bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultraviolet and blue discretely tunable double-pass fiber Raman laser
Authors: Ilev Ilko K. , Kumagai Hiroshi , Toyoda Koichi
Published: July 2002

The experimental realization of a new ultraviolet and blue tunable fiber Raman laser, based on a simple double-pass fiber laser arrangement with Littrow-prism-tuned emission, is demonstrated. The third harmonic of a Q-switched Nd:YAG laser (λp=355 nm) is used to pump a 100-μm-core-diam multimode silica UV optical fiber. As a result of a single pass of the pump laser emission through the fiber, an initial multicascade Raman Stokes spectrum that contains up to 21 Stokes orders in the spectral range 360–527 nm at 1.9 GW/cm2 input pump intensity is generated. By using the double-pass regime of the fiber Raman laser and multimode fibers with different lengths, a discretely tunable generation corresponding to the various Stokes orders over the multicascade Stokes spectrum in the ultraviolet and blue regions (360–493 nm) is obtained.
Studies of Mn valence conversion and oxygen vacancies in La1−xCaxMnO3−y using electron energy-loss spectroscopy
Authors: Wang Z. L. , Yin J. S. , Jiang Y. D. , Zhang Jiming
Published: July 2002

Using the white line intensities, electron energy-loss spectroscopy in a transmission electron microscope has been employed to characterize the valence conversion and oxygen vacancies in La1−xCaxMnO3−y. For a nominal doping composition x=0.33, the ratio of Mn4+ to Mn3+ is determined to be more than 0.25 but less than 0.5, and the content of oxygen vacancy y is no more than 0.065 (equivalent to 2.2 at. % of the oxygen content). At ymax=0.065, 60% of the residual charge introduced by Ca doping is balanced by the conversion of Mn3+to Mn4+ and 40% by oxygen vacancy.
Interference scanning optical probe microscopy
Authors: Bacsa W. S. , Kulik A.
Published: July 2002

We describe an optical scanning probe technique (Interference Scanning Optical Probe Microscopy) with enhanced resolution possibilities not limited by the aperture size of the optical probe. This is realized using a substrate in the form of a microcavity and probe collection mode in reflection geometry. The microcavity consisting of an opaque and a transparent layer, is used to shift the phase of the wave scattered from the adsorbate with respect to the incident and reflected beams. Using this technique silver island films have been detected with resolution better than 40 nm with a nominal probe aperture size of 100 nm.
Theoretical investigation on δ doping of Se atoms in GaAs
Authors: Nara Jun , Sasaki Taizo , Ohno Takahisa
Published: July 2002

We report the result of first-principles calculations on Se δ-doped GaAs and propose a carrier compensation mechanism in the region of high Se concentrations. It is found that introducing Ga vacancies near Se atoms makes the system extremely stable and semiconducting. In the high-concentration region where Se atoms are close to each other, the Se atoms are likely to get together by introducing Ga vacancies and form clusters consisting of three Se atoms and one Ga vacancy. The extra electrons of the Se atoms transfer to the dangling bonds of the Se or As atoms, and become electrically inactive.
Autocorrelation of mode-locked laser pulses based on the synchronous drift of photogenerated carriers
Authors: Obeidat Amjad T. , Khurgin Jacob B.
Published: July 2002

We show that the autocorrelation function of mode-locked laser pulses can be measured by interfering a pulse with its time-delayed replica in a conventional photoconductor to generate a pulsating grating that can be represented as a superposition of moving gratings. The resonance current peak resulting from matching the carrier drift velocity to the velocity of one of the grating components is proportional to the autocorrelation function. Unlike conventional autocorrelators, the proposed method uses no nonlinear effects and thus can be utilized for measuring weak pulses.
Phase-conjugation broad area twin-contact semiconductor laser
Authors: Vasil’ev Peter P. , White Ian H.
Published: July 2002

Experimental demonstration of lasing in a broad area twin-contact semiconductor laser which operates as a phase-conjugation (PC) mirror in an external cavity configuration is reported. This allows “self-aligned” and self-pumped spatially nondegenerate four-wave mixing to be achieved without the need for external optical signals. The external cavity laser system is very insensitive to tilt misalignments of the external mirror in the PC regime and exhibits very good mechanical stability. The resonant frequency of the external cavity lies in the GHz range which corresponds to a subnanosecond time response of phase conjugation processes in the semiconductor laser.
Fabrication of dry etched CdZnSe/ZnSe quantum wires by thermally assisted electron cyclotron resonance etching
Authors: Kümmell T. , Bacher G. , Forchel A. , Nürnberger J. , Faschinger W. , Landwehr G. , Jobst B. , Hommel D.
Published: July 2002

A low damage dry etch technology suitable for in situ processing was developed for the fabrication of ZnSe-based nanostructures. Thermally assisted electron cyclotron resonance etching combines plasma etching at low ion energies with process temperatures between 80°C and 210°C. Due to a variation of the process parameters, i.e., plasma power and sample temperature, a transition from partially physical to prevailing chemical etch properties is obtained. Therefore an accurate control of etch profile, surface morphology, and etch rate is possible. Optically active CdZnSe/ZnSe quantum wires with lateral sizes down to 20 nm were realized, indicating a significantly reduced influence of optically inactive layers compared to conventionally dry etched nanostructures. In narrow wires, a systematic blue shift of the photoluminescence signal with decreasing wire width clearly demonstrates lateral carrier confinement effects.
THz optical beat frequency generation from a single mode locked semiconductor laser
Authors: Pelusi M. D. , Liu H. F. , Novak D. , Ogawa Y.
Published: July 2002

