bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication of ultrasmall Nb-AlOx-Nb Josephson tunnel junctions
Authors: Martinis John M. , Ono R. H.
Published: July 2002

We describe a fabrication process to make Nb-AlOx-Nb edge junctions with areas as small as 0.0022 μm2 and with current densities from 10 to 24 000 A/cm2. The junction conductance was low for voltages below the superconducting energy gap which indicates good quality tunnel barriers. Coulomb gap measurements obtained when the junctions were in the normal state were used to find the junction capacitance. Junction capacitance as small as 0.18 fF has been measured.
Atomic hydrogen concentration profiles at filaments used for chemical vapor deposition of diamond
Authors: Schäfer L. , Klages C.-P. , Meier U. , Kohse-Höinghaus K.
Published: July 2002

The quantitative determination of atomic hydrogen concentrations cH in the vicinity of hot filaments is performed with two-photon laser-induced fluorescence. The measurements yield atomic hydrogen concentration profiles up to 28 mm from the filament surface with a spatial resolution of about 0.5 mm. The nonequilibrium nature of the hydrogen dissociation on the filament surface results in a saturation of hydrogen concentration profiles cH(r) for gas pressures above 10 mbar. Atomic concentrations in immediate vicinity of the filament are significantly lower than expected from thermodynamical calculations and depend on the filament diameter. Addition of methane results in a decrease of cH by more than 30% near the filament and a steeper cH(r) dependence, demonstrating the accelerated consumption of H atoms by the presence of hydrocarbon species. H concentration profiles for Ta, Ir, and W filaments show a dependence on filament materials which might be taken into account when selecting filament materials for chemical vapor deposition of diamond.
Polarization and field dependent two-photon absorption in GaAs/AlGaAs multiquantum well waveguides in the half-band gap spectral region
Authors: Tsang H. K. , Penty R. V. , White I. H. , Grant R. S. , Sibbett W. , Soole J. B. D. , LeBlanc H. P. , Andreadakis N. C. , Colas E. , Kim M. S.
Published: July 2002

We report the observation of two photon absorption which is strongly dependent on the applied electric field and the optical polarization. At 1.55 μm wavelength, the two-photon absorption coefficient of the GaAs/AlGaAs multiquantum well (MQW) waveguides for transverse-magnetic light is about seven times lower than for transverse-electric polarized light and changes by a factor of approximately 4 for a change in applied direct-current electric field of ∼140 kV/cm. Ultrafast nonlinear refraction causing phase changes of over π radians without appreciable excess loss is observed. These measurements demonstrate that GaAs/AlGaAs MQW waveguides could be successfully used for subpicosecond all-optical switching near half-band gap, at wavelengths corresponding to the 1.55 μm optical communications band.
Subpicosecond carrier lifetime in GaAs grown by molecular beam epitaxy at low temperatures
Authors: Gupta S. , Frankel M. Y. , Valdmanis J. A. , Whitaker J. F. , Mourou G. A. , Smith F. W. , Calawa A. R.
Published: July 2002

Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time-resolved-reflectance techniques, a sub-picosecond (<0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at ∼200°C and annealed at 600 °C. With the same material as a photoconductive switch we have measured electrical pulses with a full-width at half-maximum of 0.6 ps using the technique of electro-optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of ∼120–150 cm2/V s. GaAs grown by MBE at 200 °C and annealed at 600 °C is also semi-insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of subpicosecond photoconductive applications.
Single-frequency continuous-wave operation of ring resonator diode lasers
Authors: Hohimer J. P. , Craft D. C. , Hadley G. R. , Vawter G. A. , Warren M. E.
Published: July 2002

We report continuous-wave room-temperature operation of a semiconductor ring resonator diode laser with a single Y-junction outcoupling waveguide. This device with a 150 μm radius and 8-μm-wide etched-rib waveguide has a threshold current of 72 mA and emits up to 1 mW of single-frequency output. The side-mode-rejection ratio of this laser exceeds 22 dBm over a ≳50 mA current range corresponding to a stable operating mode of the laser. Studies of the emission behavior in this and other similar ring lasers suggest that the ring is lasing in ‘‘whispering gallery’’ modes, and also that the Y junction is affecting mode selection in the ring.
Ambient temperature diodes and field-effect transistors in InSb/In1−xAlxSb
Authors: Ashley T. , Dean A. B. , Elliott C. T. , McConville C. F. , Pryce G. J. , Whitehouse C. R.
Published: July 2002

InSb and related narrow-gap alloys have many potential applications in addition to the conventional one of infrared detection, provided that ambient temperature operation can be achieved. We report experimental results on multilayer InSb/In1−xAlxSb structures utilizing minority-carrier exclusion and extraction. At room temperature, diodes have R0A values several orders higher than homostructure InSb devices. Negative differential resistance associated with Auger suppression is observed under reverse bias. Enhancement-mode metal-insulator-semiconductor field-effect transistors have near classical output characteristics at 294 K, with a typical transconductance of 34 mS/mm and dynamic range of 23 dB.
High-rate, anisotropic dry etching of InP in HI-based discharges
Authors: Pearton S. J. , Chakrabarti U. K. , Katz A. , Ren F. , Fullowan T. R.
Published: July 2002

