bluuebunny/crossref_metadata_2025 huggingface dataset:
A general purpose cold finger using a vibration-free mounted He closed-cycle cryostata)
Authors: Boolchand P. , Lemon G. H. , Bresser W. J. , Enzweiler R. N. , Harris R.
Published: July 2002

A method for mounting a He closed-cycle cryostat which consists of an exchange gas envelope around the cold head to cool an independently supported sample mount as in model DE202 with a DMX-20 interface from APD Cryogenics, Inc. is described. No detectable vibration of the sample mount is observed as evaluated using 57Fe Mössbauer spectroscopy and a piezoelectric accelerometer. Using a 25 μm thick α-Fe foil a linewidth of 0.231(3) mm/s at 300 K with the refrigerator on and the same linewidth with the refrigerator off is observed. The easy optical access afforded by such a cold finger makes it an economical general purpose laboratory tool for performing low-T spectroscopic investigations, such as microwave, optical, γ-ray, x-ray and neutron-scattering measurements. Other applications include electrical transport, SIMS, RBS, and rare-gas matrix isolation.
X-ray observation of electroclinic layer constriction and rearrangement in a chiral smectic-A liquid crystal
Authors: Rappaport Aaron G. , Williams P. A. , Thomas Britt N. , Clark Noel A. , Blanca Ros M. , Walba David M.
Published: July 2002

An x-ray scattering study of electroclinic layer constriction verifies the interpretation of the electroclinic effect as field-induced molecular tilt. The tilt angles deduced from the layer spacing changes are in close agreement with those from optical measurements. Layer buckling, a consequence of the layer constriction, is also observed and may be the cause of the loss of optical contrast observed in electroclinic devices.
Large photoinduced persistent optical absorption in selenium doped AlSb
Authors: Becla P. , Witt A. , Lagowski J. , Walukiewicz W.
Published: July 2002

Low-temperature, T<90 K, illumination transfers transparent, single crystals of AlSb doped with Se to a metastable state characterized by a high optical absorption approaching 200 cm−1. The enhanced absorption spectrum extends from 0.1 to 1.5 eV and consists of a double band structure with peaks at 0.2 and 0.5 eV. It is induced by photons with a threshold energy of about 1 eV. It is persistent, i.e., it shows no measurable decay after switching off the excitation. The low absorption state is thermally recovered at 100 K. The persistent absorption is a manifestation of a DX-type bi-stability of the Se donor. The 0.2 and 0.5 bands are, respectively, due to the photoionization of electrons from the metastable hydrogenic donor level to X1 and X3 conduction band minima. A large binding energy of the Se donor is essential for the observation of persistent absorption and a lack of any free-electron related effects.
Tunable multilayer EUV/soft x-ray polarimeter
Authors: Kortright J. B. , Rice M. , Franck K. D.
Published: July 2002

A tunable polarimeter using multilayer interference coatings as both reflection polarizer and transmission phase retarder is described. The portable device bolts onto existing chambers and its rotation axis is precisely aligned to the beam. The upstream transmission phase retarder is tunable by varying its incidence angle, and the downstream polarizer is tunable by translating a laterally graded multilayer. These optical components allow an unambiguous determination of the polarization state of any collimated beam entering the device over at least the 50–800 eV range.
A direct current superconducting quantum interference device gradiometer with a digital signal processor controlled flux-locked loop and comparison with a conventional analog feedback scheme
Authors: Kung P. J. , Bracht R. R. , Flynn E. R. , Lewis P. S.
Published: July 2002

