bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion source test stand for radioactive beams (abstract)
Authors: Nolen J. A. , Decrock P. , Portillo M. , Mullen T. P. , Geraci A. A. , Barlow T. A. , Greene J. P. , Gomes I. , Batson C. H. , Saremba S. E.
Published: July 2002

A test stand for development of ion sources for radioactive beams is currently being commissioned at Argonne. It is located at the Physics Division’s Dynamitron accelerator which will be used as a neutron generator with a flux of up to 1011 neutrons per second created by reactions of 4 MeV deuterons on various targets with beam currents of up to 100 μA. The primary targets will be located adjacent to heated secondary targets inside an on-line ion source. With this neutron-generator facility it will be possible to produce radioactive beams of various isotopes, such as He6, Na24, and neutron-rich fission fragments. For example, with a secondary target of uranium carbide containing 25 g of natural or depleted uranium the yields of individual isotopes in the target will be about 107/s for isotopes such as Sn132, Xe140, and Cs142, near the peak of the fission distribution. The ion sources to be evaluated will be located within a shielded cave with walls consisting of 30 cm of steel plus 60 cm of concrete to attenuate the prompt neutron radiation by a factor of about 104. Secondary beams of radioactive fission fragments with intensities on the order of 106/s per isotope will be extracted in the 1+ charge state at energies of 20 keV and mass separated with a Danfysik mass separator. Light isotopes, such as He6 and Na24, can be produced via (n,α) and (n,p) reactions on appropriate target materials. Commissioning began with measurements of fission yields from primary targets of C, Be, BeO, and BN. A surface ionization source which is a variation of the one used in the TRISTAN on-line mass seperator facility at Brookhaven National Laboratory has been installed and tested with stable Rb and Cs beams. The isotope separator was also commissioned with these beams. The development program will include emittance measurements and source optimization, initially with stable beams, and target-delay-time and release-efficiency measurements for various target/secondary-beam systems.
Bernas ion source modifications for platinum and aluminum ion implantation (abstract)
Authors: Medulla C. , Raspagliesi M.
Published: July 2002

Technology has a continuous developmental push toward the semiconductor’s fabrication to find new solutions to improve its own production. Hence the request to implant platinum and aluminum using commercial ion implanters in scanning tunneling microscopy was born. Starting with a standard Bernas source, we modified the conventional operating mode, as follows, to adapt the requested ion implantation process on our machines. Modification for platinum implantation: Inside the arc chamber a platinum target is installed, which is electrically insulated from the source body and powered from the outside by a dedicated power supply. Using an argon plasma there are enough argon ions that are accelerated by the target negative electrical field to sputter platinum atoms; they are ionized and extracted together with argon ions. The mass analysis system is able to separate the platinum mass if the extraction voltage is not great (typically 50% of maximum range). The beam current achieved is in the range of some hundred microamperes. We can show some beam spectrum and some correlation “beam current—source parameter.” Modification for aluminum implantation: Inside the arc chamber an alumina target (aluminum trioxide) is installed. Using a gas fluoride plasma (typically silicon tetrafluoride—Si4) we etched the alumina at a high temperature. So there are many aluminum atoms to be ionized and extracted. Using a dedicated source body with no boron contamination, it is possible to have an aluminum beam pure enough for the requested applications. The beam current achieved is of several milliamperes with no energy limitations. We can show beam spectrum and source parameters. For both modifications we can also show some pictures.
New design of an ion source for accelerator mass spectrometry at the Lund Pelletron
Authors: Persson Per , Freimann Kim , Hellborg Ragnar , Håkansson Kjell , Skog Göran , Stenström Kristina
Published: July 2002

The requirements for negative ion sources designed for accelerator mass spectrometry (AMS) are, e.g., a high and stable beam current, a high efficiency, a low memory effect, and a quick system for switching between different samples. For the AMS program at the Lund 3 MV Pelletron tandem accelerator a new caesium sputtering source was constructed and installed a few years ago. Some new mechanical features of the source, as well as tests of how well the requirements are fulfilled are reported.
Application of one-dimensional Wentzel–Kramers–Brillouin approximation in microwave reflectometry of plasma density profiles
Authors: Bruskin L. G. , Mase A. , Tamano T. , Yatsu K.
Published: July 2002

The accuracy of the one-dimensional (1D) Wentzel–Kramers–Brillouin (WKB) profile reconstruction algorithm currently used in microwave reflectometry of fusion plasmas is investigated. Two experimental conditions beyond the applicability of the 1D WKB approximation are discussed: a plasma with steep density gradients and the case of strong divergence of the microwave beam. It is demonstrated, that the 1D procedure of profile reconstruction provides a fairly accurate estimate for both cases, despite the influence of abrupt density changes and multidimensional effects.
Differential phase shift interferometer for measuring axisymmetric gas distributions for high-power Z-pinch research
Authors: Weber B. V. , Stephanakis S. J. , Moosman B.
Published: July 2002