Electron cyclotron resonance HI/H2Ar discharges with additional rf-induced dc biasing of the sample have been used to obtain extremely anisotropic dry etching of InP. At a fixed ratio of 10 HI/10 H2/5 Ar (total flow rate 25 sccm) and 1 mTorr pressure, both n+ and p+ InP have etch rates of ∼875 Å×min−1 at −100 V bias and ∼3000 Å min−1 at −400 V bias. The etch rates increase rapidly with total discharge pressure, reaching 4000 Å min−1 at 20 mTorr and −100 V dc bias. Rates in excess of 1 μm min−1 are obtained with higher HI flow rates or higher biases. Features 0.5 μm wide and 13 μm high have been etched, demonstrating the promise of this gas chemistry for production of laser mesas on InP and related materials with substantially faster etch rates (typically a factor of 8–10) relative to the more conventional CH4/H2 mixtures. The etched surfaces are smooth, with no evidence for iodine-containing residues or preferential loss of either In or P. Both photoresist and SiO2 masks show minimal erosion in this mixture because of the ability to obtain practical etch rates at low pressure and low self-bias.
Low-temperature dc characteristics of S- and Si-doped Ga0.51In0.49P/GaAs high electron mobility transistors grown by metalorganic molecular beam epitaxy
Authors: Ginoudi A. , Paloura E. C. , Kostandinidis G. , Kiriakidis G. , Maurel Ph. , Garcia J. C. , Christou A.
Published: July 2002

Ga0.51In0.49P/GaAs high electron mobility transistors (HEMTs) grown by metalorganic molecular beam epitaxy have been fabricated for the first time. The typical transconductance (gm) of devices of 1.3-μm gate length at 300 K is 110 mS/mm and is independent of donor type. At 100 K the dc characteristics of Si-doped devices remain almost unchanged, while there is a decrease of 55% in gm and in the drain-source saturation current (Idss) of the S-doped devices. The degradation of the S-doped HEMTs is attributed to ‘‘DX-like’’ centers in the doped GaInP layer. All of the doped samples are characterized by a deep electron trap with an activation energy that takes values in the range 310–345 meV and causes persistent photoconductivity (PPC) in S-doped samples, while Si doping suppresses the PPC effect.
Laser-scattering measurement of temperature profiles of a free-burning arc
Authors: Murphy A. B. , Farmer A. J. D. , Haidar J.
Published: July 2002

A method of determining the temperature of a thermal plasma by laser scattering, without frequency resolution of the scattered signal, is introduced. The method gives valid measurements of the gas temperature across the complete diameter of a free-burning argon arc, even in regions in which local thermodynamic equilibrium does not exist. The results show that the maximum temperature in a 100 A arc, with a 60° conical thoriated-tungsten cathode, is over 18 000 K, in agreement with our previous spectroscopic measurements.
Nonalloyed InGaAs/GaAs ohmic contacts for self-aligned ion implanted GaAs heterostructure field effect transistors
Authors: Huang J. H. , Abrokwah J. K. , Ooms W. J.
Published: July 2002

Nonalloyed indium gallium arsenide (InGaAs) ohmic contacts were investigated and successfully applied to both n- and p-type self-aligned ion implanted heterostructure field effect transistors (HFETs). The key factor was to preserve the doping concentration and structure integrity of the InGaAs layer during implant activation. Specific contact resistances in the range of 5×10−6–2×10−5 Ω cm for n and p HFETs were realized with this structure and process.
Surface recombination in dry etched AlGaAs/GaAs double heterostructure p-i-n mesa diodes
Authors: Corbett B. , Kelly W. M.
Published: July 2002

We have measured the surface recombination in reactive ion etched mesas in an AlGaAs/GaAs p-i-n double heterostructure using the size dependence of the current density versus voltage characteristic. The recombination current is dependent on processing due to surface oxide formation. The surface recombination of the oxidized surface is independent of the crystal axis. The recombination current can be reduced by removal of the oxide and sulfur passivation which protects the surface from further oxidation.
Field-emitter array performance enhancement using hydrogen glow discharges
Authors: Schwoebel P. R. , Spindt C. A.
Published: July 2002

The changes in the current-voltage characteristics and electron emission spatial distribution of microfabricated field emitters following exposure to hydrogen low-pressure glow discharges have been investigated. The hydrogen discharge was found to result in a work function decrease typically between 0.5 and 1.5 eV following a dose of 1018–1019 ions/cm2. The net result is a reduction in the operating voltage and an improvement in the spatial uniformity of the electron emission.
Improved Al/InP Schottky barriers by coimplantation of Be/P
Authors: Tyagi Ritu , Chow T. P. , Borrego J. M. , Pisarczyk K. A.
Published: July 2002

Improved Al/InP Schottky barriers are formed on n-InP by using coimplantation of Be/P. The doped surface layer (Be) helps in retarding the surface fields, resulting in larger Schottky barrier heights. The addition of phosphorus prevents excessive P loss and enhances the percentage of Be activation. A Schottky barrier height as high as 0.64 eV has been obtained, as compared to 0.55 eV for diodes without P implantation. The ideality factor is found to be very close to unity (≊1.08). The reverse leakage current density is reduced by almost four orders of magnitude. To demonstrate the versatility of our process, the same technique is applied to p-InP samples. As expected, the barrier height is reduced by almost 0.5 eV. Furthermore, the sum of the two Schottky barrier heights also matches closely with the theoretical value of energy band gap for InP.
Band offsets at the CdS/CuInSe2 heterojunction
Authors: Wei Su-Huai , Zunger Alex
Published: July 2002

