bluuebunny/crossref_metadata_2025 huggingface dataset:
Transient temperature measurements in an ideal gas by laser-induced Rayleigh light scattering
Authors: Horton J. F. , Peterson J. E.
Published: July 2002

A laser-induced Rayleigh light-scattering (RLS) system was assembled and used to noninvasively measure the transient molecular number density in an ideal gas. This information was used to find the transient gas temperature when operating at known pressure. The laser was a 4 W argon ion operating on all lines at a total power of about 2.5 W. The theoretically predicted photon arrival rate at the photomultiplier tube detector was calculated and compared well with the observed photon rates. These rates were high enough that sampling could be averaged over a 1 s period with theoretical uncertainty due to electronic shot noise below 0.1%, and below 2% for a 0.001 s averaging time. A propagated error analysis showed uncertainty in the transient temperature due to all sources was 2–4 K. The RLS system was used to record transient air temperature at several locations above a flat plate during heating from room temperature to 475 K. Results showed buoyancy-induced fluctuations of about 3 Hz, and instabilities in temperature in addition to the overall temperature rise due to plate heating. Excellent transient temperature records were obtained, substantiating the predicted 2–4 K uncertainty.
Focusing characteristics of intense beam solenoid lens
Authors: Sun An , Sun B. H.
Published: July 2002

In this article, by solving the nonlinear envelope equation of the intense beam in the cylindrical coordinate system, we have obtained the varying relations of the beam radius and the beam divergent angle with the paramedics (the magnetic induction strength in solenoid lens, the characteristic quantity of beam intensity, the beam emittance, the initial beam radius, and initial beam divergent angle). By theoretical analysis for the relations, we have obtained the beam peak radius and beam peak position in solenoid lens, the beam waist radius and beam waist position in the case of perfect focusing. In the ideal focusing condition, the radius of intense beam solenoid lens should not be less than the beam peak radius; the length of the solenoid lens must not be less than the beam peak position; and in order to gain optimum focusing, an optimal length should be selected; the magnetic induction strength must not be less than the threshold for getting a focusing beam through solenoid lens, and its optimum value should be selected so as to obtain the minimum value of beam divergent angle at the exit port for getting an perfect focusing output beam. The conclusions in this article have some guiding significance for the design and operation of the intense beam solenoid magnetic lens.
Using the induced axial magnetic field to measure the root mean square beam size and beam density uniformity of an electron beam in an induction linac
Authors: Carlsten Bruce E.
Published: July 2002

An intense electron beam in a solenoidal axial magnetic field rotates azimuthally, and generates a diamagnetic axial field that counteracts the applied field. However, the magnetic flux within the beam pipe is conserved over sufficiently short times, and this induced diamagnetic field must lead to an increase in the axial field outside the beam, which is measurable with a loop or an optical Faraday rotator. If the applied axial magnetic field is uniform across the beam pipe, the measurement directly leads to the rms transverse beam size. However, if the axial magnetic field has sinusoidal components, measurement of the diamagnetic field at two axial positions yields both the beam’s rms transverse size and the beam’s fourth radial moment. Comparison of these moments can give a figure of merit of the beam’s density uniformity.
Stroboscopic x-ray topography in crystals under 10-μm-surface acoustic wave excitation
Authors: Zolotoyabko E. , Shilo D. , Sauer W. , Pernot E. , Baruchel J.
Published: July 2002

High-frequency stroboscopic x-ray topography is developed with the goal to visualize surface acoustic wave propagation. For this purpose the resonant frequencies of LiNbO3-based surface acoustic wave devices (290 and 355 MHz) are synchronized with the frequency of x-ray flashes (5.68 MHz) delivered by the European Synchrotron Radiation Facility (ESRF, Grenoble). X-ray topographs, taken in such a manner, revealed periodic contrast caused by a “frozen” acoustic deformation field. This technique allows for visualization of individual wavefronts of traveling acoustic waves having a wavelength in a 10 μm range. X-ray stroboscopic topographs showed a weak wavefront distortion due to the scattering of acoustic waves on linear dislocations. Secondary spherical waves were observed as a result of the strong acoustic wave interaction with submicron size density perturbations.
Setup for potential bias experiments on the Saha Institute of Nuclear Physics tokamak
Authors: Ghosh J. , Pal R. , Chattopadhyay P. K.
Published: July 2002

An experimental setup for studying the influence of the radial electric field on very low qa plasma on the Saha Institute of Nuclear Physics tokamak is presented. A high current, high voltage pulsed power supply, using a semiconductor controlled rectifier (SCR) as a dc switch is developed and used to bias a tungsten electrode inserted inside the plasma. The electrode’s exposed length and its position inside the plasma are controlled by a double bellows assembly to optimize the electrode-exposed length. We show that using the force commutation method to turn the SCR off to get the power pulse desired has good potential for carrying out similar kinds of studies, especially in a low budget small tokamak.
Toward the experimental observation of nonlinear effects in laser-electron beam scattering
Authors: Matsukado Koji , Endo Ichita , Takahashi Tohru , Tauchi Toshiaki
Published: July 2002

We constructed an experimental system to observe nonlinear effects in the Compton scattering. The electron beam is supplied by an electron gun and a photon beam comes from a table-top tera-watt laser system. As a test experiment, we measured the Compton scattering in linear regime and had a good agreement with simulations, indicating that the background level of the system was under control and the detection efficiency was well estimated. The simulation study shows that it is necessary to use a laser beam with an inclined polarization plane to obtain a maximal amount of second harmonics in photon-electron scattering.
Pneumatic repetitive injector with a porous pellet generator for steady-state plasma fueling
Authors: Viniar I. , Sudo S.
Published: July 2002