A sensitive interferometer [B. V. Weber and S. F. Fulghum, Rev. Sci. Instrum. 68, 1227 (1997)] with 10−5-wave phase resolution is used to measure gas distributions from axisymmetric, supersonic nozzles employed in high power Z-pinch experiments. The line-integrated gas density, N≡∫ndx, is measured as a function of time at one distance, z, from the nozzle and one distance, y, from the axis. The (y,z) region of interest is scanned shot to shot. The N(y) data are analyzed to find radial density distributions via Abel inversion: n(r)=−(1/π)∫r∞[(dN/dy)/y2−r2]dy. Error propagation based on the uncertainty in N results in larger relative uncertainties in n, especially near the axis. The precision may be improved using a modified version of this interferometer, where the reference beam is moved into the gas, a small distance (1–2 mm) from the scene beam. The phase difference is then a direct measurement of dN/dy, reducing the error introduced by numerically differentiating N. These two techniques are compared using two nozzles with different density distributions from DECADE Module 2 Z-pinch experiments.
Influence of interace and buffer layer on the structure of InAs/GaSb superlattices
Authors: Twigg M. E. , Bennett B. R. , Shanabrook B. V.
Published: July 2002

Using image processing algorithms based on nonlinear imaging theory, we have analyzed high-resolution transmission electron microscopy images of InAs/GaSb superlattices (SLs) grown by molecular beam epitaxy. Our analysis indicates that InSb-like interfaces have a roughness of 1 monolayer (ML), for a SL grown on a GaSb buffer layer. For GaAs-like interfaces, however, the interface roughness is found to be 2 MLs when the SL is grown on a GaSb buffer. For SLs grown on an InAs buffer, the roughness of GaAs-like interfaces (3 MLs) is also greater than that of InSb-like interfaces (2MLs). These results suggest two general observations. The first is that GaAs-like interfaces are rougher than InSb-like interfaces. This difference may be due to the high surface energy of GaAs as compared to InSb. The second observation is that interface roughness is greater for an InAs/GaSb SL grown on an InAs buffer layer than for the same SL grown on a GaSb buffer layer. This difference in interface roughness may be due to InAs SL layers being in tension when grown on a GaSb buffer layer, whereas GaSb SL layers are under compression when grown on an InAs buffer layer.
A simple instrument for ultraviolet-visible absorption spectrophotometry in high temperature molten salt media
Authors: Li Jianzhong , Dasgupta Purnendu K.
Published: July 2002

An inexpensive (<US$5000) spectrophotometer of simple design was constructed from commercially available components for absorption spectroscopic studies in molten chloride salts. The spectrometer consists of a replaceable fused silica cell of 9×9 mm square exterior cross-section housed in a temperature-controlled ceramic tubular furnace of 13 mm inner diameter. Light communication to and from the cell occurs via a pair of 1 mm core silica-on-silica optical fibers from diametrically opposite sides. The light source is a pulsed xenon flash lamp and detection is accomplished by a photodiode array spectrometer card housed in a personal computer. The system has been operated up to temperatures of 950 °C. The effective spectral range is 280–650 nm at 900 °C; higher wavelengths can be accessed at lower operating temperatures. A spectral snapshot can be acquired in as little as 1 ms. With 1 s integration time, the intrinsic system noise level is ∼2×10−4 absorbance units. The system is compact and energy efficient. Applications of the system are demonstrated with spectral studies of some metal chloride systems in an equimolar sodium and potassium chloride eutectic.
Efficient discharge pumping of a copper vapor laser using a cylindrical grid electrode
Authors: Ohzu Akira , Kato Masaaki , Maruyama Yoichiro
Published: July 2002

Experimental investigation of laser output characteristics of an elemental copper vapor laser using a cylindrical grid electrode has been carried out. The grid electrode, which is placed at the inside of a cathode and separated 2 cm away from a cathode, is used for increasing the output power by improving the discharge characteristics. When the grid electrode is coupled with a peaking capacitor, it is observed that the peak voltage (voltage hold-off) in the pulsed discharge wave forms increases by about 20%. Also, the initial part of the discharge current pulse is reduced. The laser average output power increased from 16 to 21 W. Additionally, when applying dc voltage (0–100 V) to the grid electrode, the output power increased by around 6%. The effect of employing the grid electrode and the mechanisms responsible for the improved characteristics are discussed.
Measurement of the beam size at the compact superconducting storage ring by the SR interferometer
Authors: Sakai I. , Yamamoto Y. , Mitsuhashi T. , Amano D. , Iwasaki H.
Published: July 2002

The beam size in the compact superconducting storage ring AURORA was measured using the SR interferometer developed by one of the present authors. The spatial coherence of the radiation beam in the visible range was derived from the visibility of the interferogram recorded by a CCD camera and its dependence on the spatial frequency has shown that the beam profile is described by a Gaussian. Analysis of the data has yielded the beam size to be 10.5 μm in the vertical direction, which is unexpectedly small for the storage ring having only a weak beam focusing system.
Development of three dimensional neutron tomography system and its applications
Authors: Sinha Amar
Published: July 2002

Though two dimensional (2D) neutron tomography has been used for a number of years for various industrial applications, its combination with volume visualization technique, which gives a three dimensional (3D) tomography image, has not received much attention. In order to exploit this powerful technique in the field of neutron imaging and highlight its potential applications, a 3D neutron computed tomography system has been developed for the visualization of the interior structure of the objects in their actual relative positions and orientations. The method involves first obtaining 2D neutron tomography images and then combining these images into a 3D volume data. Volume rendering techniques are used for visualization of such volume data. Industrial application of this technique in opening up an object layer by layer or cutting it at any particular angle without physical disassembly of the object is demonstrated with the help of some especially fabricated samples. Use of such a technique can overcome many of the limitations of conventional radiography or 2D neutron tomography. The experiments using this imaging setup have been conducted at a 400 KW swimming pool-type reactor APSARA situated at the Bhabha Atomic Research Centre, Bombay. The neutron flux used in these experiments is 1×106 n/cm2/s with a L/D ratio of 90 and neutron beam size of 15 cm×15 cm. A neutron imaging camera consisting of a LiF+ZnS6 based neutron scintillator, image intensifier and charge coupled device based detector has been used in these experiments. Two and three dimensional reconstruction software has been developed for interactive examination of the interior of various types of industrial objects. Experimental and theoretical methods used in this work are discussed and results of experiments conducted using the 3D reconstruction techniques are presented in the article. Potential use of this technique for various industrial applications is also discussed.
Automatization of a multipurpose astronomical spectrograph
Authors: del Rio J. Sanchez , Rojas A. Ruiz-Falco , Herpe G.
Published: July 2002

