Berger H. , Stebbings C. T. , Gillies G. T.
Published: July 2002

The response of several different scintillator screens to beams of slow neutrons has been investigated using charge-coupled device (CCD)-based and nuvicon imaging systems. The goal of the effort has been to design and develop an electronic neutron imager compatible for use with a neutron radiography system driven by an accelerator that generates intensities of ≈10 6 neutrons cm−2  s−1. The experimental results demonstrated that the LiF/ZnS class of scintillators used with a CCD camera could provide near-real-time images with a dynamic range (i.e., a functional span of optical intensities) of ≳103 and spatial resolution of 2.5 line pairs mm −1. Full details of the experimental arrangements are presented, along with discussions of the scintillator properties, the results of their use in thermal neutron beams, and an assessment of the performance characteristics of the various imaging techniques that were employed.
Streaming metal plasma generation by vacuum arc plasma guns
Authors: MacGill R. A. , Dickinson M. R. , Anders A. , Monteiro O. R. , Brown I. G.
Published: July 2002

We have developed several different embodiments of repetitively pulsed vacuum arc metal plasma gun, including miniature versions, multicathode versions that can produce up to 18 different metal plasma species between which one can switch, and a compact high-duty cycle well-cooled version, as well as a larger dc gun. Plasma guns of this kind can be incorporated into a vacuum arc ion source for the production of high-energy metal ion beams, or used as a plasma source for thin film formation and for metal plasma immersion ion implantation and deposition. The source can also be viewed as a low-energy metal ion source with ion drift velocity in the range 20–200 eV depending on the metal species used. Here we describe the plasma sources that we have developed, the properties of the plasma generated, and summarize their performance and limitations.
Design and evaluation of a low thermal electromotive force guarded scanner for resistance measurements
Authors: Jarrett Dean G. , Marshall James A. , Marshall Thomas A. , Dziuba Ronald F.
Published: July 2002

The design and testing of a low thermal electromotive force guarded scanner, developed to provide completely guarded switching when used with actively guarded resistance bridge networks, is described. The design provides a continuous guard circuit trace on the scanner circuit boards that surrounds the relay contacts and protects the measurement circuit from leakages to ground. Modification to the circuit boards and relays of the guarded scanner are explained. Several tests were developed to evaluate the guarding effectiveness, including isolating sections of the guard circuit to create a potential drop between the main and guard circuits. Calibration of standard resistors using the guarded scanner has shown relative differences less than 1×10−6, 30×10−6, and 150×10−6 for measurements made with and without the guarded scanner at nominal resistances of 1, 10, and 100 GΩ, respectively. The substitution method was used to significantly reduce the relative differences between channels to less than 0.5×10−6, 3×10−6, and 30×10−6 for nominal resistances of 1, 10, and 100 GΩ, respectively. Applications for the guarded scanner in automated direct current measurement systems are presented.
Defect density measurements of low temperature grown molecular beam epitaxial GaAs by photothermal deflection spectroscopy
Authors: Chan M. H. , So S. K. , Chan K. T. , Kellert F. G.
Published: July 2002

The subgap optical absorption of GaAs layers grown by low temperature molecular beam epitaxy is measured by photothermal deflection spectroscopy (PDS). The absorption increases as the growth temperature decreases at a fixed wavelength. Defect densities evaluated from the absorption spectra and the known absorption cross sections are between 1018 and 1019 cm−3. It is shown that complementary PDS phase spectra can be used to separate the absorption of the epitaxial layers from the bulk.
In-plane orientation control of (001)YBa2Cu3O7−δ grown on (001)MgO by pulsed organometallic beam epitaxy
Authors: Buchholz D. B. , Lei J. S. , Mahajan S. , Markworth P. R. , Chang R. P. H. , Hinds B. , Marks T. J. , Schindler J. L. , Kannewurf C. R. , Huang Y. , Merkle K. L.
Published: July 2002

Thin films of (001) YBCO are grown on epitaxially polished (001) MgO by pulsed organometallic beam epitaxy. The in-plane orientation of the film is controlled by the thickness of a BaO layer, grown in situ, prior to the YBCO growth. For thin BaO layers (<≊7×1014 Ba/cm2) the films grown [110]YBCO∥[100]MgO. For thick BaO layers (≳≊11×1014 Ba/cm2) the films grow [100]YBCO∥[100]MgO. A mechanism that relates the change in YBCO in-plane orientation to a change in the structure of the initial BaO layers with BaO thickness is described.
White organic electroluminescence devices
Authors: Jordan R. H. , Dodabalapur A. , Strukelj M. , Miller T. M.
Published: July 2002