A repetitive injector with a new porous pellet generator providing steady-state plasma fueling has been developed for fusion devices. The pellet generator can produce an unlimited number of pellets at 0.5–0.9 Hz. Several thousands of hydrogen pellets of 2.5 mm diameter and 2–4 mm length were formed in different modes and accelerated to 0.8–1.1 km/s with an average 70% reliability. The pellet generator has no moving units and contains only 3–5 pellets as reserve fuel for steady-state operation, so the injector design is compatible with and attractive for future tritium operation. A detailed design description and test results are presented.
A technique for preparing submicrometer-size liquid film: Development of a new type of optical cell
Authors: Sakaguchi Yoshifumi , Tamura Kozaburo
Published: July 2002

We present a specially designed optical cell made of quartz preparing submicrometer-size films of liquid semiconductors. Using our technique, liquid film with the thickness of 0.3 μm could be produced. We applied the technique to the laser excited transient absorption measurement for liquid sulfur and the transient direct-current (dc) conductivity measurement for liquid selenium. It was found from the transient absorption measurement for liquid sulfur that the photoinduced polymerization occurs in liquid sulfur. The result of the transient dc conductivity measurement for liquid selenium indicated a possibility of the photoinduced semiconductor-metal transition in liquid selenium.
Metallic work function measurement in the range 2–3.3 eV using a blue light-emitting diode source
Authors: Schletti Reto , Wurz Peter , Fröhlich Theo
Published: July 2002

A new photoelectric method to monitor the work function of contaminated metallic surfaces in the range 2–3.3 eV using blue light-emitting diodes (LEDs) is presented. This method was specifically developed for an application onboard a spacecraft, where the simplicity of a system is a great asset. This technique makes it possible to follow the work function of a calibrated surface LED combination with a reproducibility of ∼2% with only one blue LED and a device to count photoelectrons. The surface described in this paper is Cs/W(110). The work function changes with time due to adsorption of residual gas in the moderate vacuum environment (in the mid 10−8 mbar range). Determination of the work function with a standard photoelectric technique is used to calibrate the system. We then derive calibration curves for this specific surface LED combination which reduce the workfunction measurement to a simple current measurement.
Injection of various metallic elements into an electron beam ion trap: Techniques needed for systematic investigations of isoelectronic sequences
Authors: Nakamura Nobuyuki , Kinugawa Tohru , Shimizu Hiroshi , Watanabe Hirofumi , Ito Satoshi , Ohtani Shunsuke , Yamada Chikashi , Okazaki Kiyohiko , Sakurai Makoto , Tarbutt Michael R. , Silver Joshua D.
Published: July 2002

Several techniques for injecting various metallic elements into an electron beam ion trap are presented. They have been developed by making use of a metal vapor vacuum are source, a wire probe, an alkali ion source, gaseous metallic compounds, and so on. These techniques are needed to systematically investigate highly charged ions along isoelectronic sequences. Merits and demerits of these methods are discussed.
Axial ion charge state distribution in the vacuum arc plasma jet
Authors: Keidar M. , Brown I. G. , Beilis I. I.
Published: July 2002

We report on our experimental studies of the ion charge state distribution (CSD) of vacuum arc plasmas using a time-of-flight diagnostic method. The dependence of the CSD on the axial distance from the plasma source region was measured for a titanium vacuum arc. It was found that the axial CSD profile is nonuniform. Generally, the mean charge state increases approximately linearly with axial distance from about 1.7 at 12 cm up to 1.9 at 25 cm from the plasma source. A model for ion transport in the free boundary plasma jet is proposed which is based on the existence of an electric field in the quasineutral plasma. This model qualitatively explains the experimental results. © 2000 American Institute of Physics.
Molecular ion implanter equipped with liquid-metal alloy ion source
Authors: Gotoh Y. , Tsuji H. , Ishikawa J.
Published: July 2002

Molecular ion implanter was developed with a liquid-metal alloy ion source. Use of a liquid-metal alloy ion source enables us to generate various kinds of molecular ions. To apply liquid-metal ion source to a general ion implanter, it is necessary to converge the divergent beam. We adopted the lens system we have already developed, and examined its performance by computer simulation and experiments. An example of molecular ion implantation was demonstrated with gold-antimony ion source. 24 keV AuSb2+ was implanted into silicon, and presence of gold and antimony atoms was confirmed by Rutherford backscattering spectrometry and particle induced x-ray emission measurements.
Magnetic field modification of 18 GHz electron cyclotron resonance ion source at the Japan Atomic Energy Research Institute
Authors: Yokota W. , Saitoh Y. , Nara T. , Ishii Y. , Arakawa K.
Published: July 2002