This article deals with the study and fabrication of a system for the automation of a multipurpose spectrograph used in Astronomy. This system allows the 19 movements of the instrument to run by using high-level software, a “master” board, and 19 “slave” boards. The Astrophysique Lineal et BIdimensional par RÉseaux Optiques (ALBIREO) spectrograph has three working modes and is installed in the Observatorio de Sierra Nevada, Granada, Spain. We describe a low-cost system realized for the automatization of a multifunction spectrograph. It allows the control of a large number of motors: 19 for Albireo; the theoretical limit is 32 768! Another advantage of this architecture based on “slave” boards is to supply each motor with exactly the current it needs to reach the required torque (reliability and repetitivity of motions are obtained when the motor torque is at least twice the resistant torque). Thus, we can avoid unnecessary overheating which would be detrimental to the material as well as to the local image quality (seeing). The reliability of the system is obtained by software filtering the commands and also by using optocoupled electronics to avoid interference. For a complex instrument, the high-level software must be user friendly. The efficiency of the present software in this respect has been appreciated since first being used on the telescope. This system may be of interest to laboratories and observatories with similar needs for automation of motions. The adoption of this system will be supported by the IAA Laboratory, Granada, Spain.
Infrared electroabsorption modulation in AlSb/InAs/AlGaSb/GaSb/AlSb stepped quantum wells grown by molecular beam epitaxy
Authors: Du Q. , Alperin J. , Wang W. I.
Published: July 2002

We report the demonstration of normal incidence infrared (3–5 μm) electroabsorption modulation utilizing the Stark effect to induce Γ-L transitions in asymmetrically stepped AlSb/InAs/Al0.4Ga0.6Sb/GaSb/AlSb quantum wells grown on a GaSb substrate by molecular beam epitaxy. The measurements were performed using a Fourier transform infrared spectrometer at 77 K. The largest infrared absorption at 5 μm with an absorption coefficient of 3200 cm−1 has been obtained at 14 V reverse bias. The results indicate the potential of this novel structure for application as a normal incidence infrared modulator.
Mapping of subsurface inhomogeneities in semiconductors using differential reflectance microscopy
Authors: Tann J. , Gal M.
Published: July 2002

We have developed a sensitive optical technique that allows two-dimensional mapping of subsurface inhomogeneities of semiconductors. Using this contactless, room temperature technique, which is based on differential reflectance spectroscopy, we have been able to generate relief maps which show the spatial distribution of damage/defects in a number of III–V compounds and Si.
Phenomenological model for giant magnetoresistance in lanthanum manganite
Authors: Núñez-Regueiro J. E. , Kadin A. M.
Published: July 2002

The electrical conductivity of doped lanthanum manganite exhibits activated semiconducting behavior at high temperatures, and a transition to metallic behavior at low temperatures in the ferromagnetic state. These two regimes are combined in a new phenomenological equation for the conductivity that uses a mean-field magnetization and physically reasonable fitting parameters. This allows us to account quantitatively for reports of giant negative magnetoresistance in single crystals and thin films. We present new data on sputtered epitaxial thin films of La1−xCaxMnO3, and analyze other recent reports in the literature. The physical basis for such an expression is discussed.
Nonlinear self-phase matching of optical second harmonic generation in lithium niobate
Authors: Orlov Sergei , Yariv Amnon , Segev Mordechai
Published: July 2002

We show that the nonlinear index perturbation due to light-induced photovoltaic space-charge field in LiNbO3 can give rise to self-phase matching of second harmonic generation. Increase of the conversion efficiency is accompanied by formation of stationary and nonstationary patterns in the spatial structure of the generated second harmonic. The space-charge field can be induced either by the initially non-phase-matched second harmonic or by an external seed.
Photoemission assignments of HxSiO4−x fragments at the Si/SiOx interface
Authors: Lee Sunghee , Banaszak Holl Mark M. , Hung Wei Hsiu , McFeely F. R.
Published: July 2002

Hydrogenation of the silicon/silicon oxide interface is shown to cause changes in every feature spectroscopically discernible by soft x-ray photoemission in addition to depinning the Fermi level. A new feature is observed to grow in at 3.6 eV with respect to bulk Si upon exposure to atomic hydrogen at the expense of interface states intermediate between 3.6 and 0 eV. Assignments of photoemission features are discussed in the context of previous model studies of the interface and issues of electronic versus chemical reversibility are discussed.
Nonlinear optical limiting of C60, platinum poly-yne, and tetrabenzporphyrin in the near infrared
Authors: Guha Shekhar , Roberts William T. , Ahn B. H.
Published: July 2002

Nonlinear optical limiting behavior of carbon 60 (C60), platinum poly-yne (PPY), and tetrabenzporphyrin (TBP) was evaluated using lasers with μs and ns pulse widths at the wavelength of 750 nm in an f/5 optical setup. PPY had the lowest clamping level for ns pulses, and TBP had the lowest clamping level for μs pulses. Possible explanations for nonlinear optical behavior are provided.
Dielectric function of biaxially strained silicon layer
Authors: Lee Hosun , Jones E. D.
Published: July 2002