The traditional explanation for the successful electron-hole separation in CdS/CuInSe2 solar cells rests on the assumption of a type-II band lineup: The conduction-band minimum is assumed to be on the CdS window while the valence-band maximum is assumed to be localized on the CuInSe2 absorber. This picture of negative conduction-band offset ΔEc<0 was supported by the electron affinity rule, but was sharply contradicted by the more recent photoemission experiments of Nelson et al. for CdS/CuInSe2 yielding ΔEc=+1.08 eV. Our first principles calculations yield for CdS/CuInSe2 ΔEc=+0.31 eV, hence, a type-I band alignment. We challenge the published experimental value as being in error and point to the need of revising current solar cell device models that assume ΔEc<0.
Paschen’s law for a hollow cathode discharge
Authors: Eichhorn H. , Schoenbach K. H. , Tessnow T.
Published: July 2002

An expression for the breakdown voltage of a one-dimensional hollow cathode discharge has been derived. The breakdown condition which corresponds to Paschen’s law contains, in addition to the first Townsend coefficient, and the secondary electron emission coefficient two parameters which characterize the reflecting action of the electric field and the lifetime of the electrons in the discharge. The breakdown voltage for a hollow cathode discharge in helium was calculated and compared to that of a glow discharge operating under similar conditions.
Critical conditions for achieving blue light emission from porous silicon
Authors: Wang Xun , Shi G. , Zhang F. L. , Chen H. J. , Wang W. , Hao P. H. , Hou X. Y.
Published: July 2002

By observing the luminescence micrographic images and measuring the decay behaviors of photoluminescence spectra, it is found that the blue light-emitting porous silicon obtained by boiling water treatment behaves very similarly to the red light-emitting sample. It is thus believed that the blue light emission is originated from the porous silicon skeleton rather than impurity contaminations. The achievement of blue light emission requires the proper control of the size of the Si nanostructures, effective passivation of the internal surfaces of porous silicon layer, and keeping a mechanically strong Si skeleton. A theoretical estimation and the experiments show that the simultaneous fulfillment of these conditions is quite critical, which explains the poor reproducibility of achieving blue emission experimentally.
Hard x-ray sputtered-sliced phase zone plates
Authors: Bionta Richard M. , Skulina Kenneth M. , Weinberg Jonathan
Published: July 2002

Al/Cu sputtered-sliced phase zone plates, designed to operate with a focal length of 18 cm at 8.05 keV, were tested at the Stanford Synchrotron Radiation Laboratory. First-, second-, and third-order peaks are measured, along with a defocused condition. The first-order focusing efficiencies ranged from 13% to 19% for the two lenses presented here. Monte Carlo simulations are compared to the data.
Low dark current dual band infrared photodetector using thin AlAs barriers and Γ-X mixed intersubband transition in GaAs quantum wells
Authors: Liu H. C. , Wilson P. H. , Lamm M. , Steele A. G. , Wasilewski Z. R. , Li Jianmeng , Buchanan M. , Simmons J. G.
Published: July 2002

A low dark current dual band quantum well infrared photodetector is demonstrated by adding thin AlAs barriers to the usual detector structure, which consists of Si-doped GaAs wells separated by thick AlGaAs barriers. The advantages of adding the thin AlAs barriers to clad the quantum wells are that (a) the detector displays a low dark current and (b) intersubband photocurrents result from transitions from both the Γ ground to the first excited state, and from the Γ ground to a mixed Γ-X excited state because the X-valley band edge forms a well in AlAs and intrinsic Γ-X mixing occurs. The spectral peaks of these two transitions, which occur at 8.5 and 5.5 μm in our test structure, can be varied by changing device parameters during growth.
Laser assisted Pd seeding for electroless plating on SiO2
Authors: Schrott A. G. , Braren B. , Saraf R.
Published: July 2002

Excimer laser pulses with wavelengths of 248 and 308 nm were used to selectively seed Pd on SiO2 surfaces, making them suitable for electroless plating. This novel seeding process for insulating materials is accomplished with the sample immersed in the seeding solution, and occurs only on the areas of the substrate that are illuminated (through the liquid) by the laser light. The Pd content of the seeded samples increased with the number of pulses, but was rather independent of repetition rate. The deposition rate showed a dependence with wavelength consistent with a defect driven mechanism for electron excitation through the band gap of SiO2. These electrons then reduce the Pd ions in the solution in contact with the surface.
Thermal control of near-infrared and visible electroluminescence in alkyl-phenyl substituted polythiophenes
Authors: Berggren M. , Gustafsson G. , Inganäs O. , Andersson M. R. , Wennerström O. , Hjertberg T.
Published: July 2002

We report electroluminescence from a regioregular alkyl-phenyl substituted polythiophene. The polymer film exists in two forms, giving widely different optical absorption, as well as photoluminescence and electroluminescence spectra. In the low-bandgap form, we observe high emission intensity centered at 1.55 eV (800 nm), well into the infrared, while the high-bandgap form gives a maximum at 1.85 eV (670 nm). The conversion from the high-bandgap form to the low-bandgap form can be done by thermal treatment of the polymer light emitting diodes.
Low-temperature homoepitaxial growth on Si(111) mediated by thin overlayers of Au
Authors: Wilk G. D. , Martinez R. E. , Chervinsky John F. , Spaepen Frans , Golovchenko J. A.
Published: July 2002

High quality homoepitaxial growth of Si on Si(111) through an overlayer of Au is shown to occur at 450–500 °C, far below the temperature required for growth of Si of similar quality on bare Si(111). Films of unlimited thickness can be obtained with excellent crystalline quality, as revealed by Rutherford backscattering spectrometry ion channeling measurements (χmin=2.2%). A distinct range of Au coverage (0.4–1.0 monolayer) results in the best quality epitaxy, with no measurable amount of Au trapped at either the interface or within the grown films. Cross-sectional transmission electron microscopy reveals that in films grown with Au coverages below and above the optimum range, the predominant defects are twins on (111) planes and Au inclusions, respectively.
Corona-assisted flame synthesis of ultrafine titania particles
Authors: Vemury Srinivas , Pratsinis Sotiris E.
Published: July 2002