An 18 GHz electron cyclotron resonance ion source under development since. 1994 has a solenoid coil between a pair of mirror coils to vary the mirror ratio over a wide range. The solenoid coil of the original design formed a small bump in the bottom of the mirror field profile when the source parameters were optimized for high charge states. Source performance in generating highly charged ions improved with a number of minor changes on the vacuum, the gas feed, and the beam extraction system. The maximum beam current of Ar16+ was 2 enA, which was much lower than that expected from the microwave frequency and the magnetic field strength. Little effect of a biased disk on intensities of highly charged ions and dependence of the charge state distribution on the solenoid coil current suggested that the bump divided plasma into two regions and limited the source performance. The bump was removed by halving the solenoid coil length, and the beam current of Ar16+ increased to 1.3 eμA as a result.
Change of diborane ion species depending on the magnitude of magnetic field using 100 MHz very high frequency discharge
Authors: Sakai Shigeki , Takahashi Masato , Tanjyo Masayasu
Published: July 2002

In order to fabricate next generation semiconductor devices, low energy and high current doping ions are required to be transported. In particular the energy of boron ions is required to be less than 1 keV at a current of several mA. When hydrogen-diluted diborane is used as the ion source gas, usually hydrogen ions are also generated as well as borone hydride ions. But if the generation of the hydrogen ions is suppressed, mass separation may be unnecessary. Then the distance between the ion source and the target wafer can be shortened. As a result it becomes easy to transport low energy, high current ion beams. This work is concerned with a new method of plasma production which can suppress the hydrogen ion ratio in the very high frequency (VHF) discharge. We have improved the magnitude of multicusp magnetic field by changing the permanent magnet arrangement at the periphery of the plasma chamber, because the ratio of hydrogen ions strongly depends on the amount of high energy electrons which are produced through a relation between the VHF field and the multicusp field. Optimizing the magnitude of the magnetic field, the ratio of hydrogen ions can be reduced to less than 1% of the total ions.
High-current ion source development for the Korea Multipurpose Accelerator Complex
Authors: Cho Y. S. , Choi B. H. , Hong I. S. , Hwang Y. S.
Published: July 2002

The Korea Multipurpose Accelerator Complex (KOMAC) project has been initiated to develop and build a high current proton/H− linear accelerator capable of delivering an 1 GeV cw proton beam with an intensity of 20 mA in the final stage. The major proton beam will be used for nuclear-waste transmutation, energy production, and nuclear physics experiments while utilizing the minor negative hydrogen beam for basic science research and medical therapy. A Duoplasmatron proton source for the KOMAC linear accelerator has been built at Korea Atomic Energy Research Institute. The hydrogen beam currents of up to 50 mA at the extraction voltage of 50 kV are routinely obtained. A low normalized rms emittance of 0.2 π mm mrad and a proton fraction of over 80% are obtained in this source. With the 100% duty factor of ion source operation, the arc filaments of the source have survived over 40 h. Except for the filament lifetime, the achieved parameters of the proton beam source are satisfying most requirements of the KOMAC ion source. The ion sources can extract the 10 mA nitrogen beam and 3 mA argon beam for the industrial surface modification processes.
Performance test of electron cyclotron resonance ion sources for the Hyogo Ion Beam Medical Center
Authors: Sawada K. , Sawada J. , Sakata T. , Uno K. , Okanishi K. , Harada H. , Itano A. , Higashi A. , Akagi T. , Yamada S. , Noda K. , Torikoshi M. , Kitagawa A.
Published: July 2002

Two electron cyclotron resonance (ECR) ion sources were manufactured for the accelerator facility at the Hyogo Ion Beam Medical Center. H2+, He2+, and C4+ were chosen as the accelerating ions because they have the highest charge to mass ratio among ion states which satisfy the required intensity and quality. The sources have the same structure as the 10 GHz ECR source at the Heavy Ion Medical Accelerator in Chiba except for a few improvements in the magnetic structure. Their performance was investigated at the Sumitomo Heavy Industries factory before shipment. The maximum intensity was 1500 μA for H2+, 1320 μA for He2+, and 580 μA for C4+ at the end of the ion source beam transport line. These are several times higher than required. Sufficient performance was also observed in the flatness and long-term stability of the pulsed beams. These test results satisfy the requirements for medical use.
Application of compact microwave ion source to low temperature growth of transition metal nitride thin films for vacuum microelectronics devices
Authors: Gotoh Y. , Tsuji H. , Ishikawa J.
Published: July 2002

A compact microwave ion source was applied to the low temperature growth of transition metal nitride thin films for the cathode of vacuum microelectronics devices. An ion beam assisted deposition system consisted of a compact microwave ion source and an electron beam evaporator was developed. Depositions of zirconium nitride and niobium nitride thin films were performed and the film properties were investigated. As a result, it was found that polycrystalline films of zirconium nitride and niobium nitride were prepared at the substrate temperature as low as 500 °C, which was almost 200 °C lower than the results shown in the literature. The reason for this reduction of substrate temperature might be attributed to low gas pressure during deposition, due to the use of a single aperture ion source. The control of film composition by controlling the ion-atom arrival rate ratio achieved the control of work function. It was concluded that the ion beam assisted deposition with microwave ion source provides a possible process of cathode deposition.
The production and use of ultralow energy ion beams
Authors: Goldberg R. D. , Armour D. G. , van den Berg J. A. , Cook C. E. A. , Whelan S. , Zhang S. , Knorr N. , Foad M. A. , Ohno H.
Published: July 2002