We demonstrate ultrahigh repetition rate beat signal generation by filtering two modes from the optical spectrum of a 110 GHz pulse train produced by a 1.56 μm wavelength, subharmonic hybrid mode locked monolithic semiconductor distributed Bragg reflector laser. The beat signal repetition rate can be tuned to any higher harmonic of the 110 GHz frequency up to 1.34 THz. Furthermore, the filtering technique facilitates low phase noise and synchronization with these signals.
Direct measurement of population-induced broadening of quantum well intersubband transitions
Authors: Xu Yuanjian , Almogy Gilad , O’Brien John , Shakouri Ali , Xu Weihua , Salvatore Randal A. , Yariv Amnon
Published: July 2002

The dependence of the absorption spectral linewidth of quantum well intersubband transitions on the electron population in the well is experimentally demonstrated. We show that the dependence of the spectral linewidth on the population is substantial and accounts for some of the broadening previously attributed to donor scattering.
Si
1−y
C
y
/
Si
(001)
heterostructures made by sublimation of SiC during silicon molecular beam epitaxy
Authors: Joelsson K. B. , Ni W.-X , Pozina G. , Radamson H. H. , Hansson G. V.
Published: July 2002

Preparation of pseudomorphic Si1−yCy/Si(001) heterostructures using Si molecular beam epitaxy with C obtained from SiC sublimation in a high-temperature cell has been studied. Thick (≈2000 Å) homogenous Si1−yCy layers, y⩽1.5%, and Si1−yCy/Si multiple quantum well (MQW) structures, y⩽8%, have been prepared. There is a growth temperature dependent surface roughness accumulating during the growth sequence that can lead to reduction of C induced strain. Temperature modulation during growth has been used to suppress this effect. Near band gap photoluminescence is reported from Si1−yCy/Si MQW structures.
Higher order magnetoresistance commensurability oscillations in low aspect ratio antidot lattice and focusing structures
Authors: Moon J. S. , Simmons J. A. , Reno J. L.
Published: July 2002

We observe unusually strong magnetoresistance commensurability oscillations in two different types of artificial arrays of scattering centers, multiparallel focusing slits and a square antidot lattice, both fabricated on a two-dimensional electron gas by electron-beam lithography and damageless wet etching. The strength of the magnetoresistance peaks is attributed to a high reflection specularity and a small effective antidot cross section. The absence of commensurability effects near Landau level filling factor υ=3/2 is attributed to the composite fermion mean free path being smaller than the slit and antidot spacings. The mechanism for the observed commensurability is discussed.
Model of coherent transport in metal–insulator–midband gap semiconductor–insulator–semiconductor structure
Authors: Abramov I. I. , Danilyuk A. L.
Published: July 2002

A kinetic model of coherent transport with self-organized carrier transfer via midband gap semiconductor states in metal–insulator–midband gap semiconductor–insulator–semiconductor structure at room temperature is proposed. The coherent transport at room temperature can be a result of continuous oscillations of charge carriers at midband gap semiconductor states.
Harmonic generation limitations in gain-switched semiconductor lasers due to distributed microwave effects
Authors: Sum K. C. , Gomes N. J.
Published: July 2002

A large-signal, time domain semiconductor laser model that incorporates propagation effects within the cavity and a microwave circuit simulator are used to investigate distributed microwave effects on gain-switching performance. It is shown that the position of the feed point to the top electrode can have a significant influence on the harmonic content of the generated pulses, which is an important consideration in microwave/millimeter-wave signal generation schemes.
High quality superconducting tunnel junctions on Nb and Ta single crystals for radiation detection
Authors: Bruijn M. P. , Kiewiet F. B. , van den Berg M. L. , Luiten O. J. , de Korte P. A. J. , Hamster A. W. , Brons C. G. S. , Flokstra J.
Published: July 2002

High quality Nb/Al-based tunnel junctions, fabricated with a superconducting interface onto thick single crystal x-ray absorbers of Nb and Ta are discussed. Current-voltage characteristics, recorded at 0.5 K, show a subgap current which is still dominated by thermally excited quasiparticles. The quality parameter Rsubgap/Rnormal reaches a value of several million, which is unequalled for nonepitaxially sputtered tunnel junctions. The fabrication process and some development steps, such as preparation of ultrasmooth crystal surfaces are described. Observations of x-ray photons absorbed in Nb and Ta single crystals detected by the superconducting tunnel junctions are also presented.
Estimation of the second crossover in insulators using the electrostatic mirror in the scanning electron microscope
Authors: Wong W. K. , Phang J. C. H. , Thong J. T. L.
Published: July 2002

A technique for measuring the second secondary emission crossover in insulators was developed. The method utilizes the behavior of an electrostatic mirror formed on insulating samples to estimate the current equilibrium point as well as the mirror potential. Current results show that the value of the second crossover for glass passivation obtained using this method is in general agreement with previous techniques.
Measurement of the depth distribution of ion beam etching-induced damage in AlGaAs/GaAs multiple quantum well structure
Authors: Frost F. , Otte K. , Schindler A. , Bigl F. , Lippold G. , Gottschalch V.
Published: July 2002

The defect depth distribution caused by a 500 eV nitrogen ion beam etching (IBE) of an Al0.35Ga0.65As/GaAs multiple quantum well (MQW) structure was investigated by confocal photoluminescence (PL) measurements on a beveled section of the sample. The beveled section with an extremely small inclination angle necessary for the high depth resolution was fabricated by the IBE itself. Compared to other ion beam or plasma assisted etching processes reported, e.g., Ar-IBE, the 500 eV nitrogen IBE yields a very low defect density. A model including diffusion effects for the description of the profile gives a value of 4×10−15 cm2 /s for the defect diffusion coefficient.
Photoluminescence of ozone oxidized and HF etched porous silicon and the multiple source quantum well model
Authors: Jia L. , Wong S. P. , Wilson I. H. , Hark S. K. , Zhang S. L. , Liu Z. F. , Cai S. M.
Published: July 2002