Dielectric functions of strained silicon layers grown on very thick, relaxed Si1−xGex (x=0.115, 0.3, and 0.37) layers are presented. The effect of tetragonal biaxial strain to the dielectric function is observed to be large near the E1 gap region. We compare the strain-induced change in the dielectric function of silicon layers with that of SiGe layers and the elasto-optical (or piezo-optical) constants of silicon which were measured from bulk Si under uniaxial compressive stress.
Organic light-emitting diodes with radio frequency sputter-deposited electron injecting electrodes
Authors: Suzuki Hiroyuki , Hikita Makoto
Published: July 2002

We report a successful fabrication of organic light-emitting diodes (LEDs) with electron injecting electrodes prepared by the radio frequency (rf) magnetron sputtering technique. These LEDs consist of a poly(methylphenylsilane) and a tris-(8-hydroxyquinoline) aluminum (Alq3) layer successively formed on an indium–tin–oxide coated glass, and either an aluminum or magnesium electron injecting electrode. Substantial improvements were observed in such device characteristics as intensity and durability of electroluminescence for LEDs with an rf-sputter-deposited electrode as compared to those with a vacuum-heat-deposited electrode.
Surface Fermi level position of hydrogen passivated Si(111) surfaces
Authors: Miyazaki S. , Schäfer J. , Ristein J. , Ley L.
Published: July 2002

Core and valence band photoemission data of hydrogen passivated Si(111):H surfaces yield surface Fermi level positions that are indicative of a near-surface depletion layer for n- as well as p-type samples. The bulk Fermi level positions are attained after annealing at ∼400 °C. These observations are explained in terms of a hydrogen induced passivation of donors and acceptors in a surface layer of the order of a μm as a result of the wet-chemical etching procedure.
Surface structure of 3C-SiC(111) grown on Si(111) surface by C60 precursor
Authors: Hu C.-W. , Kasuya A. , Suto S. , Wawro A. , Nishina Y.
Published: July 2002

The surface structure of cubic 3C-SiC(111) films prepared by thermal reaction of a Si(111) substrate with C60 molecules has been studied by combined in situ measurements of scanning tunneling microscopy and high resolution electron energy loss spectroscopy (HREELS-STM). The (2×n) surface reconstructions such as (2×2), (2×3) were observed under low reaction temperatures (≤900 °C), and the Si-terminated SiC(111)-(3×3) was obtained by annealing the sample at higher temperatures (∼1100 °C). Optical surface phonon energies of 113±2 meV for SiC prepared at low temperatures and 116±2 meV for the films with (3×3) surface reconstruction were measured. The diffusivity of Si atoms from the substrate through the SiC film at various temperatures is suggested as the reason for the formation of different surface reconstructions of the SiC.
Photoconductive switches for time-resolved transport measurements at low temperatures and high magnetic fields
Authors: Ernst G. , Haug R. J. , Klingenstein M. , Kuhl J. , von Klitzing K. , Eberl K.
Published: July 2002

Metal-semiconductor-metal photodiodes were optimized as pulse generators for time-resolved electrical measurements at low temperatures and high magnetic fields, using a laser diode as the light source. For the photoconductor, a heterostructure consisting of high-quality GaAs and low-temperature grown GaAs was used. The dependence of the photocurrent pulses on the applied magnetic field was determined. A two-dimensional electron gas in a magnetic field served as the device under test for the time-resolved measurements. We observed a splitting of the incident pulse into two transmitted pulses resulting from modal dispersion in the two-dimensional electron gas.
Contact imaging in the atomic force microscope using a higher order flexural mode combined with a new sensor
Authors: Minne S. C. , Manalis S. R. , Atalar A. , Quate C. F.
Published: July 2002

Using an atomic force microscope (AFM) with a silicon cantilever partially covered with a layer of zinc oxide (ZnO), we have imaged in the constant force mode by employing the ZnO as both a sensor and actuator. The cantilever deflection is determined by driving the ZnO at the second mechanical resonance while the tip is in contact with the sample. As the tip-sample force varies, the mechanical boundary condition of the oscillating cantilever is altered, and the ZnO electrical admittance is changed. Constant force is obtained by offsetting the ZnO drive so that the admittance remains constant. We have also used the ZnO as an actuator and sensor for imaging in the intermittent contact mode. In both modes, images produced by using the ZnO as a sensor are compared to images acquired with a piezoresistive sensor.
Evidence of two-species nucleation of InAs islands on (100) and (111)B GaAs substrates
Authors: Welser R. E. , Guido L. J.
Published: July 2002

The nucleation of InAs islands on (100) and (111)B GaAs substrates during metalorganic chemical vapor deposition has been investigated via scanning electron microscopy. The measured values of island density are 4–5 orders of magnitude smaller than those predicted using conventional one-species nucleation theory. In addition, the effect of an increase in arsine flow on island density is opposite in sense for the two substrate orientations. To explain these observations, we further develop a two-species picture of nucleation—originally proposed by Stoyanov [Appl. Phys. A 50, 349 (1990)]—in which the InAs island density is an explicit function of both the indium flux and the arsenic partial pressure at the growth interface. This more realistic physical model is in good agreement with the complete set of experimental data.
Fabrication of 15 nm wide trenches in Si by vacuum scanning tunneling microscope lithography of an organosilane self-assembled film and reactive ion etching
Authors: Perkins F. Keith , Dobisz Elizabeth A. , Brandow Susan L. , Calvert Jeffrey M. , Kosakowski John E. , Marrian Christie R. K.
Published: July 2002