An ion accelerator, purpose built to produce beams at energies down to 10 eV with current densities in the 10–100 μA cm−2 range, is described. Fitted with dual ion source assemblies, the machine enables ultralow energy ion implantation and the growth of films and multilayers to be carried out under highly controlled conditions. The accelerator delivers ion beams into an ultrahigh vacuum chamber, containing a temperature controlled target stage (range −120 to +1350 °C), where they are used to study the fundamental physics relating to the interaction of ultralow energy ions with surfaces. This knowledge underlies a wide range of ion-beam and plasma-based technologies and, to illustrate its importance, results are presented from investigations designed to determine the optimum conditions for the growth of diamond-like and aluminum films by ion-beam deposition and the formation of ultrashallow junctions in semiconductors by 2.5 keV As+ implantation. The later investigation shows how transient arsenic diffusion, which occurs during post-implant thermal processing, can be controlled by manipulating the substrate temperature during implantation.
A 2.45 GHz microwave negative hydrogen ion source
Authors: Takagi A. , Mori Y.
Published: July 2002

A 2.45 GHz microwave plasma sputter-type of negative hydrogen ion source has been developed at KEK. The microwave discharge was produced by a 2.45 GHz microwave source. In this ion source, negative hydrogen ions are produced at the surface of a molybdenum converter electrode which is placed in a hydrogen plasma confined by a cusp magnetic field. In preliminary experiments, a H− beam current was obtained in pulsed mode operation (0.2 ms, 20 Hz). Impurities (O− and OH−) of 27% in the total negative ion beam current were observed.
Numerical simulation and optimization of multicomponent ion beam from RIKEN electron cyclotron resonance ion sources
Authors: Alexandrov V. , Nakagawa T. , Shevtsov V. , Shirkov G.
Published: July 2002

A new version of a program library for numerical simulation of ion beam transportation is presented. The library is aimed at high current and low energy multicomponent beams from ion sources and based on the macroparticle method. It has been used to simulate and optimize the transport line between the 18 GHz ECR ion source and RFQ linac at RIKEN. It is shown that it is possible to improve the ion transportation efficiency to a factor of 2 or 3 using the solenoid instead of the present einzel lens as a first element of the line.
Ion energy width of a dc metal arc plasma under high vacuum
Authors: Tamba Moritake , Amemiya Hiroshi
Published: July 2002

A finite energy width of the ion beam after being extracted from a metal ion source has been measured by using a mass spectrum analyzer of magnetic prism type. The method of analysis is described in which effects of velocity dispersions in longitudinal and lateral directions, slit width, scan effect, and prism angle are taken into account. The energy width of Cu ions has been determined by considering the width of the spectrum at a maximum slope under magnetic field scanning. The energy width is found to be of the order of the pre-sheath depth, the ion temperature effect having a secondary effect. The method has the merit of causing no disturbance on the plasma and will be useful for the beam diagnostics and for designing an ion source for a three dimensional ion implantation.
Calculations of stray electron trajectory for designing negative ion accelerators of neutral beam injector
Authors: Asano Shiro , Okuyama Toshihisa , Suzuki Yasuo , Kaneko Osamu
Published: July 2002

A simulation code named “STELOC2” has been developed to calculate the stray electron trajectories of negative ion sources for neutral beam injectors (NBIs). The electrons stripped from the negative ions were treated as one of the main sources of stray electrons. This code was applied to a negative hydrogen ion accelerator for LHD-NBI No. 1 (neutral beam injector 1 for a large helical device). Calculation results showed that stripped electrons and electrons secondarily produced by stripped electron impacts on the extraction grids account for the main part of accelerated electrons. By using STELOC2, the effect of electron suppression grid was confirmed.
Study of ion source emittance of an ELS-PIG source used for ion implantation
Authors: Swenson D. R. , Saadatmand K. , Zeinoun I.
Published: July 2002

Optimizing transmission of beam in an ion implanter used for the manufacture of semiconductor devices leads to improved source lifetime, better reliability, and reduces contamination. With this goal, we are studying the emittance of the beam produced by the ELS-PIG ion source for beams from a BF3 plasma extracted at 90 kV with currents as high as 50 mA. The four-dimensional emittance is measured using a unique pepper-pot emittance gear. We have studied the effect on emittance of the plasma emission aperture dimensions, the extraction gap length, the extracted current and other parameters.
Parametric study of negative ion production in cesium seeded hydrogen plasmas
Authors: Fukumasa Osamu , Monji Hideki
Published: July 2002

Effects of cesium vapor injection on H− production in a tandem volume source are studied numerically as a function of plasma parameters. Model calculation is performed by solving a set of particle balance equations for steady-state hydrogen discharge plasmas. Here, the results with a focus on gas pressure dependence of H− volume production are presented and discussed. Considering H− surface production due to H atoms and positive hydrogen ions, enhancement of H− production and pressure dependence of H− production observed experimentally are qualitatively well reproduced in the model calculation, where stripping loss in the extraction and acceleration regions is taken into account. For pressure dependence of H− production with cesium, i.e., decrease in optimum pressure, H atoms play an important role. For enhancement of H− production, effect of so-called electron cooling is also briefly discussed.
Alignment system for crossed parabolic x-ray mirrors
Authors: Matyi R. J. , Moran P. D. , Hagquist W. W. D. , Volz H. M.
Published: July 2002

We describe the implementation of a crossed graded parabolic x-ray mirror system that is suitable for incorporation into both conventional and high resolution x-ray diffraction systems. A novel optical alignment system has been devised that permits independent adjustment of the positions and the angular settings of the mirrors. This alignment system is simple and inexpensive to construct, yet is mechanically stable and easy to use. The crossed mirror system has been successfully introduced into both protein crystallography and high resolution x-ray diffraction systems.
Real-time imaging of semiconductor space-charge regions using high-spatial resolution evanescent microwave microscope
Authors: Tabib-Azar M. , Akinwande D.
Published: July 2002