Thin film organic electroluminescent devices are described which employ a blue-emitting species as a layer sandwiched between the hole transporter bis(triphenyl)diamine and the light emitting electron transporter tris(8-hydroxyquinoline)aluminum to obtain bright (≳4700 cd/ m2) efficient (0.5 lm/W) white emission.
Mixed-mode twisted nematic liquid crystal cells for reflective displays
Authors: Wu Shin-Tson , Wu Chiung-Sheng
Published: July 2002

A mixed-mode twisted nematic (MTN) liquid crystal cell is developed for both reflective-mode projection and direct-view displays. This MTN cell exhibits two times higher brightness and eliminates the parallax problem as commonly observed in reflective displays using a nominal transmissive TN cell. In the meantime, the MTN structure preserves advantageous characteristics of the TN structure such as low operation voltage and high contrast ratio. Confirming computer simulations using Jones matrix method agree with the experimental results well.
New near-infrared defect luminescence in GaN doped with vanadium by ion implantation
Authors: Kaufmann B. , Dörnen A. , Härle V. , Bolay H. , Scholz F. , Pensl G.
Published: July 2002

We report a new photoluminescence (PL) spectrum of wurzite-type GaN intentionally doped with vanadium by ion implantation. A group of several broad intense near-infrared PL lines is observed at 820 meV. The whole PL spectrum can be observed up to room temperature. The samples were grown by low-pressure metalorganic vapor phase epitaxy on sapphire substrate. After ion implantation the samples were annealed under growth conditions at 920 °C.
Deposition of polycrystalline β-SiC films on Si substrates at room temperature
Authors: Cheng Kuan-Lun , Cheng Huang-Chung , Lee Wen-Horng , Lee Chiapyng , Liu Chih-Chien , Yew Tri-Rung
Published: July 2002

Polycrystalline β-SiC, with grain size up to 1500 Å, has been room-temperature-deposited on Si substrates by electron cyclotron resonance chemical vapor deposition. Microwave power and the hydrogen carrier gas are the key parameters to lower the deposition temperature. According to the results of the cross-sectional transmission electron microscopy, the grain size appeared to be in the same scale as that deposited at 500 °C while a large amount of plasma-induced defects were observed in the Si substrate for the room-temperature-deposited samples. Hence, a CH4-plasma treatment prior to the β-SiC film growth was adopted, forming a SiC-like interfacial layer to suppress the substrate damages.
Low temperature plasma process based on CO-rich CO/H2 mixtures for high rate diamond film deposition
Authors: Lee Joungchel , Collins R. W. , Messier R. , Strausser Y. E.
Published: July 2002

A low temperature process (350 °C<T<500 °C) for nanocrystalline diamond film growth by microwave plasma-enhanced chemical vapor deposition has been developed that yields high rates (up to 2.5 μm/h) at relatively low plasma powers (0.5 kW). In contrast to the widely used H2-rich mixtures of CH4 or CO and H2 that exhibit monotonic decreases in the diamond growth rate as T is reduced from 800 to 400 °C, CO-rich mixtures of CO and H2 exhibit an increase and sharp maximum as T is reduced. The results suggest a different diamond growth mechanism from the CO-rich mixtures.
Small-signal modulation and differential gain of single-mode self-organized In0.4Ga0.6As/GaAs quantum dot lasers
Authors: Kamath K. , Phillips J. , Jiang H. , Singh J. , Bhattacharya P.
Published: July 2002

We report small-signal modulation bandwidth and differential gain measurements of a single-layer self-organized In0.4Ga0.6As/GaAs quantum dot laser grown by molecular beam epitaxy. The 3 dB bandwidth of single-mode ridge waveguide lasers was measured to be 7.5 GHz at 100 mA under pulsed measurements, demonstrating the possibility of high speed operation of these devices. The differential gain was measured to be 1.7×10−14 cm2.
Raman characterization of aligned carbon nanotubes produced by thermal decomposition of hydrocarbon vapor
Authors: Li Wenzhi , Zhang Hao , Wang Chaoying , Zhang Yun , Xu Liwen , Zhu Ke , Xie Sishen
Published: July 2002