Organosilane precursor molecules, here (aminoethylaminomethyl)phenethyltrimethoxysilane, or PEDA, are chemisorbed onto a Si surface forming a monolayer thick film. These films are patterned using the scanning tunneling microscope to locally modify the chemical reactivity and are then used as a template for selective electroless plating of a thin Ni film. Reactive ion etching in a C2F6/O2 mixture transfers this pattern into the substrate. Improvement over previous lithographic performance is achieved by growing the films on a passivated and lightly oxidized Si surface, optimizing the patterning conditions, and improving the metallization and etch chemistry. In this way, we have generated deep trenches on the order of 15±4 nm width, with edge roughness of 3 nm. We believe this demonstrates the resolution limiting factors of this lithographic process.
Thermal stability of dry etch damage in SiC
Authors: Pearton S. J. , Lee J. W. , Grow J. M. , Bhaskaran M. , Ren F.
Published: July 2002

The introduction of dry etch damage into n-type SiC has been measured by monitoring the sheet resistance after exposure to Ar plasmas under both reactive ion etching and electron cyclotron resonance conditions. The threshold rf powers for measurable resistance changes in 1 μm thick films are ∼250 W for reactive ion etching (RIE) conditions, and ∼150 W for electron cyclotron resonance (ECR) conditions. A major annealing stage occurs with an activation energy of ∼3.4 eV, but some damage remains even after 1050 °C annealing.
The role of ambient gas scattering effect and lead oxide formation in pulsed laser deposition of lead–zirconate–titanate thin films
Authors: Ma C. S. , Hau S. K. , Wong K. H. , Chan P. W. , Choy C. L.
Published: July 2002

The angular distribution of lead in films deposited by pulsed laser irradiation of lead–zirconate–titanate and lead targets are studied as a function of ambient gas (argon or oxygen), gas pressure, and substrate temperature. When the substrate is kept in vacuum and at room temperature, a dip in the lead content attributable to the intrinsic resputtering of lead is observed at the position of the target surface normal. In the presence of an ambient gas, the dip disappears and the lead content increases at all angles. These results are attributed to a reduction of resputtering arising from scattering of the ablated species by ambient gas molecules. Under ambient oxygen and at high substrate temperature, the retention of lead content in the deposited films is largely due to the formation of lead oxide.
Photovoltaic effect in PbS/PbTiO3/Si heterostructures
Authors: Pintilie L. , Alexe M. , Pintilie I. , Botila T.
Published: July 2002

PbS/PbTiO3/Si heterostructures were prepared by successive depositions of a PbTiO3 thin film using the sol-gel method and of a PbS thin film using the coprecipitation method on a bulk silicon wafer. The current–voltage characteristic of the heterostructure is of a diode type. The heterostructure is sensitive over a broad range of wavelengths, from 0.35 to 3 μm, when illuminated with continuous light or with modulated light. Three sensitivity peaks are observed, at 2.7, 0.95, and 0.38 μm. The measured signals are attributed to a photovoltaic effect which appears in the studied heterostructure.
Surface morphology of homoepitaxial silicon thin films grown using energetic supersonic jets of disilane
Authors: Pacheco K. A. , Ferguson B. A. , Banerjee S. , Mullins C. B.
Published: July 2002

The effect of different chemical mechanisms for adsorption, which depend on the incidence energy of disilane, on the film morphology is investigated by comparing deposition by high (∼2 eV) kinetic energy disilane jets, direct chemisorption; low (∼0.09 eV) kinetic energy disilane jets and ultra high vacuum chemical vapor deposition, trapping-mediated chemisorption. For substrate temperatures of 500–550 °C the mechanism for adsorption does not influence the film morphology as observed for films up to 3300 Å thick, which are comparable in smoothness to the starting substrate, as determined by atomic force microscopy.
Properties of DX center in Te-doped In1−xG axAsyP1−y/GaAs0.61 P/d0.39
Authors: Jeon Byung-Deuk , Kwon Ho Ki , Choe Byung-Doo
Published: July 2002

We report that the Te donor forms a deep level and acts as a DX center in In1−xGaxAsyP1−y/GaAs0.61P0.39. The characteristics of Te-related deep level is investigated by deep-level transient spectroscopy and thermally stimulated capacitance measurements. The deep level has a thermal activation energy of 0.14±0.01 eV regardless of the alloy composition of In1−xGaxAsyP1−y. Persistent photoconductivity is observed at low temperature in all Te-doped In1−xGaxAsyP1−y/GaAs0.61P0.39 samples. The compositional dependence of deep trap density is explained by the energy difference between the conduction band minimum and the DX level.
Low-temperature synthesis of Ge nanocrystals in zeolite Y
Authors: Miguez H. , Fornés V. , Meseguer F. , Marquez F. , Lopez C.
Published: July 2002

A method for obtaining Ge nanocrystals has been developed using the porous structure of zeolite Y as a matrix. Samples were obtained by reduction of GeO2 nanocrystals, previously grown in zeolite α cages, in H2 at 470 °C. This is apparently the lowest GeO2 reduction temperature reported and is probably due to the influence of the host. Ge crystallites so formed present an absorption edge at 2.2 eV, which is shifted ∼1.4 eV with respect to the bulk value. A softening and an asymmetric broadening of the Raman peak were also observed. We can estimate an average diameter of 3 nm for the Ge nanocrystals from both results.
Interface states at ultrathin oxide/Si(111) interfaces obtained from x-ray photoelectron spectroscopy measurements under biases
Authors: Kobayashi H. , Yamashita Y. , Nakato Y. , Komeda T. , Nishioka Y.
Published: July 2002