A high-resolution evanescent microwave probe (EMP) was used to detect and image depletion regions in solar cell p-n junctions in real time. The EMP uses a microwave resonator operating around 10 GHz that is coupled to a thin wire probe. Unable to travel beyond the waveguide discontinuity, the microwave fields set up evanescent fields in the tip’s vicinity. When coupled to an object nearby, these evanescent fields are modified and change the resonant characteristics of the resonator. The microwave conductivity of the nearby object affects the extent of the modification of the probe’s output which is monitored as the probe is scanned over the object. Using these EMP scans, steady-state and transient expansions/contractions of the p-n junction’s depletion region under dc and pulsed reverse/forward biases are mapped. These experimental data along with the conductivity calibration of the EMP were then used to quantitatively calculate doping concentrations, diffusion lengths, and carrier recombination lifetimes in the junction. Junctions are one of the most crucial building blocks of semiconductor devices and these studies clearly show the ability of the EMP in quantitative and nondestructive evaluations of electronic devices and circuits.
Design and performance of a highly efficient mass spectrometer for molecular beams
Authors: DeKieviet M. , Dubbers D. , Klein M. , Pieles U. , Schmidt C.
Published: July 2002

We present a very efficient 90° sector magnet mass spectrometer detector, based on electron impact ionization, especially adapted for the use with atomic and molecular beams. In this setup, a cylindrically symmetric magnetic field compresses electrons emitted from a large filament into a long, narrow volume containing primarily the molecular beam. This has the advantage of both producing as little as possible background from residual gases and increasing the interaction region between beam and ionizer. Experimental tests using thermal He atoms show that the detector’s efficiency and sensitivity are very high, amounting to η⩾0.70±0.04% and S=0.22±0.03 A mbar−1, respectively. The unconventionally long ionization volume, however, may restrict the use of this mass spectrometer to experiments with no, or only moderate timing requirements.
Relaxation mechanism of Ge islands/Si(001) at low temperature
Authors: LeGoues F. K. , Tersoff J. , Reuter M. C. , Hammar M. , Tromp R.
Published: July 2002

Strained layers generally relax by dislocation glide. Here, using UHV-TEM, we study growth of Ge islands on Si(001) at ≲350 °C. We find that, although conventional relaxation (i.e., via glide of 60° dislocations) is suppressed, the islands grow relaxed from the outset, by direct incorporation of sessile 90° dislocations into the edge of the growing island. Paradoxically, the low-temperature islands are more fully relaxed than those grown at higher temperature.
Transmission electron microscopy of 〈100〉 dark line defects in CdZnSe quantum well structures
Authors: U’Ren G. D. , Haugen G. M. , Baude P. F. , Haase M. A. , Law K. K. , Miller T. J. , Wu B. J.
Published: July 2002

The initial degradation of II–VI light emitting devices, namely the 〈100〉 dark line defect formation in CdZnSe quantum well structures formed during laser or light emitting diode (LED) operation, has been investigated. Optically degraded quantum well structures exhibiting dark line defects were investigated via transmission electron microscopy. The observable dislocation networks have been determined to be conglomerations of dislocation loops confined to the quantum well region having an associated Burgers vectors of a[100] and a[010]. The discovery and identification of initial dark line defects may prove useful in identifying the defect mechanism in II–VI light emitters.
Epitaxial electro-optical SrxBa1−xNb2O6 films by single-source plasma-enhanced metalorganic chemical vapor deposition
Authors: Zhu L. D. , Zhao J. , Wang F. , Norris Peter E. , Fogarty G. D. , Steiner B. , Lu P. , Kear B. , Kang S. B. , Gallois B. , Sinclair M. , Dimos D. , Cronin-Golomb M.
Published: July 2002

SrxBa1−xNb2O6 films have been epitaxially grown on MgO substrates by a single-source plasma-enhanced chemical vapor deposition (PE-CVD). Exceptionally high quality of the epitaxial films was observed as indicated by high-resolution synchrotron x-ray diffraction imaging. The films exhibit waveguiding behavior with values of refractive index and the linear electro-optic coefficient r51 close to those of bulk crystals.
Determination of the effective mass of GaN from infrared reflectivity and Hall effect
Authors: Perlin P. , Litwin-Staszewska E. , Suchanek B. , Knap W. , Camassel J. , Suski T. , Piotrzkowski R. , Grzegory I. , Porowski S. , Kaminska E. , Chervin J. C.
Published: July 2002

Infrared reflectivity and Hall effect measurements were performed on highly conducting n-type GaN (n≊6×1019 cm−3) bulk crystals grown by the high-pressure high-temperature method. Values of electron-plasma frequency and free-electron concentration were determined for each sample of the set of seven crystals. It enabled us to calculate the perpendicular effective mass of electrons in the wurtzite structure of GaN as m*=0.22±0.02 m0. Effects of nonparabolicity and a difference between parallel and perpendicular components of the effective mass are small and do not exceed the experimental error.
Laser irradiation effects on tunneling properties of n-type GaAs and InAs by scanning tunneling microscopy
Authors: Takahashi Takuji , Yoshita Masahiro
Published: July 2002