Raman characterization of aligned carbon nanotubes of average diameter 10–15 nm, produced by chemical vapor deposition on a mesoporous substrate, has been carried out. The resonance behavior and higher-order Raman bands up to fourth order have been observed and compared with those of carbon nanotubes produced by arc discharge and highly oriented pyrolytic graphite, as well as pyrolytic graphite. The phonon properties have been analyzed with the help of high-resolution transmission electron microscope studies.
Adsorption of tertiarybutylarsine on GaAs(001)-c(4×4) surface studied by molecular beam scattering
Authors: Cui Jie , Ozeki Masashi , Ohashi Masafumi
Published: July 2002

The scattering properties of tertiarybutylarsine (TBAs) on c(4×4) GaAs(001) surfaces were studied by supersonic-molecular beam scattering. The sticking coefficient measurement reveals that TBAs molecules chemisorb nondissociatively on c(4×4) surface through precursor-mediated mechanism. The desorption of TBAs molecules from the chemisorption well was observed at temperatures higher than 100 °C. The activation energies of the desorption (10.9 and 7.7 kcal/mol) are small, reflecting that the chemisorption wells of TBAs molecules on c(4×4) surface are shallow.
Room temperature observation of single electron tunneling effect in self-assembled metal quantum dots on a semiconductor substrate
Authors: Park Kang-Ho , Ha Jeong Sook , Yun Wan Soo , Shin Mincheol , Park Kyoung-Wan , Lee El-Hang
Published: July 2002

We report on the observation of room-temperature single electron tunneling phenomena in a metal-insulator-metal-semiconductor double-junction structure. The nanosized Ag dots were self-assembled on a Sb-terminated Si(100) surface, and the Coulomb gap and staircases were observed in the local current–voltage (I–V) measurements using scanning tunneling microscopy. The I–V characteristics exhibiting the single electron tunneling behavior vary significantly with the variation of the measurement position within the same Ag droplet. These phenomena are well described by the tip-dot(Ag)-Si double-junction picture.
Transition from diffusive to localized regimes in surface corrugated optical waveguides
Authors: Garcı́a-Martı́n A. , Torres J. A. , Sáenz J. J. , Nieto-Vesperinas M.
Published: July 2002

Exact calculations of the transmittance of surface corrugated optical waveguides are presented. The elastic scattering of diffuse light or other electromagnetic waves from a rough surface induces a diffusive transport along the waveguide axis. As the length of the corrugated part of the waveguide increases, a transition from the diffusive to the localized regime is observed. This involves an analogy with electron conduction in nanowires, and hence, a concept analogous to that of “resistance” can be introduced. We show an oscillatory behavior of both the elastic mean-free path and the localization length versus the wavelength.
Nano-compact disks with 400 Gbit/in2 storage density fabricated using nanoimprint lithography and read with proximal probe
Authors: Krauss Peter R. , Chou Stephen Y.
Published: July 2002

Nano-compact disks (Nano-CDs) with 400 Gbit/in2 topographical bit density (nearly three orders of magnitude higher than commercial CDs) have been fabricated using nanoimprint lithography. The reading and wearing of such Nano-CDs have been studied using scanning proximal probe methods. Using a tapping mode, a Nano-CD was read 1000 times without any detectable degradation of the disk or the silicon probe tip. In accelerated wear tests with a contact mode, the damage threshold was found to be 19 μN. This indicates that in a tapping mode, both the Nano-CD and silicon probe tip should have a lifetime that is at least four orders of magnitude longer than that at the damage threshold.
Scanning Joule expansion microscopy at nanometer scales
Authors: Varesi J. , Majumdar A.
Published: July 2002

We report a new technique called scanning Joule expansion microscopy that can simultaneously image surface topography and material expansion due to Joule heating with vertical resolution in the 1 pm range and lateral resolution similar to that of an atomic force microscope. By coating the sample with a polymer film, we demonstrate that sample temperature distribution can be directly measured without the need of fabricating temperature-sensing scanning probes.
Solid-state metathesis reactions under pressure: A rapid route to crystalline gallium nitride
Authors: Wallace Charles H. , Kim Sang-Ho , Rose Garry A. , Rao Lin , Heath James R. , Nicol Malcolm , Kaner Richard B.
Published: July 2002