The energy distribution of interface states for Si(111)-based metal–oxide–semiconductor (MOS) devices with an ultrathin chemical oxide layer is obtained from measurements of x-ray photoelectron spectra under biases. All the observed interface states have discrete energy levels. The energy deviation of the interface state levels from the midgap is smaller than that for the Si(100)-based MOS devices; this is attributed to weaker interaction of the Si dangling bond with Si and oxygen atoms in the oxide layer. The densities of the interface states at the Si(111)/ultrathin oxide interface are comparable to those at the Si(100)/ultrathin oxide interface.
Below threshold conduction in a-Si:H thin film transistors with and without a silicon nitride passivating layer
Authors: Slade H. C. , Shur M. S. , Deane S. C. , Hack M.
Published: July 2002

We report temperature measurements of inverted staggered amorphous silicon thin film transistor subthreshold conductance for devices with and without a top silicon nitride passivating layer. Subthreshold conductance activation energies clearly show the different conductance paths in the active layer of these devices. Transistors with no top nitride layer conduct in the bulk amorphous silicon, whereas the devices with a top nitride layer conduct at the interface between the amorphous silicon and the top nitride (a ‘‘back’’ channel). Gate bias stressing and light soaking experiments uphold the existence of the back channel. We also present two-dimensional simulations that support our interpretation of the experimental data.
Mode behavior and high resolution spectra of circularly-symmetric GaAs-AlGaAs air-post vertical cavity surface emitting lasers
Authors: Epler J. E. , Gehrsitz S. , Gulden K. H. , Moser M. , Sigg H. C. , Lehmann H. W.
Published: July 2002

The mode behavior of symmetric air-post vertical cavity surface emitting lasers is investigated using a unique tandem triple-pass Fabry–Perot interferometer as well as standard characterization techniques. A series of high resolution spectra taken at various current levels demonstrates relaxation oscillations of the fundamental mode and the lifting of mode degeneracy by crystal birefringence and structural anisotropy. Near field images and L–I measurements identify the discontinuous jumps in mode number and polarization.
X-ray diffraction study of alternating nanocrystalline silicon/amorphous silicon multilayers
Authors: Wu X. L. , Tong S. , Liu X. N. , Bao X. M. , Jiang S. S. , Feng D. , Siu G. G.
Published: July 2002

Structural properties of alternating nanocrystalline silicon/amorphous silicon multilayers with visible light emission at room temperature were examined by means of x-ray diffraction. According to the linewidths and intensities of the diffraction peaks in the low- and high-angle ranges, we have determined the effective interface thickness, the mean crystallite sizes, and the internal strains, which are closely related to the photoluminescence in this material. In addition, the existence of the voids or holes was also observed, indicating that the improved electrical properties of this kind of hydrogenated nanocrystalline materials are due to the inhomogeneous structure of the material.
Raman study of the relationship between nanoparticles and photoluminescence in spark-processed silicon
Authors: Rupp S. , Quilty J. , Trodahl H. J. , Ludwig M. H. , Hummel R. E.
Published: July 2002

Spark-processed silicon (sp-Si) that strongly photoluminesces (PL) in the blue and green regions of the visible spectrum has been analyzed by Raman scattering, in order to probe a possible relationship between the presence and size of Si nanoparticles and PL emission wavelengths. Spatially resolved Raman spectra were measured across spark-processed regions, scanning areas which vary in PL intensity but not wavelength. The observed small shifts and broadenings of the Raman signals indicate the presence of Si particles having diameters of about 15 nm in the central, photoluminescing section of the sp region. Slightly smaller Si crystallites with sizes of about 8–9 nm were found in the halo region which does not contribute to PL. Furthermore, the Raman peak shifts and broadenings are essentially identical for blue and green photoluminescing sp-Si. These results suggest that the PL of sp-Si is not caused by a quantum-size effect which is coupled to the presence of Si nanoparticles.
Comparison of degradation caused by dislocation motion in compound semiconductor light-emitting devices
Authors: Sugiura Lisa
Published: July 2002

Dislocation glide velocities in GaAlAs/GaAs, InGaAsP/InP, and GaN-based light-emitting devices are estimated. These results are consistent with device degradation rates related to dislocation motion. It is clarified that the long lifetime of GaN-based devices with high dislocation density is principally due to extremely small dislocation mobility, partly due to small shear stress for dislocation motion, and due little to the radiation enhancement effect.
A novel approach to electrochromism in WO3 thin film using piezoelectric ceramics for power supply
Authors: Xu C. N. , Akiyama M. , Sun P. , Watanabe T.
Published: July 2002

Electrochromism was newly realized in a WO3–Pb(Zr0.52Ti0.48)O3 (PZT) system which utilized the piezoelectric property of PZT ceramics for power supply. The electric power produced by Mn-doped PZT ceramics enabled the WO3 film to color blue. High piezoelectricity with a peak voltage of 35 V and peak current of 1.2 mA on a 30 kΩ circuit was observed at a pressure of 30 MPa for the present system. This study shows the possibility to actuate the electrochromic WO3 film with PZT ceramics.
Stimulated far-infrared emission from copper-doped germanium crystals
Authors: Sirmain G. , Reichertz L. A. , Dubon O. D. , Haller E. E. , Hansen W. L. , Bründermann E. , Linhart A. M. , Röser H. P.
Published: July 2002