Porous silicon (PS) samples were alternately oxidized in an ozone atmosphere and etched in HF solution. The variation of the corresponding photoluminescence (PL) implies that there is more than one origin responsible for the PL of PS. These results are explained by a multiple source quantum well model, where the nanoscale Si units and their covering oxide layers both contribute to the PL in different situations.
Solid-phase epitaxial growth of AlxGa1−xAs alloys as a function of Al content
Authors: Hogg S. M. , Llewellyn D. J. , Tan H. H. , Ridgway M. C.
Published: July 2002

The solid-phase epitaxial growth (SPEG) of amorphized AlxGa1−xAs alloys has been investigated as a function of Al content (0⩽x⩽0.46). Time-resolved reflectivity and Rutherford backscattering spectrometry in combination with channeling (RBS/C) were used to determine the recrystallization kinetics over the temperature range 220–310 °C. Residual disorder was characterized with RBS/C and cross-sectional transmission electron microscopy. At a given temperature, both the average velocity and nonplanarity of the amorphous/crystalline interface increased as the Al content increased. In contrast, an Al-content dependence was not observed for residual disorder—for all compositions, single-crystal regrowth was apparent for ∼25 nm followed by the onset of twinning. The SPEG kinetics exhibited an Arrhenius-type temperature behavior with an average activation energy Eact value of 1.60 eV independent of Al content. Also, the validity of a recrystallization mechanism based on the collective reorganization of groups of atoms was verified—Debye frequency values derived from experimental Eact results were in excellent agreement with theoretical predictions.
Evolution of ion charge states in vacuum arc plasmas
Authors: Demokan O.
Published: July 2002

A model, based on ion transport along the plasma in vacuum arcs, is proposed to explain the evolution of charge states. Physical arguments implementing such a model are given, and the charge state fluxes are evaluated. The systematic trends of the particle current fractions are seen to be alike for the theoretical and experimental values, and good quantitative agreement is found for electron temperatures, higher than the generally accepted values. Comments regarding the future work and the effect of a magnetic field are stated.
An ion-beam-assisted process for high-Tc Josephson junctions
Authors: Huang M. Q. , Chen L. , Zhao Z. X. , Yang T. , Nie J. C. , Wu P. J. , Xiong X. M.
Published: July 2002

We have developed a non-ion-etching ion-beam-assisted-deposition (IBAD) process for fabricating high critical-temperature (Tc) grain boundary Josephson junctions through a photoresist liftoff mask. The YBa2Cu3O7 (YBCO) junctions fabricated through this process exhibited the resistively-shunted-junction (RSJ)-like I–V characteristics. The well-defined Shapiro steps have been seen on the I–V curves under microwave radiation. The magnetic modulation of critical current of a 4 μm width YBCO junction tallied with the prior simulated Fraunhofer diffraction pattern of a Josephson junction with a spatially homogeneous critical current density. The maximum peak-to-peak modulation voltage across the dc superconducting quantum interference device (SQUID) fabricated by using these junctions reached up to 32 μV at 77 K. The magnetic modulation of the SQUID exhibited periodic behavior with the observed modulation period of 5.0×10−4 G.
Effects of isolation materials on facet formation for silicon selective epitaxial growth
Authors: Tseng H.-C. , Chang C. Y. , Pan F. M. , Chen J. R. , Chen L. J.
Published: July 2002

Effects of isolation materials on facet formation for low temperature Si selective epitaxial growth (SEG) using Si2H6 have been demonstrated by ultrahigh vacuum-chemical molecular epitaxy (UHV-CME) system. The activation energy of the Si epitaxial growth is 47.35 Kcal/mol. The isolation material includes silicon nitride, wet oxide, and tetraethylorthosilicate (TEOS) oxide. Different isolation materials have different faceting behaviors for the Si epilayer. A silicon nitride mask and a wet oxide mask result in a preferred faceting plane of (111) and (311), respectively. Using a TEOS oxide mask, we obtain a low temperature Si SEG layer without facet and twin formation. The preferred faceting plane orientation for different isolation materials is ascribed to the difference in the interface free energy between the Si layer and the isolation material.
Use of carbon black to eliminate surface charging effects in photoelectron spectroscopy measurements of powders
Authors: Obrovac M. N. , Gao Yuan , Richard M. N. , Dahn J. R.
Published: July 2002

The photoelectron spectra of SiO2 on silicon powder is measured with and without the addition of carbon black. Samples without carbon black show significant charging and peak deformation due to photoelectron emission. Samples containing carbon black show no evidence of charging. Furthermore, the contribution from carbon black can be subtracted from the overall signal in the valence band region to give valence band spectra of insulating powders. It is suggested that mixing carbon black with nonconducting powders may be an effective way to eliminate surface charging effects while at the same time providing a useful calibration standard.
Diamond synthesis by using very high frequency plasmas in parallel plate electrodes configuration
Authors: Matsumoto Seiichiro , Asakura Yoshiki , Hirakuri Kenji
Published: July 2002