Synthesis of ultrafine titania particles is investigated in a diffusion flame aerosol reactor in the presence of a gaseous electric discharge (corona) created by two needle electrodes. The corona wind flattens the flame and reduces the particle residence time at high temperatures, resulting in smaller primary particle sizes and lower level of crystallinity. Increasing the applied potential from 5 to 8 kV reduces the particle size from 50 to 25 nm and the rutile content from 20 to 8 wt %. Coronas provide a clean and simple technique that facilitates gas phase synthesis of nanosized materials with controlled size and crystallinity.
Magnetic force microscopy of the submicron magnetic assembly in a magnetotactic bacterium
Authors: Proksch R. B. , Schäffer T. E. , Moskowitz B. M. , Dahlberg E. D. , Bazylinski D. A. , Frankel R. B.
Published: July 2002

A magnetic force microscope (MFM) was used to image topography and magnetic forces from a chain of submicron single magnetic domain particles produced by and contained in isolated magnetotactic bacteria. The noncontact magnetic force microscope data were used to determine a value for the magnetic moment of an individual bacterial cell, of order 10−13 emu, consistent with the average magnetic moment of bacteria from the same sample, obtained by superconducting quantum interference device magnetometry. The results represent the most sensitive quantification of a magnetic force microscope image to date.
Force microscope using a fiber-optic displacement sensor
Authors: Rugar D. , Mamin H. J. , Erlandsson R. , Stern J. E. , Terris B. D.
Published: July 2002

A force microscope is described which uses a fiber-optic interferometer as the cantilever displacement sensor. Low thermal drift and reduced susceptibility to laser frequency variation are achieved due to the small (several micrometer) size of the interferometer cavity. A sensitivity of 1.7×10−4 Å/(Hz)1/2 is observed for frequencies above 2 kHz. The drift rate of the sensor is on the order of 3 Å/min. As an initial demonstration, laser-written magnetic domains in a thin film sample of TbFeCo were imaged.
Uncooled high-speed InSb field-effect transistors
Authors: Ashley T. , Dean A. B. , Elliott C. T. , Pryce G. J. , Johnson A. D. , Willis H.
Published: July 2002

InSb enhancement-mode, metal-insulator-semiconductor, field-effect transistors with 1 μm gate lengths have been fabricated. When operated at room temperature with less than 0.5 V applied between the source and drain, the transistors have a static dynamic range in excess of 20 dB, a cut-off frequency (fT) of 14 GHz and a transconductance, at 1 GHz, of 230 mS mm−1. Analysis of the parasitic capacitances indicates an intrinsic fT of about 90 GHz. The static electron mobility in the channel is 2×104 cm2 V−1 s−1, so a carrier velocity of about 3.7×107 cm s−1 should be attained. This leads to a predicted frequency response of 84 GHz, in reasonable agreement with the intrinsic microwave data.
Silicon avalanche photodiode detector circuit for Nd:YAG laser scattering
Authors: Hsieh C. L. , Haskovec J. , Carlstrom T. N. , DeBoo J. C. , Greenfield C. M. , Snider R. T. , Trost P.
Published: July 2002

A silicon avalanche photodiode with an internal gain of about 50 to 100 is used in a temperature-controlled environment to measure the Nd:YAG laser Thomson scattered spectrum in the wavelength range from 700 to 1150 nm. A charge-sensitive preamplifier has been developed for minimizing the noise contribution from the detector electronics. Signal levels as low as 20 photoelectrons (S/N=1) can be detected. Measurements show that both the signal and the variance of the signal vary linearly with the input light level over the range of interest, indicating Poisson statistics. The signal is processed using a 100 ns delay line and a differential amplifier which subtracts the low-frequency background light component. The background signal is amplified with a computer-controlled variable gain amplifier and is used for an estimate of the measurement error, calibration, and Zeff measurements of the plasma. The signal processing has been analyzed using a theoretical model to aid the system design and establish the procedure for data error analysis.
X-ray detection characteristics of microchannel plates using synchrotron radiation in the energy range from 0.06 to 0.6 keV
Authors: Hirata M. , Yamaguchi N. , Cho T. , Takahashi E. , Kondoh T. , Miyoshi S. , Aoki S. , Maezawa H. , Yagishita A.
Published: July 2002

The characteristics of microchannel plates (MCPs) for x-ray detection have been investigated with continuous x-ray energy variation using synchrotron radiation in the XUV and soft x-ray region (from 60 to 600 eV). The current response data have shown x-ray absorption fine structure (EXAFS or XANES) near the oxygen K absorption edge as well as a jump structure near the silicon L edge; these structures are explained by the surface composition of channel walls (SiO2). The data show that the surface layer of a microchannel is essential for x-ray detection by MCPs in this energy range. The precise current response curve has been completed for the wide x-ray energy range from 0.06 to 82 keV combined with the previously reported data [Kondoh et al., Rev. Sci. Instrum. 59, 252 (1988); Cho et al., ibid. 59, 2453 (1988); Yamaguchi et al., ibid. 60, 368, 2307 (1989)].
Visible charge exchange spectroscopy at JET
Authors: von Hellermann M. G. , Mandl W. , Summers H. P. , Weisen H. , Boileau A. , Morgan P. D. , Morsi H. , Koenig R. , Stamp M. F. , Wolf R.
Published: July 2002