We have detected stimulated far-infrared emission from copper-doped germanium single crystals. By varying the magnetic field between 1 and 2.3 T, we have achieved emission in the range of 70–120 cm−1. Laser action was observed for crystals with a copper acceptor concentration as high as 1.5×1015 cm−3, a doping level that is considerably higher than that of any single or double acceptor doped Ge laser. Stimulated emission from Ge crystals with such a high Cu concentration is possible because only a small fraction of the copper acceptors is ionized during operation.
Surface roughening and columnar growth of thin amorphous CuTi films
Authors: Geyer U. , von Hülsen U. , Thiyagarajan P.
Published: July 2002

Different growth stages and the microstructure of amorphous CuTi films are investigated by scanning tunneling microscopy and small angle neutron scattering. During film growth at room temperature, the initially smooth films show increasing surface roughening and finally a change to a columnar growth mode with column diameters of about 20 nm. The interfacial energies associated with the column boundaries are higher than those of grain boundaries in crystalline systems. The column boundaries might be the origin of high intrinsic tensile stresses measured before in the amorphous CuTi films.
Suppression of the increase of high-temperature coercivity in MnBi thin films by Al interlayers
Authors: Rüdiger U. , Roos T. , Güntherodt G. , Holländer B. , Fumagalli P.
Published: July 2002

By tailoring the microcrystalline structure of MnBi films, using Al interlayers, a reduction of the high-temperature coercivity by a factor of 3 is achieved. The separation of Bi/Mn bilayers by Al interlayers acts as a diffusion barrier perpendicular to the surface. After annealing, the MnBi layers contain single-domain particles surrounded by an Al matrix exhibiting no significant increase of the coercive field with increasing temperature.
A mechanism of the exchange anisotropy in Co1−xMnx/Mn bilayers: Interfacial influences
Authors: Zhao H. W. , Lu M. , Sui Y. X. , Zhai H. R. , Feng Duan , Chen Y. , Dong G. S. , Jin X. F.
Published: July 2002

The magnetic exchange anisotropy of a series of single crystal Co1−xMnx/Mn bilayers (0⩽x⩽ 0.37) is studied as a function of the Mn content. By means of ferromagnetic resonance, it is found that the exchange anisotropy shows up when 0.30⩽x⩽0.37 and disappears when x⩽0.20 at room temperature. The phenomenon of Mn enrichment is observed at the Co–Mn surface by the technique of varying angle x-ray photoelectron spectroscopy, which may lead to the formation of antiferromagnetic Mn-rich interface regions and, hence, results in the exchange interaction between the ferromagnetic Co1−xMnx film and the antiferromagnetic Mn-rich interface region.
Interface pinning in high Tc-high Jc Nd1+xBa2−xCu3Oy directionally solidified in air
Authors: Yu R. , Sandiumenge F. , Martı́nez B. , Vilalta N. , Obradors X.
Published: July 2002

Single domain bars of NdBa2Cu3O7/Nd4Ba2Cu2O10 (Nd123/422) superconducting composites have been fabricated in air by using a directional solidification technique. Tc values of 96.5 K can be obtained after an Ar heat treatment and further oxygenation thus showing that reduced oxygen partial pressure during solidification is not a fundamental requirement to prepare high-Tc Nd123 bulk superconductors. The temperature dependence of the critical currents point to a limited efficiency of the Nd123/422 interfaces to pin vortices. High resolution electron microscopy observations clearly reveal smooth Nd123/422 interfaces thus suggesting that interface pinning may not be as relevant as compared to Y123/211 composites.
Electron beam irradiation of Y1Ba2Cu3O7−x grain boundary Josephson junctions
Authors: Tafuri F. , Shokhor S. , Nadgorny B. , Gurvitch M. , Lombardi F. , Di Chiara A.
Published: July 2002

The properties of the Y1Ba2Cu3O7−x biepitaxial Josephson junctions were reproducibly modified by a focused electron beam irradiation of the interface region. The junctions were fabricated by depositing Y1Ba2Cu3O7−x thin film by cylindrical magnetron sputtering technique on the (110) SrTiO3 substrate, partially covered by a pregrown MgO seed layer. The junction parameters can be adjusted controllably by applying an appropriate dose. Electron irradiation decreased the critical current of the junctions IC and increased the normal state resistance times area to values of the order of 1(μ Ω cm2). Some other effects, such as the disappearance of the excess current, were also observed. The original properties of the junctions could be partly restored by isothermal annealing. We also speculate that some aspects of the nature of the grain boundary barriers can be better understood from the study of the properties of irradiated junctions.
Multiple optical paths in a self-seeding scheme for multiwavelength short pulse generation
Authors: Wang D. N. , Shu C.
Published: July 2002

A simultaneous generation of multiwavelength short pulses is demonstrated by self-injection seeding of a laser diode. The wavelengths are selected by the use of multiple optical paths in an external cavity consisting of a grating, a beam splitter, and two mirrors. By adjusting the grating and either one of the mirrors, the wavelengths and their spacings can be readily tuned in a flexible manner. The system is simple, robust, and is of low cost.
Titanium thin film growth on small and large misfit substrates
Authors: Huth M. , Flynn C. P.
Published: July 2002