We have studied the effect of laser irradiation on the tunneling properties of n-type GaAs and InAs by scanning tunneling microscopy. We measured the changes of tip height between on and off cycles of the laser light at different wavelengths and laser power. The laser-induced tip-sample separation is observed to be very large in GaAs compared with InAs when above-band-gap laser light is used. This result can be explained in terms of the shrinkage of the surface depletion layers by generated photocarriers.
Ultrafast optical pump THz-probe spectroscopy on silicon
Authors: Zielbauer Jürgen , Wegener Martin
Published: July 2002

We investigate the dynamics of photoexcited carriers in silicon after interband excitation with optical femtosecond pulses by probing intraband transitions with time-delayed THz pulses. The experimental data at various temperatures are compared with numerical, self-consistent solutions of the intraband polarization and Maxwell’s equations. In addition, we deduce the carrier scattering rate from the comparison between experiment and theory.
75 Å GaN channel modulation doped field effect transistors
Authors: Burm Jinwook , Schaff William J. , Eastman Lester F. , Amano Hiroshi , Akasaki Isamu
Published: July 2002

A III–V nitride modulation doped field effect transistor (MODFET) layer structure grown by organometallic vapor phase epitaxy (OMVPE) on a sapphire substrate was employed for transistor fabrication. The MODFET layer structure contained a 75 Å GaN channel, 50 Å Al0.16Ga0.84N spacer, 20 Å Si doped charge supply layer, 130 Å Al0.16Ga0.84N barrier, and 60 Å Al0.06Ga0.94N cap layer. The thin channel (75 Å) was chosen to improve the carrier confinement in the channel. The fabricated MODFET’s had 0.25 μm long gates, and utilized a Au–Si alloy Ohmic metal and a Ti/Pd/Au gate metal. The measured transconductance was 40 mS/mm. From the microwave measurements on devices with 0.25 μm long gates, ft and fmax were determined to be 21.4 and 77.5 GHz, respectively.
Scanning tunneling microscopy of undoped GaAs/AlGaAs heterostructures under laser irradiation
Authors: Takahashi T. , Yoshita M. , Sakaki H.
Published: July 2002

Scanning tunneling microscopy (STM) of cleaved surfaces of undoped GaAs/AlGaAs heterostructures has been performed under laser irradiation, by which the tunneling current is strongly enhanced via photocarrier generation. Clear STM images are obtained with high contrast between the GaAs and the AlGaAs layers when the laser wavelength is 800 nm which creates photocarriers only in the GaAs regions. It is found that the STM tip height difference between the GaAs and the AlGaAs regions reaches as large as 25 nm in the constant-current mode.
Reciprocal-space analysis of photoluminescence and photoluminescence excitation spectra
Authors: Yoo S. D. , Aspnes D. E. , Rhee S. J. , Woo J. C.
Published: July 2002

We show that energy positions of features in photoluminescence and photoluminescence excitation (PLE) spectra can be obtained more accurately by Fourier transforming segments of these spectra and analyzing the resulting coefficients in reciprocal space than by using conventional real-space analysis. Fourier transform analysis is particularly advantageous where base line effects are significant, as in PLE.
Comparison of thermally oxidized metal–oxide–semiconductor interfaces on 4H and 6H polytypes of silicon carbide
Authors: Shenoy Jayarama N. , Cooper James A. , Melloch Michael R.
Published: July 2002

The 4H polytype of silicon carbide (SiC) has a wider band gap and higher electron mobility than either the 6H or 3C polytypes. We show here that similar oxidation rates and interfacial quality can be obtained on 4H-SiC and 6H-SiC by thermal oxidation. This makes the 4H polytype an attractive choice for developing SiC power metal–oxide–semiconductor field effect transistors. Postoxidation annealing in helium increases fixed charge and interface state densities in both 4H and 6H metal–oxide–semiconductor capacitors.
Polarized neutron reflectometry study of an exchange biased Fe3O4/NiO multilayer
Authors: Ball A. R. , Leenaers A. J. G. , van der Zaag P. J. , Shaw K. A. , Singer B. , Lind D. M. , Fredrikze H. , Rekveldt M.Th.
Published: July 2002

Polarized neutron reflectometry was performed on a [Fe3O4/NiO]x15 multilayer, molecular beam epitaxy grown on a (001) MgO substrate, in both exchange biased and nonexchange biased states. The Fe3O4 layers exhibit a depth-dependent magnetic profile characterized by a change in the magnetization near the Fe3O4/NiO interfaces. In the exchange biased state, measurements performed in the two saturated states of the magnetic hysteresis loop reveal magnetic differences in the Fe3O4, possibly due to the interfacial domain wall formation in the ferromagnetic layer.
Photodecay mechanisms in side chain nonlinear optical polymethacrylates
Authors: Vydra Jan , Beisinghoff Hanno , Tschudi Theo , Eich Manfred
Published: July 2002

The chemical and physical properties of photobleached nonlinear optical (NLO)-polymethacrylates were studied in order to investigate photobleaching mechanisms. We have shown by three key measurement techniques (infrared-spectroscopy, mass spectrometry, and gel permeation chromatography) that the predominant bleaching process is an irreversible decomposition of the NLO moieties accompanied by a broadening of the polymer molecular weight distribution. The quantum yield of the chromophore degradation determined by the evaluation of the bleaching kinetics fit is in agreement with typical data from comparable chromophores. It was found that bleaching causes an increase in surface roughness of NLO-polymer films, causing an increase in waveguide loss. Smoothing of the film surface by an annealing step leads to a lowering of bleaching induced waveguide loss.
High-volume production of laser-polarized 129Xe
Authors: Driehuys B. , Cates G. D. , Miron E. , Sauer K. , Walter D. K. , Happer W.
Published: July 2002