By using very high frequency (100 MHz) plasma, diamond was synthesized with only rf power in parallel plate electrodes configuration from a mixture gas of methane and hydrogen. Deposition conditions are similar to those used so far in medium pressure methods, i.e., the pressure, methane concentration, substrate temperature, and rf power used are 90–20 Torr, 5–0.5%, 900 °C, and 400–1000 kW, respectively. This plasma is very stable and the power density used is small. Furthermore, diamond formed at a pressure lower than that used in dc plasma chemical vapor deposition. These results suggest the possibility of the application of this method to large area deposition of diamond. However, the quality of the diamonds obtained are not yet very high. Methods of improvement are also discussed.
Hydrogen-induced light emission from an organic electroluminescent device
Authors: Okuyama Sumio , Ito Yasuaki , Sugawara Toshiyuki , Okuyama Katsuro , Matsushita Koichi , Kido Junji
Published: July 2002

An organic electroluminescent device consisting of ITO/aromatic amine/tris(8–quinolinolato)aluminum(Alq3)/Pd was fabricated. The light emission was found to occur when hydrogen gas was present in the ambience and ceased when hydrogen was removed from the Pd electrode. The light emission induced by hydrogen gas was attributed to the work function lowering of the Pd electrode at the Pd–Alq3 interface by adsorption of hydrogen.
Photothermal reflectance investigation of ion implanted 6H–SiC
Authors: Muratikov K. L. , Usov I. O. , Walther H. G. , Karge H. , Suvorov A. V.
Published: July 2002

The photoreflection method is applied to detailed characterization of the ion implanted silicon carbide. Experimentally and theoretically, it is shown that the photoreflection method can be an effective tool for the determination of the dose of implantation and the depth of the implanted layer. It is demonstrated that the photoreflection method also has a high sensitivity to the measurement of optical parameters of the implanted layer. Experimental results are presented for the case of Lely grown 6H–SiC crystals implanted by 37 keV He+ ions.
Optical evaluation of the ionized EL2 fraction in proton (24 GeV) irradiated semi-insulating GaAs
Authors: Ferrini R. , Galli M. , Guizzetti G. , Patrini M. , Nava F. , Canali C. , Vanni P.
Published: July 2002

Semi-insulating SI GaAs samples from a zone refined crystal were irradiated with high energy protons (24 GeV/c, fluences up to 1.64×1014 p/cm2). Optical spectra in transmittance and reflectance were accurately measured in the energy range of 0.6–1.4 eV to determine, through the absorption coefficient, the concentrations of both neutral and ionized EL2 defects as a function of the proton fluence. Both these concentrations have been shown to increase linearly with the proton fluence; this behavior well explains the remarkable decrease of the charge collection efficiency observed in proton irradiated GaAs detectors at doses associated with high luminosity beams at a new particle collider accelerator (e.g., the LHC at the CERN laboratory).
Novel features in the strain profile and gate oxide capacitance of through-gate-oxide implanted structures
Authors: Zaumseil P. , Kar S.
Published: July 2002

Oxidized silicon samples were implanted with O, Si, or Ar ions. The samples were characterized by x-ray triple crystal diffractometry and metal-oxide-semiconductor admittance spectroscopy to reveal information on the nature of the ion-induced damage. The experimental results on the strain profile, gate oxide capacitance, etc., exhibited novel features. These interesting results suggest ion-damage-induced precipitation of SiO2 particles in silicon and oxidation of the silicon subsurface by the injection of the recoil O atoms from the gate oxide into the silicon subsurface, both at room temperature.
Carbon-doped GaInP/GaAs heterojunction bipolar transistors grown by metalorganic chemical vapor deposition using nitrogen as the carrier gas
Authors: Hsu C. C. , Yang Y. F. , Ou H. J. , Yang E. S. , Lo H. B.
Published: July 2002

The use of nitrogen as the carrier gas in metalorganic chemical vapor deposition (MOCVD) for the growth of carbon-doped GaInP/GaAs heterojunction bipolar transistors (HBTs) is reported. The material quality grown using a nitrogen carrier gas is the same as that of using a hydrogen carrier gas. High carbon doping and hole concentrations of 3×1020 and 2×1020 cm−3 in GaAs were obtained. The fabricated HBTs showed very good DC and RF performances indicating that nitrogen can be a promising carrier gas for MOCVD growth.
An extended external cavity for the generation of tunable multi-wavelength pulses from a self-seeded laser diode
Authors: Yam S. P. , Shu C.
Published: July 2002

We have developed a technique to generate tunable multi-wavelength picosecond pulses using a self-seeded gain-switched laser diode. The external cavity allows simultaneous feedback of up to four wavelengths into the laser diode for mode selection. A tunable spectral spacing of about 20 nm among the wavelength channels can be obtained. The optimized output side-mode suppression is higher than 20 dB. Different channels in the output are separated temporally and can be individually accessed for modulation.
Formation of elemental Ag precipitates in AlGaAs by ion implantation and thermal annealing
Authors: Chang J. C. P. , Ye J. , Melloch M. R. , Crouse D. T. , Nolte D. D.
Published: July 2002

We report transmission electron microscopy (TEM) studies of precipitation in Ag-implanted and annealed Al0.3Ga0.7As. Silver was chosen because it does not form compounds with Ga and/or As when precipitating based on bulk thermodynamics arguments. TEM confirmed the formation of an elemental metal/semiconductor composite, which consists of nanometer-sized Ag (fcc structure) precipitates dispersed in the matrix. The precipitates are nonspherical and have an orientation relationship to Al0.3Ga0.7As of (200)Ag//(200)AlGaAs, (02-2)Ag//(02-2)AlGaAs, and [011]Ag//[011]AlGaAs. High temperature (900 °C) anneals transform the phase of the precipitate to hexagonal, Ag3(GaAl) compounds. This shape distribution has been correlated with inhomogeneous broadening of the optical absorption.
Off-axis holography of laser-induced shock wave targets
Authors: Werdiger M. , Eliezer S. , Henis Z. , Arad B. , Horovitz Y. , Shpitalnik R. , Maman S.
Published: July 2002