The influence of lattice misfit on the growth of Ti (0001) is investigated in the limit of small negative (−1%) and large positive (+6.8%) misfit by choosing MgO (111) and Al2O3 (0001) as substrate materials. Reflection high energy electron diffraction imaging and intensity measurements during growth reveal two-dimensional nucleation of islands on MgO, in contrast to three-dimensional nucleation on Al2O3. X-ray analysis of 30-nm-thick films on MgO shows a two-component line shape in transverse scans of the (0002) and (0004) reflections, pointing to a high degree of structural coherence in the weak disorder limit. The surface morphology of films grown on MgO depends strongly on the substrate temperature during growth.
Avalanche breakdown mechanism originating from Γ–X–Γ transfer in GaAs/AlAs superlattices
Authors: Hosoda M. , Tominaga K. , Ohtani N. , Kuroyanagi K. , Egami N. , Mimura H. , Kawashima K. , Fujiwara K.
Published: July 2002

An avalanche breakdown mechanism in GaAs/AlAs type-I superlattices is demonstrated. This mechanism shows its power at a bias voltage where both of the following two conditions are met. One is electron transfer from the Γ ground state to the X ground state (Γ1-X1), and the other is the escape of electrons from the X1 state to the second Γ state (X1-Γ2). Under both conditions, because the AlAs barriers become transparent for electron transport due to the Γ1-X1-Γ2 path, the drift speed (i.e., the acceleration of electrons) grows, and then the superlattice shows the phenomenon of avalanche breakdown. From our experimental results for various GaAs/AlAs superlattices, it is thought that such avalanche breakdown frequently occurs when type-I GaAs/AlAs superlattices have thick barrier widths.
Demonstrated low radiative loss of a quadrupole ultrashort electrical pulse propagated on a three strip coplanar transmission line
Authors: McGowan R. W. , Grischkowsky D. , Misewich J. A.
Published: July 2002

We present an approach for reducing the absorption and dispersion due to Cherenkov radiation on coplanar transmission lines, which does not require the reduction of the permittivity mismatch between the substrate and the superstrate of the optoelectronic chip. Exciting only the odd mode of a three strip coplanar transmission line, a quadrupole ultrashort electrical pulse is generated and propagates with very low radiative loss over long distances (>5 mm). The quadrupole’s measured frequency dependent absorption and dispersion, are compared to those measured for the even mode (dipole) pulse on the same transmission line. The radiative loss for the quadrupole pulse is shown to be <1/10 that of the dipole pulse, and for equal transverse dimensions it is <1/10 the radiative loss of the two strip transmission line and of the two slot coplanar waveguide.
Temperature independent performance of organic semiconductor lasers
Authors: Kozlov V. G. , Bulović V. , Forrest S. R.
Published: July 2002

Characteristics of lasing in optically pumped, vacuum deposited organic semiconductor thin films of tris-(8-hydroxyquinoline) aluminum doped with DCM laser dye are studied as a function of DCM concentration and ambient temperature. In contrast to inorganic laser diodes, the lasing wavelength, output power, differential quantum efficiency, and threshold pump energy of organic lasers are found to be independent of temperature in the range from 0 to 140 °C. In addition, no degradation in laser performance was observed at temperatures approaching 160 °C.
Evolution of defect-related structure in the x-ray absorption spectra of buried SiNx films
Authors: Paloura E. C.
Published: July 2002

Near-edge x-ray absorption fine structure measurements at the N-K edge are used to monitor the evolution of defect-related structure in the spectra of buried SiNx films as a function of the implantation dose. The buried SiNx films were fabricated with implantation of 35 keV N14+ ions in Si in the dose range 2×1017–2×1018 cm−2. The defect-related resonances RL1 and RL2 appear at 401.1±0.3 and 403.3±0.1 eV, respectively. The RL1 is characteristic of a defect structure in the low and intermediate implantation doses and can be annealed out with an activation energy of 0.5 eV. RL2, which is the signature of excess N in N-rich films and is attributed to transitions of 1s-electrons to unfilled states with p component at a defect site containing a N dangling bond, can be annealed out only after prolonged annealing at 1150 °C.
Josephson effect in Nb/two-dimensional electron gas structures using a pseudomorphic InxGa1−xAs/InP heterostructure
Authors: Schäpers Th. , Kaluza A. , Neurohr K. , Malindretos J. , Crecelius G. , van der Hart A. , Hardtdegen H. , Lüth H.
Published: July 2002

Superconducting Nb–In0.53Ga0.47As/In0.77Ga0.23As/InP–Nb contacts with an electrode separation of 450 nm were fabricated. Due to the high mobility of the two-dimensional electron gas, the current transport can be described within the clean limit regime. At 0.3 K a critical current of 3.8 μA was obtained for 6 μm wide contacts leading to a characteristic voltage of 190 μV. The decrease of the critical current with increasing temperature can be explained by a theory developed by A. Chrestin, T. Matsuyama, and U. Merkt [Phys. Rev. B 49, 498 (1994)], which takes δ-shaped barriers at the superconductor/semiconductor interfaces into account.
Enhancement of nonlinear optical response due to cavity-induced double resonance in a semiconductor ultrathin film
Authors: Ishihara Hajime , Cho Kikuo
Published: July 2002

A mechanism for a large enhancement of nonlinear response due to microcavity effect is theoretically proposed for excitons coupled strongly with radiation field. For an appropriate choice of microcavity parameters, the energy of a coupled mode of cavity and exciton coincides with the transition energy between exciton and biexciton states. This scheme leads to a remarkable enhancement of a degenerate pump-probe signal due to the simultaneous enhancement of the internal field and nonlinear susceptibility.