A method is described for producing several liters of nuclear spin polarized 129Xe gas via spin exchange with an optically pumped Rb vapor. We use a 140 W AlGaAs laser diode array whose broad spectral output is efficiently absorbed by employing ∼10 atm of 4He to pressure broaden the Rb D1 absorption profile. 129Xe is polarized in a continuous gas flow and is then cryogenically accumulated and stored. Extensions of this technique should enable the production of tens of liters of 129Xe with a nuclear spin polarization of order 50%. Production of laser-polarized 129Xe in liter quantities is important for the continued development of magnetic resonance imaging using spin-polarized 129Xe.
Excimer laser crystallization and doping of source and drain regions in high quality amorphous silicon thin film transistors
Authors: Al-Nuaimy E. A. , Marshall J. M.
Published: July 2002

A method has been proposed to fabricate a bottom gate hydrogenated amorphous silicon thin film transistor with highly conductive polycrystalline silicon source and drain regions. Dopant atoms were supplied during selective crystallization of the amorphous silicon layer, via a spin-on-dopant film coated on top of it. A suitably patterned thin layer of silicon nitride served to restrict laser-induced crystallization and doping of the amorphous silicon to the required source and drain regions. The sheet resistance of the resulting doped polycrystalline areas of the film was found to be 1.8 kΩ/⧠. The fabricated transistors had a room temperature field effect mobility of up to 1.35 cm2 V−1 s−1 and featured on/off current ratios of more than 106.
A simple and robust electron beam source from carbon nanotubes
Authors: Collins Philip G. , Zettl A.
Published: July 2002

A simple fabrication method is described to produce reliable and robust (self-repairing) field emission sources from randomly aligned carbon nanotubes. The sources display stable and reproducible current–voltage characteristics with sharp turn on near 100 V and emission currents up to 400 mA/cm2 at 200 V. Two successful methods are described for patterning the emission source area into emitting and nonemitting regions.
Solid phase immiscibility in GaInN
Authors: Ho I-hsiu , Stringfellow G. B.
Published: July 2002

The large difference in interatomic spacing between GaN and InN is found to give rise to a solid phase miscibility gap. The temperature dependence of the binodal and spinodal lines in the Ga1−xInxN system was calculated using a modified valence-force-field model where the lattice is allowed to relax beyond the first nearest neighbor. The strain energy is found to decrease until approximately the sixth nearest neighbor, but this approximation is suitable only in the dilute limit. Assuming a symmetric, regular-solutionlike composition dependence of the enthalpy of mixing yields an interaction parameter of 5.98 kcal/mole. At a typical growth temperature of 800 °C, the solubility of In in GaN is calculated to be less than 6%. The miscibility gap is expected to represent a significant problem for the epitaxial growth of these alloys.
Controlling filamentation in broad-area semiconductor lasers and amplifiers
Authors: Marciante John R. , Agrawal Govind P.
Published: July 2002

We show that with the introduction of a new pair of epitaxial layers, sandwiched between the active and cladding regions, self-defocusing can play an important role in stabilizing the lateral mode in broad-area semiconductor lasers and amplifiers. Under certain conditions, it can be used to eliminate filamentation and provide a nearly flat mode profile by adjusting the band gap of the new self-defocusing layers. We discuss the use of a strained multiple-quantum-well laser for producing such a stable lateral mode.
High temperature adduct formation of trimethylgallium and ammonia
Authors: Thon A. , Kuech T. F.
Published: July 2002

High temperature gas phase reactions between trimethylgallium (TMG) and ammonia were studied by means of in situ mass spectroscopy in an isothermal flow tube reactor. The temperature, pressure, and reaction time were chosen to emulate the gas phase environment typical of the metal–organic vapor phase epitaxy (MOVPE) of GaN. The main gas phase species is [(CH 3)2Ga:NH2]x, where most probably x=3, resulting from the very fast adduct formation followed by elimination of methane. The further gas phase decomposition of this species proceeds through the stepwise elimination of methane. These studies indicate that little TMG exists within the growth ambient under most MOVPE growth conditions. The further gas phase reaction of [(CH3)2Ga:NH 2]x may be responsible for the strong dependence of the MOVPE GaN growth rate and uniformity commonly observed.
Growth and characterization of (InSb)m(InP)n short period superlattices
Authors: Utzmeier T. , Armelles G. , Postigo P. A. , Briones F. , Castrillo P. , Sanz-Hervas A. , Aguilar M. , Abril E. J.
Published: July 2002

Short period superlattices of (InSb)m(InP)n were grown on semi-insulating (001) InP substrates by atomic layer molecular beam epitaxy. High resolution x-ray diffractometry was used to study the structural quality of the superlattices. Raman spectroscopy, in conjunction with theoretical calculations, provided information about intermixing at the interfaces.
Large photoinduced refractive index changes of a polymer containing photochromic norbornadiene groups
Authors: Kinoshita Koichi , Horie Kazuyuki , Morino Sin’ya , Nishikubo Tadatomi
Published: July 2002