Shock waves of the order of hundreds of kilobars are produced in a tin foil 50 μm thick by a Nd:YAG laser system with a wavelength of 1.06 μm, pulse width of 7 ns [full width at half-maximum (FWHM)] and irradiance in the range (1.4–2.4)×1013 W/cm2, focused to a spot of 200 μm. Off-axis holographic measurements of the target, after 0.5 and 1 μs from the shock arrival, are reported. The hologram is produced by backscattering of a pulse, 6.5 ns (FWHM) of green laser light, from the ejected material from the target.
Real time observation of surface kinetics during the self-assembly of Ga chains on Si(112)
Authors: Glembocki O. J. , Prokes S. M.
Published: July 2002

Single wavelength reflectance difference anisotropy (RDA) has been used to study the kinetics of the self-assembly of Ga atom chains on faceted Si(112) surfaces. The formation of the chains is followed from the initial deposition through changes in the surface reconstruction from (5×1) to (6×1). We present a simple Monte Carlo model to account for the time evolution of the RDA signal as a function of temperature and experimentally determined kinetic parameters.
Reducing threshold voltage shifts in amorphous silicon thin film transistors by hydrogenating the gate nitride prior to amorphous silicon deposition
Authors: Tsai Jun-Wei , Huang Chun-Yao , Tai Ya-Hsiang , Cheng Huang-Chung , Su Feng-Cheng , Luo Fang-Chen , Tuan Hsing-Chien
Published: July 2002

A short H2 plasma treatment of the gate SiNx before depositing amorphous silicon (a-Si:H) is found to significantly decrease the threshold shifts in the bias stress, inverted a-Si:H thin film transistors (TFTs). The reduced threshold voltage shift is attributed to a plasma induced reconstruction of SiNx precursors leading to the removal of the weak bonds. A prolonged plasma treatment, however, degraded the TFT characteristics; this was traced H2 plasma damage which eventually generated a rough a-Si:H/SiNx interface.
Determination of the glass transition temperature of nonlinear optical planar polymer waveguides by attenuated total reflection spectroscopy
Authors: Bock Harald , Christian Stefan , Knoll Wolfgang , Vydra Jan
Published: July 2002

We present an experimental technique for nondestructive measurement of the glass transition temperature (Tg) of thin nonlinear optical (NLO) polymer films by attenuated total reflection spectroscopy. At Tg the thermal expansion coefficient and temperature dependence of the refractive index ∂n/∂T change discontinuously. This change can be detected with high sensitivity by recording thickness and refractive index of a polymer waveguide coated on a glass/gold substrate as a function of temperature. Measurements were made on various high Tg NLO polymers (Tg up to 170 °C). The resulting Tg values are compared with those determined by differential scanning calorimetry of the bulk material.
Microstructural origin of 1/f noise in high Tc bicrystal SQUID magnetometers
Authors: Huang Y. , Merkle K. L. , Lee L. P. , Teepe M. , Char K.
Published: July 2002

The origin of noise in YBa2Cu3O7−x (YBCO) bicrystal SQUID magnetometers on SrTiO3 substrates is investigated by comparing the microstructure of actual low-noise and high-noise devices. The most obvious difference in the microstructure is the presence of a-axis oriented particles in the high-noise devices, whereas the low-noise devices consist exclusively of c-axis oriented YBCO films. The growth of the a-axis particles in the YBCO films induces many defects, including amorphous regions, distortion in c-axis lattice planes and extra a-c interfaces. The quality of the junction boundary is also degraded by the a-axis particles. The existence of these defects are expected to affect the superconducting current and the motion of the magnetic flux in the films and hence generate extra noise in the devices.
Observation of wavelength-dependent generation efficiency of laser-induced ultrasonic surface acoustic waves on ceramic materials
Authors: Flannery C. M. , Kelly P. V. , Beechinor J. T. , Crean G. M.
Published: July 2002

In this work, the efficiency of laser generation of surface acoustic waves (SAWs) in ceramic materials is reported to be wavelength dependent. A pulsed Nd:YAG laser operating at wavelengths of 1064 or 532 nm was used to generate SAWs on ceramic (silicon nitride and silicon carbide) and metal substrates. It was observed that 1064 nm radiation is more efficient than 532 nm radiation for SAW generation on ceramics, whereas the opposite is the case for metals. While the wavelength dependence of SAW generation efficiency in metals is due to stronger optical absorption at the shorter wavelength, the dependence for the ceramics is attributed to a longer absorption length at the longer wavelength.
Low-temperature, chemically driven atomic-layer epitaxy: In situ monitored growth of CdS/ZnSe(100)
Authors: Luo Yi , Slater David , Han Ming , Moryl Joseph , Osgood R. M.
Published: July 2002

A chemical reaction scheme for room-temperature atomic-layer epitaxial growth of II–VI semiconductors is described and demonstrated. Growth is accomplished by dosing in an ultra-high-vacuum chamber, and the surface structure and composition is probed in situ by a variety of electron and ion probes.
Effect of interface roughness and well width on differential reflection dynamics in InGaAs/InP quantum wells
Authors: Zhao Y.-G. , Zou Y.-H. , Wang J.-J. , Qin Y.-D. , Huang X.-L. , Masut R. A. , Bensaada A.
Published: July 2002