Recent developments and results of the JET CXRS diagnostic are reported. The measurements of radial profiles of ion temperatures and densities are based on CXR spectra of fully stripped ions of either carbon or beryllium. Considerable effort has been expended in ensuring consistency between radial profiles of low Z impurity densities and those from other diagnostics. The contributions of the main light impurities are used to reconstruct radial profiles of Zeff which can be compared with Abel-inverted signals from visible bremsstrahlung or soft x-ray emission. Active Balmer-Alpha spectroscopy (ABAS) is being introduced as a diagnostic tool providing data on local magnetic fields, neutral beam densities, and dilution factors. The effects of collision-energy-dependent CXR cross sections on observed CXR spectra are calculated. Corrections for the values of deduced ion temperatures, toroidal velocities, and impurity densities are discussed for the case of plasmas with high ion temperatures and high toroidal rotation velocities. Some recent results of the JET 1989 operation illustrating the CXRS diagnostic potential are given.
An electron-impact ionization time-of-flight mass spectrometer using a simple high-voltage square pulse generator
Authors: Jung Kwang Woo , Choi Sung Seen , Jung Kyung-Hoon
Published: July 2002

A versatile electron-impact time-of-flight mass spectrometer (EI-TOFMS) in a pulsed molecular beam has been constructed. The instrument contains a fast rise time, square pulse generator, biased at high voltage, which simplifies the implementation of EI-TOFMS. Power field effect transistors are used as the active switching element, which afford compact size and simplified circuit construction. The effect of the shape of the repelling pulse on the flight time and space focusing condition is estimated by numerical calculation. Preliminary test results are provided for pulsed molecular beams of ammonia and hydrogen iodide.
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n−A program for the simulation of positive ion extraction including magnetic fields
Authors: Becker R. , Herrmannsfeldt W. B.
Published: July 2002

i g u n is a program for the simulation of positive ion extraction from plasmas. It is based on the well known program egun for the calculation of electron and ion trajectories in electron guns and lenses. The mathematical treatment of the plasma sheath is based on a simple analytical model, which provides a numerically stable calculation of the sheath potentials. In contrast to other ion extraction programs, igun is able to determine the extracted ion current in succeeding cycles of iteration by itself. However, it is also possible to set values of current, plasma density, or ion current density. Either axisymmetric or rectangular coordinates can be used, including axisymmetric or transverse magnetic fields.
Soft x-ray multilayer phase shifter
Authors: Yamamoto M. , Yanagihara M. , Nomura H. , Mayama K. , Kimura H.
Published: July 2002

Phase shift versus incident angle characteristics of soft x-ray multilayer mirrors have been studied for the first time by rotating analyzer ellipsometry at 97 eV photon energy. Synchrotron radiation was used as a linear polarization source and a multilayer polarizer of 97% polarizance was used as the analyzer. By the reflection at two multilayers mounted in the double crystal configuration, circular polarization was produced. This confirmed a total phase shift of 90° and proved the usefulness of the multilayer as a phase shifter for polarization evaluation.
Stroboscopic optical boxcar technique for the determination of fluorescence lifetimes
Authors: James Douglas R. , Siemiarczuk Aleksander , Ware William R.
Published: July 2002

This work describes a stroboscopic optical boxcar technique for the determination of fluorescence lifetimes which achieves performance comparable to techniques such as time-correlated single photon counting or phase modulation. The stroboscopic technique is based on the use of a traveling wave injected into a delay line connecting the dynodes of a photomultiplier tube. The transient potential difference created between two adjacent dynodes results simultaneously in significant amplification and the generation of a ‘‘gate’’ for the amplification process. Accurate control of the timing between the flashing of the gated lamp and the computer controlled delayable triggering of the photomultiplier tube pulser allows the gate to be placed at any time position within the range of the digital delay generator. The intensity of the fluorescence emission can thus be measured as a function of time relative to the excitation flash yielding data which is very similar to that from time-correlated single photon counting. Data analysis is done by iterative reconvolution in a procedure very similar to the analysis of time-correlated single photon counting data. Fluorescence lifetimes from a few hundred picoseconds to hundreds of nanoseconds can be determined to an accuracy and precision of better than ±2%.
Rocket-borne downconverter system for measuring space plasma turbulence
Authors: Haas D. G. , Siefring C. L. , Baumback M. M. , Rodriguez P. , McNutt D. P.
Published: July 2002

For research into space plasma turbulence, we have developed a number of space-flight instruments for making in situ measurements of plasma wave activity. One of these instruments, the NRL downconverter, is used to measure plasma waves in the 2- to 20-MHz frequency range by downconverting a 100-kHz band of frequencies to the baseband. Input frequency tuning is controlled by an imbedded microcomputer, and can be programmed to scan in any desired frequency pattern. The in-phase and quadrature receiver outputs, each having a 50-kHz bandwidth, are processed externally by digital signal processors (DSP). The receiver achieves 35 dB of sideband rejection using a hybrid quadrature mixer design, which incorporates an analog front end with digital signal processing. This design yields 15 dB of improved sideband rejection over more conventional analog techniques. The microcomputer control, DSP processing, and modular design contribute to the flexibility of the receiver, which has allowed the system to be used on a number of different missions.
TFTR natural diamond detectors based D–T neutron spectrometry system
Authors: Krasilnikov A. V. , Azizov E. A. , Roquemore A. L. , Khrunov V. S. , Young K. M.
Published: July 2002