We prepared a polymer containing pendant norbornadiene (NBD) moieties, P(MMA0.43-co-GMA0.57-PNCA), and measured absorbance and refractive index spectra before and after photoisomerization of norbornadiene moieties. Large photoinduced refractive index changes of ∼0.01 were obtained at 632.8 nm, the region far from the absorption band. A quantum yield of 0.50 was obtained for the photoisomerization of NBD moieties in this polymer. These values are sufficient to make efficient channel waveguides by photoisomerization.
Cutting of a thin epitaxial iron film into low dimensional structures by dislocation engineering
Authors: Ressier L. , Diaz J. , Peyrade J. P.
Published: July 2002

The “Atomic saw” method, initially developed for semiconductor heterostructures, has been successfully used to fabricate low dimensional iron structures. Dislocations, generated by plastic deformation, are used to cut a 2 nm iron film, epitaxially grown onto a (001) MgO substrate, into one dimensional iron structures (called “stripes”) or zero dimensional structures (called “boxes”). Atomic force microscopy observations of the created magnetic structures demonstrate the applicability of this simple method. A statistical analysis of these observations quantifies the distributions of the widths and the shifts of the created adjacent stripes and reveals that these two parameters can be controlled by the choice of the plastic strain.
Misfit dislocations generated from inhomogeneous sources and their critical thicknesses in a InGaAs/GaAs heterostructure grown by molecular beam epitaxy
Authors: Zou J. , Cockayne D. J. H. , Russell-Harriott J. J.
Published: July 2002

Misfit dislocations generated from inhomogeneous sources in InGaAs/GaAs strained-layer heterostructures grown by molecular beam epitaxy were investigated using transmission electron microscopy (TEM) with the lift-off technique. These inhomogeneous sources are suggested to be particulates. TEM investigations show that (1) the critical thicknesses of misfit dislocations generated from inhomogeneous sources are the same as those generated from preexisting threading dislocations, and (2) each inhomogeneous source may generate a number of misfit dislocations. These observations suggest that these inhomogeneous sources are a prime source of misfit dislocation generation.
Degenerate layer at GaN/sapphire interface: Influence on Hall-effect measurements
Authors: Look D. C. , Molnar R. J.
Published: July 2002

Temperature-dependent Hall-effect measurements in hydride vapor phase epitaxial GaN grown on sapphire can be well fitted over the temperature range 10–400 K by assuming a thin, degenerate n-type region at the GaN/sapphire interface. This degenerate interfacial region dominates the electrical properties below 30 K, but also significantly affects those properties even at 400 K, and can cause a second, deeper donor to falsely appear in the analysis. However, by using a two-layer Hall model, the bulk mobility and carrier concentration can be accurately ascertained.
Overdamped Josephson junctions with Nb/AlOx/Al/AlOx/Nb structure for integrated circuit application
Authors: Maezawa Masaaki , Shoji Akira
Published: July 2002

We present characteristics of overdamped Josephson junctions consisting of Nb/AlOx/Al/ AlOx/Nb structures. The junctions were fabricated using a well-developed Nb/AlOx/Nb-junction technology and showed well-defined Josephson characteristics at 4.2 K. The characteristic voltage Vc [the product of the critical current Ic and the effective normal resistance Rn(eff)] of junctions, which determines high-frequency performance of the junction, was in the range of 0.1–0.5 mV, and the critical current density Jc in the range of 102–104 A/cm2. Maximum-to-minimum variations in Ic over a wafer were ±4% for junctions with Vc=0.15 mV and ±13% for junctions with Vc=0.5 mV.
Efficient defect passivation by hot-wire hydrogenation
Authors: Plieninger R. , Wanka H. N. , Kühnle J. , Werner J. H.
Published: July 2002

Atomic hydrogen, produced at a hot wire, passivates bulk defects in polycrystalline silicon without damaging surface regions. Solar cells from such polycrystalline silicon respond much more favorably to hot-wire hydrogenation than to low-energy ion implantation or a direct-current plasma treatment. Hot-wire passivation yields a hydrogen concentration close to the surface of 8×1019 cm−3 and improves the minority carrier diffusion length of solar cells by up to 100%. Implantation as well as conventional plasma treatment result in lower hydrogen concentration and, consequently, in much smaller improvements of diffusion lengths.
Comparison of photorefractive damage effects in LiNbO3, LiTaO3, and Ba1−xSrxTiyNb2−yO6 optical waveguides at 488 nm wavelength
Authors: Eknoyan O. , Taylor H. F. , Matous W. , Ottinger T. , Neurgaonkar R. R.
Published: July 2002

Photorefractive-induced damage effects at 488 nm wavelength in LiNbO3, LiTaO3, and Ba1−xSrxTiyNb2−yO6 (BSTN) optical waveguides are compared for the extraordinary mode of polarization. The damage effects are strongest in LiNbO3 and weakest in BSTN. At an input power density of 3000 W/cm2, the output power from passive channel waveguide in LiNbO3 is more than 12 dB lower than in BSTN. Optically induced index changes measured using a Fabry–Perot interferometric method show a nonlinear behavior with increasing input intensity for all these materials. For incident power densities greater than 200 W/cm2 on the waveguides, the optically induced index-change damage increases at a rate of 2.8×10−3 cm2/μW in LiNbO3, 4.9×10−4 cm2/μW in LiTaO3, and 3.1×10−4 cm2/μW in BSTN.