We have observed differential reflection dynamics in In0.518Ga0.492As/InP multiple quantum wells, using the pump–probe technique, and examined the photoluminescence spectra to determine the interface quality for the samples studied. Our results show that the interface quality and well width of the quantum wells (QWs) strongly influence the differential reflection dynamics. The experimental results provide a direct evidence to demonstrate that photoexcited carrier diffusion in cap layer and barriers along the direction perpendicular to sample surface plays a dominant role in determining the differential reflection dynamics of the QWs.
Fiber array targets for producing long media of high gain in recombination-pumped soft-x-ray lasers
Authors: Ozaki Tsuneyuki , Kuroda Hiroto
Published: July 2002

We demonstrate a target for recombination-pumped soft-x-ray lasers, consisting of an equally spaced array of parallel fibers. This target is capable of producing long media of several centimeters, and can also generate high gain due to the initial two-dimensional expansion of the plasma. Numerical simulations show that amplification occurs after the plasma expansion changes from a cylindrical to a slablike one, as a result of collisions with plasma produced from neighboring fibers. Initial gain measurements using graphite-coated fiber array targets reveal Balmer α gain coefficients of 4.1 cm−1.
Poole–Frenkel effect assisted emission from deep donor level in chromium doped GaP
Authors: Ajjel R. , Zaidi M. A. , Alaya S. , Brémond G. , Guillot G. , Bourgoin J. C.
Published: July 2002

The electrical properties of chromium-related defects in GaP are investigated. Using deep-level transient spectroscopy, a related deep level is observed in p-type GaP exhibiting an activation energy, associated with hole emission, of 0.5 eV. Detailed capacitance transient investigations were undertaken to study the electric field dependence. This emission rate which is found to have a field dependence can be fitted by a Poole–Frenkel model. Evidence is given that the trap is the Cr4+/3+ deep donor level in GaP caused by substitutional Cr on Ga sites. This trap seems to be well adapted to compensate donors for the growth of the semi-insulating GaP.
Deep levels in Er-doped liquid phase epitaxy grown silicon
Authors: Cavallini A. , Fraboni B. , Pizzini S.
Published: July 2002

The optical activity of Er-doped silicon is related to the presence of defects which enhance the typical radiative transition at 0.8 eV but whose nature and properties are still largely unknown. We have investigated the deep levels present in Er-doped liquid phase epitaxy silicon to identify the traps possibly related to the luminescence. Among the observed levels, two minority carrier traps labeled E2 (EC−0.20 eV) and E3 (EC−0.39 eV), and one majority carrier trap labeled H1 (Ev+0.32 eV) are good candidates for the material optical activity. The onset of luminescence occurs after a thermal treatment and is accompanied by a high space charge density localization at the epilayer-substrate interface.
A high-speed free-space traveling wave photodetector
Authors: Merlet Thomas , Dolfi Daniel , Collot Philippe , Nagle Julien , Huignard Jean-Pierre
Published: July 2002

We present and experimentally demonstrate a traveling wave photodetector adapted to the detection of optically carried microwave signals. The synchronous propagation of the detected modulated laser beam with the photogenerated microwave signal is obtained with a diffraction grating. This concept provides a large detection volume which could be suitable for the optical generation of high power/large dynamic range microwave signals. The operating principle of the device is detailed and experimentally demonstrated, over a 0–5 GHz bandwidth, using a coplanar metalsemiconductor-metal waveguide on a GaAs molecular beam epitaxially grown layer.
c
-axis oriented ferroelectric thin films of Si-substituted PbTiO3 on Si(100) by pulsed laser deposition: Boost for nonvolatile memory application
Authors: Purandare S. C. , Palkar V. R. , John J. , Multani M. S. , Pinto R.
Published: July 2002

It has been demonstrated that Si substitution in a PbTiO3 target helps to avoid the formation of a pyrochlore phase, which otherwise occurs during fabrication of the films directly on Si(100) by the pulsed laser deposition technique. The films are perfectly c-axis oriented. Moreover, the ferroelectric properties of PbTiO3 are not affected by Si substitution. As silicon helps the realization of the films at low substrate temperature, the Pb/Ti ratio is maintained close to 1 and the loss factor tan δ in the range of 0.02–0.05. Thus, the integration of nonvolatile ferroelectric random access memory on semiconductors is shown to be possible in a reliable, cost effective, and simple manner.
Angular dependence of the magnetoresistivity in (Bi,Pb)2Sr2Ca2Cu3O/Ag tape
Authors: Han G. C. , Ong C. K. , Li C. S. , Xi Z. P.
Published: July 2002

Measurements of the magnetoresistance R(H) as functions of the temperature and the angle θ between the applied magnetic field and the plane of a (Bi,Pb)2Sr2Ca2Cu3O/Ag tape have been performed in a low magnetic field region below Tc. Contrary to the two-dimensional vortex pancake model, the data do not scale well with H sin(θ) for small angles. Instead, our results show a general three-dimensional scaling behavior based on the anisotropy in the upper critical field Hc2 over entire angular range. From the scaling, the dissipation anisotropy is found to be unchanged at lower temperatures and decreases with increasing temperature at temperatures close to Tc.
Modification of band offsets by a ZnSe intralayer at the Si/Ge(111) interface
Authors: Pan M. , Wilks S. P. , Dunstan P. R. , Pritchard M. , Williams R. H. , Cammack D. S. , Clark S. A.
Published: July 2002