Three natural diamond detectors (NDD) based spectrometry systems have been developed and used on TFTR to perform D–T neutron spectrum and flux measurements. DT neutrons interact with the NDD through the 12C(n, alpha)9Be reaction to produce a narrow peak in the pulse height distribution which has 2%–3% energy resolution and is well isolated from other reactions by ∼2 MeV in energy. The NDD detector is also highly radiation resistant (5×1014 n/cm2) and compact in size (diameter ∼4 mm, thickness ∼0.2 mm). Three detectors have been installed near TFTR. One, installed in a central sight line of the neutron collimator views perpendicularly from below. Two other detectors placed inside radiation shields in the TFTR test cell have tangential and perpendicular cones of view. These three spectrometers provide spectrometry of D–T neutrons escaping the tokamak with angles with respect to plasma current in the ranges close to 90°, 110°–180°, and 60°–120° correspondingly. The fastest standard electronics have been used to reduce the influence of pulse pileup during spectrometry measurements at count rates up to 3×105 counts/s. Results of the first D–T neutron spectra and flux dynamics measurements made with the three NDD based spectrometers during TFTR high power neutral beam injection are presented.
Achromatically filtered diamond photoconductive detectors for high power soft x-ray flux measurements
Authors: Turner R. E. , Landen O. L. , Bell P. , Costa R. , Hargrove D.
Published: July 2002

A 1-mm-square diamond photoconductive detector (PCD) has been installed on the LLNL Nova laser system, for use as a broadband soft x-ray power diagnostic. The PCD is installed behind an array of pinholes, which cast multiple, overlapping images of the source onto the diamond. This allows reduction of the x-ray intensity, to avoid saturation problems, while avoiding the spectral dependency of thin film filters. The diode current is read out on a 5 GHz bandwidth scope. The system is calibrated by comparison to an absolutely calibrated array of filtered vacuum x-ray photodiodes (“dante”). The time response of the PCD and its bias electronics have been characterized using the 5th harmonic (210 nm) of a short pulse (<1 ps) Ti: sapphire laser. The data show a fast rise, limited by the 5 GHz scope bandwidth, and a slower fall off, characterized by an RC time of order 200 ps.
Subpicosecond carrier lifetimes in arsenic-ion-implanted GaAs
Authors: Ganikhanov Feruz , Lin Gong-Ru , Chen Wen-Chung , Chang C.-S. , Pan Ci-Ling
Published: July 2002

We have investigated photoexcited carrier lifetimes in arsenic-ion-implanted semi-insulating GaAs by time-resolved reflectivity measurements. Subpicosecond carrier lifetimes (220 to 550 fs) which do not exhibit apparent dosage dependence for samples bombarded with 200 keV arsenic ions at increasing dosages in the range of 1012 and 1016 ions/cm2 are reported. The shortest carrier lifetime was observed for the sample irradiated at 1013 ions/cm−2. These are the shortest lifetimes ever observed for ion-damaged GaAs and comparable to those of low-temperature molecular beam epitaxially grown GaAs, which is also nonstoichiometric with excess-arsenic-related, deep-level defects.
Structure of cross-tie wall in thin Co films resolved by magnetic force microscopy
Authors: Löhndorf M. , Wadas A. , van den Berg H. A. M. , Wiesendanger R.
Published: July 2002

We have studied the magnetic domain structure of a thin polycrystalline Co film by magnetic force microscopy (MFM). Domain walls of the cross-tie type have been observed for a Co film of 50 nm thickness. Due to the high lateral resolution of MFM we have been able to study the magnetic structure of a single cross tie. We have determined locations of Bloch lines within a domain wall comparing the experimental data with a theoretical model of a cross-tie wall. In order to explain our experimental results we have proposed a model for the interaction between a MFM tip and a cross-tie wall.
Voltage-tunable-color multilayer organic light emitting diode
Authors: Kalinowski J. , Di Marco P. , Cocchi M. , Fattori V. , Camaioni N. , Duff J.
Published: July 2002

Organic electroluminescent diodes with a double-heterostructure of indium-tin-oxide (ITO) substrate/aromatic diamine (TPD)/8-hydroxyquinoline aluminum (Alq3)/perylene bisimide pigment (PBP)/Mg/Ag have been fabricated by successive vapor deposition. Their electroluminescence (EL) for thin layers of PBP varies from the orange dominated by the red emission of PBP to greenish-yellow with a strong contribution of the green emission of Alq3 as voltage increases. The relative contribution of the red emission increases with increasing voltage for the structures with thick PBP layers. The tuning of the EL color is explained in terms of electric field-induced quenching of excited states and voltage evolution of the recombination zone in PBP.
Boron implantation/in situ annealing procedure for optimal p -type properties of diamond
Authors: Fontaine F. , Uzan-Saguy C. , Philosoph B. , Kalish R.
Published: July 2002

Record values for high Hall mobility and for low compensation ratio of boron doped diamond by ion implantation are reported. These are achieved, following the suggestion by Prins, by low dose Boron implantation into cold diamond (−97 °C) and in situ rapid heating (1050 °C for 10 min) and by a further anneal at higher temperature (1450 °C for 10 min). Detailed evaluation of Hall effect data and of the temperature dependence of the resistivity over a wide temperature range (200 to 700 K) prove that this implantation/annealing scheme yields p-type behavior of the implanted layer with the highest hole mobility (385 cm2/V s, at room temperature) and the lowest compensation ratio (0.05) ever reported for diamond doped by ion implantation.
Pseudomorphic InAsSb multiple quantum well injection laser emitting at 3.5 μm
Authors: Kurtz S. R. , Biefeld R. M. , Allerman A. A. , Howard A. J. , Crawford M. H. , Pelczynski M. W.
Published: July 2002