High pressure chemistry has traditionally involved applying pressure and increasing temperature until conditions become thermodynamically favorable for phase transitions or reactions to occur. Here, high pressure alone is used as a starting point for carrying out rapid, self-propagating metathesis reactions. By initiating chemical reactions under pressure, crystalline phases, such as gallium nitride, can be synthesized which are inaccessible when initiated from ambient conditions. The single-phase gallium nitride made by metathesis reactions under pressure displays significant photoluminescence intensity in the blue/ultraviolet region. The absence of size or surface-state effects in the photoluminescence spectra show that the crystallites are of micron dimensions. The narrow lines of the x-ray diffraction patterns and scanning electron microscopy confirm this conclusion. Brightly luminescent thin films can be readily grown using pulsed laser deposition.
Determination of the shape of self-organized InAs/GaAs quantum dots by reflection high energy electron diffraction
Authors: Lee Hao , Lowe-Webb Roger , Yang Weidong , Sercel Peter C.
Published: July 2002

We report a reflection high energy electron diffraction study of InAs self-organized quantum dots grown on GaAs (001). We observe facet reflections along the [31̄0] and [13̄0] azimuths, which indicate that the quantum dot shape is pyramidal with bounding facets corresponding to a family of four {136} planes. The determined structure, which possesses C2v symmetry, is quite different both from square-base pyramidal geometries which have been assumed in recent electronic structure calculations, and from previously proposed structures which have been based upon incomplete reflection high energy electron diffraction data.
Arsenic surfactant effects and arsenic mediated molecular beam epitaxial growth for cubic GaN
Authors: Okumura H. , Hamaguchi H. , Feuillet G. , Ishida Y. , Yoshida S.
Published: July 2002

Small amounts of As residual pressure were found to affect the structure of cubic GaN growing surfaces in molecular beam epitaxy growth, i.e., modification of surface reconstruction structures, stabilization of reconstructed flat surfaces at high substrate temperatures, and preferential growth of the cubic phase. These As surfactant effects are discussed in relation to the atomic arrangement of the As-passivated surface of GaN. It was shown that the quality of cubic GaN epilayers can be improved by utilizing a small amount of As residual pressure.
Electroluminescence efficiency of 1.3 μm wavelength InGaAs/GaAs quantum dots
Authors: Huffaker D. L. , Deppe D. G.
Published: July 2002

Data are presented characterizing the spectral emission and the electroluminescence efficiency dependence on growth conditions of 1.3 μm wavelength InGaAs/GaAs quantum dots. We show that highly efficient 1.3 μm room temperature electroluminescence can be achieved with only ten total deposited monolayers with an averaged In content of 50%. Atomic force microscopy shows that the 1.3 μm wavelength quantum dots form with a density of ∼1.3×1010 cm−2.
Indium transient enhanced diffusion
Authors: Griffin P. B. , Cao M. , Vande Voorde P. , Chang Y.-L. , Greene W. M.
Published: July 2002

Indium, an acceptor dopant in silicon, is a large atom with a low diffusion coefficient potentially suitable for doping the channel of transistors. Systematic experiments are described which measure the susceptibility of indium to transient enhanced diffusion caused by ion implant damage introduced during the transistor fabrication process. We find that indium diffusion is dramatically enhanced by a source of interstitials and that the amount of enhancement is comparable to that seen for boron. Indium is preferable as a channel dopant not because of its diffusion behavior, but rather because a narrow ion implanted distribution can be obtained using the heavy indium ion, giving a more steeply retrograde profile than can be achieved by boron doping. These results help clarify the physics of indium and boron doping in small devices.
In situ reflectance difference spectroscopy of N-plasma doped ZnTe grown by molecular beam epitaxy
Authors: Stifter D. , Schmid M. , Hingerl K. , Bonanni A. , Garcia-Rocha M. , Sitter H.
Published: July 2002

In situ reflectance difference spectroscopy (RDS) has been performed during growth and nitrogen-doping of ZnTe thin films fabricated by molecular beam epitaxy. The doping level of the ZnTe samples can be determined by evaluating the RD spectra in the vicinity of the E1 and E1+Δ1 transitions. RDS features in this spectral range were used to optimize online the doping performance of the N-plasma cell. Furthermore, doping-induced surface processes have been investigated, like surface saturation with activated N species and surface Fermi level pinning occurring at ambient pressure.