In this letter, the use of an ordered ultrathin ZnSe dipole layer to significantly modify the band discontinuity at the Si/Ge(111)-c(2×8) heterojunction is reported. Soft x-ray photoemission spectroscopy (SXPS) was utilized to monitor the evolution of the interface. The ZnSe intralayer increased the valence band offset by ∼0.57 eV, as compared to a negligible valence band offset for the Si/Ge(111) junction. This dramatic modification is interpreted in terms of the charge transfer at the interface.
Atomistic studies of segregation and diffusion in Al-Cu grain boundaries
Authors: Liu X.-Y. , Xu Wei , Foiles S. M. , Adams J. B.
Published: July 2002

The segregation of Cu atoms at Al [110] Σ11 and [001] Σ5 tilt grain boundaries was studied. Cu atoms were found to segregate to asymmetric sites at the Σ11 boundary and form zig-zag planar aggregates at the interface. Segregation is dominated by atomic size and local hydrostatic stress. Cu atoms prefer to occupy the prime diffusion path sites at both grain boundaries. Cu segregation raises the vacancy formation and diffusion activation energies at Σ11 grain boundaries, thus slowing the rate of atomic diffusion.
A comparative study of uniaxial pressure effects in intraband AlGaAs/GaAs and interband InAs/AlSb/GaSb resonant tunneling diodes
Authors: Mutamba K. , Sigurdardóttir A. , Vogt A. , Hartnagel H. L. , Li E. H.
Published: July 2002

We report on the effects of uniaxial pressure on (001)-oriented AlGaAs/GaAs and InAs/AlSb/GaSb double barrier resonant tunneling diodes (RTDs). The current–voltage characteristics of the AlGaAs/GaAs RTDs shift asymmetrically due to stress-induced piezoelectric fields in the barriers and well structures. Although all the materials involved are piezoelectric, the interband InAs/AlSb/GaSb resonant tunneling device surprisingly shows, in contrast to the AlGaAs/GaAs one, a symmetrical behavior for the same orientation [110] of the applied pressure. We explain the observed differences considering the different tunneling paths involved in the conduction mechanism of the two heterostructure device types as well as their pressure dependencies.
Optical unwinding and reentrance phenomena in chiral smectic-C liquid crystals
Authors: Chen Shu-Hsia , Chen Ching-Yih
Published: July 2002

We optically measure the molecular reorientation induced by an argon laser in homeotropically aligned chiral smectic-C films. Optical unwinding of the helix is observed and the critical unwinding intensity is determined. We have also developed a theory which not only explains the observed effect but also predicts a novel reentrance phenomenon of the unwinding state.
Differential thermal analysis of magnetic transitions at high pressure: Néel temperature of NiO up to 8 GPa
Authors: Sidorov V. A.
Published: July 2002

The method of differential thermal analysis is applied to the study of magnetic transitions at high hydrostatic pressure up to 9 GPa. It is very useful for detecting the antiferroparamagnetic transition temperature in insulating solids under high pressure, which is difficult to observe by other methods. The Néel temperature of NiO was found to increase under pressure at a rate dTN/dP=7.33 K/GPa. This result is in good agreement with the high pressure two-magnon Raman scattering experiments.
Relaxation times in InAs/AlSb quantum wells
Authors: Markelz A. G. , Asmar N. G. , Gwinn E. G. , Brar B.
Published: July 2002

We have measured energy relaxation and longitudinal-optical (LO) phonon emission times in modulation-doped InAs quantum wells driven by high dc fields, and by intense ac fields at frequencies from 0.49 to 0.66 terahertz. We find that for electron temperatures between 50 and 200 K, LO phonon emission dominates energy relaxation. We determine a large net LO phonon emission time, indicating a strong LO phonon bottleneck both in high dc and in intense terahertz fields.
Area-selective nucleation of copper on fluorocarbon–resin surface using ArF excimer laser-induced chemical modification
Authors: Okoshi M. , Murahara M.
Published: July 2002

Area-selective nucleation of copper onto fluorocarbon–resin surface was demonstrated by using an ArF excimer laser and copper–sulfate (CuSO4) aqueous solution. Pyramid-typed shaped nuclei of copper were initially formed on even an inert surface of fluorocarbon–resin in the same way as semiconductor or metal surfaces. Interface of copper nuclei and fluorocarbon–resin was chemically bonded through oxygen which was photodissociated from water in copper–sulfate aqueous solution under ArF excimer laser irradiation. The modification cannot be successful without our previous defluorination efforts using ArF excimer laser-induced chemical reaction.
Guiding and antiguiding effects in epitaxially regrown vertical-cavity surface-emitting lasers
Authors: Oh T.-H. , McDaniel M. R. , Huffaker D. L. , Deppe D. G.
Published: July 2002

The fabrication and optical mode characteristics of epitaxially regrown vertical-cavity surface-emitting lasers are presented. Modeling suggests that a highly resistive AlGaAs current blocking layer grown at low temperature by molecular beam epitaxy can be used to form an antiguiding structure, and antiguiding effects are observed in 10 μm diameter devices. Additional influence on the optical mode arises due to discontinuities associated with the etch and regrowth.
Interface flattening and optical characteristics of GaAs/Al0.3Ga0.7As quantum wells grown on finite patterns on a GaAs(111)B substrate
Authors: Nishida Toshio , Kobayashi Naoki
Published: July 2002