A pseudomorphic, compressively strained InAs0.94Sb0.06 multiple quantum well injection laser, emitting in the 3.5–3.6 μm range is reported. The device was grown by metalorganic chemical vapor deposition, and x-ray and optical characterization indicate that the active region has a very low dislocation density. In pulsed mode, the laser operated at 135 K and displayed a characteristic temperature of 33 K, equaling the highest value reported for molecular-beam epitaxy grown, InAsSb/InAlAsSb active region lasers of comparable wavelength. Factors limiting the performance of these lasers are discussed.
Atomic scale study of the interaction between misfit dislocations at the GaAs/Si interface
Authors: Vilà A. , Cornet A. , Morante J. R.
Published: July 2002

High resolution electron microscopy (HREM) has been applied to study the interaction between misfit dislocations at the atomic layer molecular beam epitaxy (ALMBE) GaAs/Si interface at the atomic scale. The structure of Lomer dissociations has been found to be highly sensitive to the presence of neighboring 60° ones, resulting in dislocations and delocalization of the extra {111} half planes. The dissociation has also been analyzed and qualitatively interpreted on the basis of equilibrium forces of nearby defects.
Nucleation and growth of PrBa2Cu3O7−δ barrier layers on ramps in DyBa2Cu3O7−δ studied by atomic force microscopy
Authors: Verhoeven M. A. J. , Moerman R. , Bijlsma M. E. , Rijnders A. J. H. M. , Blank D. H. A. , Gerritsma G. J. , Rogalla H.
Published: July 2002

We studied the microstructure of Ar ion-beam etched ramps in epitaxial DyBa2Cu3O7−δ films by atomic force microscopy. Generally, ramps were well aligned (φ≊0°) with one of the crystal axes of the (001) SrTiO3 substrate. In those cases we observed a surface reconstruction into a regular pattern of very long (up to 0.5 μm), about 20 nm wide facets, parallel to the ramp edge. For high misorientation angles (φ≳0°), the reconstruction appeared much more complex and less regular. Furthermore, we investigated the nucleation and growth of very thin barrier layers of PrBa2Cu3O7−δ on well aligned ramps. We found that nucleation takes place in the shallow trenches in the ramp, where facets meet. When more material is deposited, islands coalesce to form closed domains parallel to the trenches. This causes a thickness modulation close to 100% for barrier layer thickness up to 6 nm. The conclusions drawn from this research allow for an important improvement of the technology for the fabrication of ramp-type junctions.
Selective growth of gallium nitride layers with a rectangular cross-sectional shape and stimulated emission from the optical waveguides observed by photopumping
Authors: Tanaka Toshiaki , Uchida Kenji , Watanabe Akiyoshi , Minagawa Shigekazu
Published: July 2002

Selective growth of GaN layers on sapphire substrates is investigated for low-loss optical waveguide structures of active and passive photonic devices. When growth conditions, such as nitridation of the substrate surface and growth temperature, are adjusted to achieve high-density uniform nucleation on the substrate, we found that the cross-sectional shape of GaN waveguides has a rectangular structure with smooth top and side surfaces. At 77 K, we observed stimulated emission from the cleaved facet of an optically pumped selectively grown GaN waveguide with a width of 6 μm and a cavity length of 800 μm. Line narrowing was also achieved: with the full width at half-maximum of the emission spectra decreasing from 11 to 2 nm by increasing a pump power density from 0.013 to 0.05 MW/cm2.
Nanolithographic templates from diblock copolymer thin films
Authors: Mansky P. , Harrison C. K. , Chaikin P. M. , Register R. A. , Yao N.
Published: July 2002

We describe a technique for creating a thin polystyrene film containing a periodic array of cylindrical holes, with a hole size of approximately 13 nm and a lattice constant of 27 nm. The starting material is a polystyrene-polybutadiene diblock copolymer, which self-assembles into a hexagonally packed array of polybutadiene cylinders embedded in a polystyrene matrix. A technique described previously is used to orient the cylinders normal to the plane of the film. The polybutadiene domains are then removed by reaction with ozone, which attacks the double bonds in the polybutadiene backbone. Films of this type could potentially be used as templates for nanolithography on a scale not readily accessed by other techniques.
Nitridation of sapphire. Effect on the optical properties of GaN epitaxial overlayers
Authors: Grandjean N. , Massies J. , Leroux M.
Published: July 2002

GaN layers were grown by gas-source molecular beam epitaxy on sapphire substrates using ammonia as a nitrogen source. The nitridation of an Al2O3 surface prior to the GaN growth was followed in situ by reflection high-energy electron diffraction. A strong variation of the surface lattice parameter was observed corresponding to the formation of an AlN relaxed layer. The nucleation of GaN on such a nitridated surface is facilitated, as checked by atomic force microscopy. Optimization of the nitridation procedure is achieved by investigating the photoluminescence properties of GaN thin films grown for different nitridation times. It is found that the band-edge and the yellow-band luminescences are strongly dependent on the nitridated starting surface. Finally, the optimized nitridation process is used to grow high-quality GaN epitaxial layers.
A mechanism for low-power all-optical switching in multiple-quantum-well structures
Authors: Knorr C. , Wilhelm U. , Härle V. , Ottenwälder D. , Scholz F. , Hangleiter A.
Published: July 2002