The formation of an atomically flat quantum well is studied by using metalorganic vapor phase epitaxy on a GaAs(111)B substrate. The surface of a 1-nm-thick GaAs layer on Al0.3Ga0.7As, corresponding to the upper interface of a quantum well, becomes stepfree on a 3.5-μm-wide mesa, and the surface of an Al0.3Ga0.7As barrier becomes stepfree on a 2.5-μm-wide mesa. On 15-μm-wide mesas, the stepfree area becomes 4–6 μm for the GaAs surface and 3–4.5 μm for Al0.3Ga0.7As, which are a few times wider than those obtained on planar substrates. The photoluminescence from different thickness quantum wells are spatially separated.
Electron-yield x-ray absorption spectroscopy with gas microstrip detectors
Authors: Rayment Trevor , Schroeder Sven L. M. , Moggridge Geoffrey D. , Bateman J. Edmond , Derbyshire Gareth E. , Stephenson Richard
Published: July 2002

It is shown that gas microstrip detectors developed for high-energy physics applications afford a considerable improvement of the surface sensitivity of electron-yield x-ray absorption fine-structure spectroscopy at ambient pressures. Data with excellent signal-to-noise characteristics can be collected using photon fluxes typical of bremsstrahlung x-ray sources and of synchrotron storage rings operating with reduced (“single bunch”) currents. Possible applications for this detection technology are discussed.
Microcanonical temperature and its Arrhenius relation to lifetimes in isomerization dynamics of clusters
Authors: Takatsuka Kazuo , Yanao Tomohiro
Published: July 2002

In a previous study of isomerization dynamics of clusters as a chaotic conservative system, we proposed a temperature, called the microcanonical temperature [C. Seko and K. Takatsuka, J. Chem. Phys. 104, 8613 (1996)], which is expected to characterize a phase space distribution on a constant energy plane. In contrast to the standard view of equal a priori distribution in phase space, we note a fact that this distribution usually becomes sharply localized with a single peak, if projected onto the potential energy coordinate. The microcanonical temperature is defined as a kinetic energy at which this projected distribution takes the maximum value. Then the most probable statistical events should be dominated by those components in vicinity of the peak, provided that the projected distribution is singly and sharply peaked and the associated dynamics is ergodic. The microcanonical temperature can be similarly redefined in the individual potential basins. Here in the present article a numerical fact is highlighted that the inverse of the lifetime of an isomer bears an Arrhenius-type relation with thus defined local microcanonical temperature assigned to the corresponding potential basin. We present an analysis of how the Arrhenius relation can arise.
Coarse-grained dynamics of one chain in a polymer melt
Authors: Akkermans Reinier L. C. , Briels W. J.
Published: July 2002

In this study we present the coarse-graining of one polymer chain in a melt to a single dimer. By using the projector operator formalism we derive the equation of motions for the dimer. The different forces that occur in this equation of motion are calculated from molecular dynamics simulations of the microscopic model, using constraint forces to fix the dimer configuration. The mean constraint force serves as the conserved part of the interaction, whereas the time correlation of the constraint force fluctuation leads to the nonconserved interactions: the dissipative and fluctuating forces. Using the configurational dependent coarse-grained interactions we have performed stochastic dynamics simulations of the dimer. Dimer properties of the microscopic and the coarse-grained model are shown to be in reasonable agreement. We also discuss the application of the framework to coarse-graining polymer melts into more detail, i.e., beyond the dimer.
Nonstoichiometry and electrocoloration due to injection of Li+ and O2− ions into lithium niobate crystals
Authors: Bredikhin S. , Scharner S. , Klingler M. , Kveder V. , Red’kin B. , Weppner W.
Published: July 2002

The high oxygen and lithium ion conductivity in LiNbO3 was investigated and interpreted in terms of lithium and oxygen vacancies being intrinsically present in congruently grown single crystals. As a result of this, it was found that the stoichiometry of lithium niobate crystals may be changed with respect to lithium and oxygen. The optical and electrical properties of electrically colored LiNbO3 crystals were studied and it was shown that the absorption spectra of thermally reduced and electrocolored samples are identical. Therefore, the origin of the absorption processes is considered to be the same in both cases. The formation of regions with different stoichiometry due to the injection of additional lithium or oxygen into the LiNbO3 crystals was also observed and investigated. The motion of these stoichiometric domains through a LiNbO3 crystal from one electrode to the other was studied and described in terms of electrodiffusion of the ions and electrons. A model is proposed which considers the injection of oxygen and lithium vacancies for the generation of concentration profiles in the originally homogeneous material. The numerical calculation of the concentration profiles shows good agreement with the experimental results.
Spin transport in nanotubes (invited)
Authors: Alphenaar B. W. , Tsukagoshi K. , Wagner M.
Published: July 2002

We study electron spin transport through carbon nanotubes contacted by ferromagnetic electrodes. The resistance of a ferromagnetically contacted multiwalled nanotube switches hysteretically as a function of applied magnetic field, with a maximum resistance change of 9% at 4.2 K. Magnetoresistance measurements of carbon nanotubes having one cobalt contact and one platinum/gold contact, however, show no switching. In addition, we present calculations of the magnetoresistance ratio for the ferromagnetic nanotube device, and predict that a resistance change of 25% is possible.
Fiber optic interferometry for precision measurement of the voltage and frequency dependence of the displacement of piezoelectric tubes
Authors: Chen F. , Mohideen U.
Published: July 2002

Piezoelectric tube scanners used in scanning microscopy show nonlinear displacements with applied voltage. We have used an optical fiber based interferometer to make precision measurements of these nonlinearities. We have made the measurement of the sensitivity coefficient (displacement/voltage) for a wide range of different scanning voltage ranges and scanning rates, and find that the sensitivity coefficient has nonlinearities of order 4% as a function of the applied voltage. A 3% frequency dependence of the sensitivity coefficient is also measured. A phenomenological model is used to characterize the frequency dependence on the basis of a single relaxation time constant.