We demonstrate a novel type of optical switching mechanism in pin separate confinement multiple quantum well (SCMQW) structures. By introducing additional large barriers into conventional InGaAs(P)/InP SCMQW structures, the transport of photogenerated holes can be controlled in such a way that they accumulate in the intrinsic region. This positive space charge leads to a local screening of the internal field in the optical confinement layer and to an enhancement of the internal field in the MQW region. We characterize the optical nonlinearity, which is based on the quantum confined Stark effect (QCSE), experimentally and theoretically. As the nonlinearity is observed at input powers <1 W/cm2 in the basic nonoptimized structures presented here, we propose to use our structure especially for low-power optical switches.
Interwell carrier transport in InGaAsP multiple quantum well laser structures
Authors: Fröjdh K. , Marcinkevičius S. , Olin U. , Silfvenius C. , Stålnacke B. , Landgren G.
Published: July 2002

We present direct measurements of interwell carrier transport in InGaAsP quantum well (QW) laser structures performed by time-resolved photoluminescence. Conditions of originally empty and filled wells are explored. In both cases, the time for the hole transport across the structure is found to be of the order of tens of picoseconds. Comparison of experimental results and simulations allowed us to develop an adequate interwell carrier transport model that includes thermionic capture/emission over the QW interfaces and drift/diffusion in the barrier regions. We show that dynamic consideration of carrier densities and band bending for each QW are essential.
Negative differential conductance in three-terminal silicon tunneling device
Authors: Koga Junji , Toriumi Akira
Published: July 2002

Negative differential conductance based on lateral band-to-band tunneling is demonstrated in a three-terminal silicon tunneling device. The device is fabricated with the current silicon ultra-large scale integration (Si ULSI) process, taking care of the field isolation to reduce the excess tunneling current that flows over some intermediate states. It is observed that the forward biased band-to-band tunneling current is largely controlled by the gate bias which modulates the tunneling barrier width. The three-terminal Si tunneling device is promising as the post complementary metal–oxide-semiconductor device in future Si ULSI.
Error rate of a superconducting circuit
Authors: Herr Quentin P. , Feldman Marc J.
Published: July 2002

The bit-error rate of the Josephson junction single-flux-quantum comparator was measured as a function of bias current offset, at clock rates up to 10 GHz. The bit-error rate versus offset is a smooth curve, measured over 16 decades of incidence, which linearly extrapolates to 10−49 for optimal bias. The lowest rate actually measured was 5×10−17, corresponding to 4 errors counted in 130 h.
Photoluminescence from optical waveguiding LiNbO3 film formed on crystalline SiO2 substrate by pulsed laser deposition
Authors: Wu X. L. , Guo X. L. , Liu Z. G. , Jiang S. S. , Feng D. , Siu G. G.
Published: July 2002

Optical waveguiding LiNbO3 film was grown on (001)-oriented crystalline SiO2 substrate by pulsed laser deposition. The film is highly (012) textured and has smooth surface. Its photoluminescence (PL) spectra were obtained by using the 514.4 nm line excitation of Ar+ laser, displaying a broad band peaked at 2.2 eV at room temperature and three well-resolved luminescent bands peaked at 2.3, 2.2, and 2.14 eV at liquid nitrogen temperature with different polarization properties. Spectral analysis indicates that these obtained PL bands arise from the self-trapped excitons and the E′ defect pairs connected with oxygen vacancies in the LiNbO3 film.
Densities of Si determined by an image digitizing technique in combination with an electrostatic levitator
Authors: Ohsaka K. , Chung S. K. , Rhim W. K. , Holzer J. C.
Published: July 2002

We have determined the densities of Si in the liquid, ρl(T), and solid, ρs(T), states as a function of temperature, T, by employing an image digitizing technique and numerical calculation methods in combination with an electrostatic levitator. The obtained density data can be fitted with the following equations: ρl(T)=ρl(Tm)−1.71×10−4(T−Tm)−1.61×10−7(T−Tm)2(g/cm3); ρs(T)=ρs(Tm)−2.63×10−5(T−Tm)(g/cm3),where Tm is the melting point, 1687 K, and ρl(Tm) and ρs(Tm) are 2.580 and 2.311 (g/cm3), respectively. The error involved in the determination is estimated to be ±0.006 (g/cm3). The ρl(T) value smoothly varies through Tm and does not indicate a reported anomalous density variation. The ρl(Tm) value is 2% larger than the literature value and the coefficient of the linear temperature dependence is approximately half of a reported value. The ρs(Tm) value closely agrees with the literature value.
Extended x-ray absorption fine structure study of AlxGa(1−x)N films
Authors: Miyano K. E. , Woicik J. C. , Robins Lawrence H. , Bouldin C. E. , Wickenden D. K.
Published: July 2002

Extended x-ray absorption fine structure above the Ga–K edge has been used to study the local structure of AlxGa1−xN films grown by metal organic chemical vapor deposition. With increasing Al content, x, the Ga–N bond length decreases, but much less than the average bond length. On the other hand, the x dependence of the Ga–Ga and Ga–Al distances does follow the variation of the average cation–cation distance. We conclude that bond angle distortions accommodate the differences between the Ga–N and Al–N bond lengths.
Observation of biexcitonic effect in single ZnCdSe quantum well under very low excitation density
Authors: Kuang G. , Gebhardt W. , Griebl E. , Sube K. , Kastner M. , Wörz M. , Reisinger T.
Published: July 2002

Under an excitation density of 0.4 W/cm2, a strong biexcitonic peak was observed in the photoluminescence (PL) spectrum of a single quantum well which was grown in submonolayer deposition mode. Its intensity was found to increase superlinearly with excitation density. The shape of the biexcitonic peak was found to be similar to that of biexcitons in CuCl and CdS crystal and a binding energy of 9.8 meV was obtained. When the temperature increased from 10 to 31 K, the biexcitonic peak decreased considerably whereas the excitonic peak increased slightly.