A double-washer dc superconducting quantum interference device (SQUID) gradiometer with a flux-locked loop (FLL) based on a digital signal processor (DSP) has been developed for biomagnetic applications. All of the analog electronics in the conventional FLL are replaced and implemented by the DSP except for the low-noise field-effect transistor preamplifier at the front end of the signal recovery components. The DSP performs the signal demodulation by synchronously sampling the recovered signals and applying the appropriate full wave rectification. The signals are then integrated, filtered, and applied to the output. At 4.2 K, the white flux noise of the gradiometer measured in a DSP FLL mode is about 4μΦ0/√Hz and the noise at 1 Hz is 13 μΦ0/√Hz. The corresponding noise levels in the gradiometer operated by the conventional FLL are 1.8 and 3μΦ0/√Hz. The poorer system performance in the DSP FLL compared to the analog FLL is mainly caused by the ambient field noise and interference signals picked up through the connecting cables. Additional noise is also added to the overall noise floor by the instruments employed in the DSP system in the present prototype setup. Further improvement in the noise characteristics and the dynamic behavior of the DSP SQUID gradiometer is expected when a better configuration of DSP with the associated I/O devices is implemented. Additional improvements of the DSP programs are expected by incorporating higher-order integration, adaptive control, and noise reduction schemes.
Spatially imaged inhomogeneous spontaneous emission spectra of high power in InGaAsP/InP Fabry–Pérot lasers
Authors: Bethea C. G.
Published: July 2002

Real-time infrared imaging acquisition was used to measure the steady state intracavity spatial spontaneous emission profile along the active stripe of bulk Fabry–Pérot InGaAsP/InP lasers. The direct observation of spatially nonhomogeneous spontaneous emission profiles (relating to the spatial nonhomogeneity and redistribution of the quasi-Fermi level along the gain guiding medium) is demonstrated. The three-dimensional image mapping of the spatial redistribution of electrons along the active stripe well above lasing threshold is also discussed. We will demonstrate that the threshold saturation of spontaneous emission does not always occur along the entire length of the lasing cavity. Above threshold it was observed that the spatial redistribution of the spontaneous emission is highly wavelength dependent and that this observed phenomenon is a direct result of the optical cavity’s asymmetry.
Apparent superstructures in scanning tunneling microscopy
Authors: Li J. T. , Berndt R. , Schneider W.-D.
Published: July 2002

Apparent superstructures have been observed with scanning tunneling microscopy (STM) on well-ordered Ag(110) surfaces. These superstructures are due to the discrete sampling of the atomic corrugation in STM. Such sampling effects may explain superstructures observed previously with STM on graphite surfaces.
Space charge analysis in quantum well structures leading to spontaneous pulsing
Authors: Perera A. G. U. , Matsik S. G.
Published: July 2002

We present a model describing the basis for spontaneous pulsing behavior in a GaAs/AlAs quantum well structure at 300 K. This model is based on the accumulation of space charge in the well due to tunneling of electrons out of the well leading to a sharp increase in current and hence a pulse. The basic concepts used in the model are verified by comparison with experimental data for a single AlGaAs (or AlAs) barrier between two contacts. These structures will allow the use of neuron-like pulsing phenomena in IR detectors, image processors, neural networks, artificial neurons, etc. at much higher temperatures than the 10 K limit for pulsing observed in silicon p-i-n diodes.
Liquid target pulsed laser deposition
Authors: Rong Fu-Xiao
Published: July 2002

A novel pulsed laser deposition (PLD) technique using a liquid target has been developed. The technique overcomes the problem of solid target deterioration under high power laser irradiation and prevents splashing of large particulates on the substrate. Choosing an organic liquid containing carbon and hydrogen as a testing target, we have obtained high quality hydrogenated carbon films with a large amount of tetrahedral (diamond-like) sp3 bonding.
Temperature controlled microstage for an atomic force microscope
Authors: Mus̆evic̆ I. , Slak G. , Blinc R.
Published: July 2002

A temperature controlled microstage has been developed for an atomic force microscope (AFM). The stage is based on a small, temperature regulated heater, which is attached to the piezo-scanner of the AFM. The operating performances of the AFM have been tested in air at elevated temperatures. It was found that atomic resolution can routinely be achieved up to 80 °C, whereas beyond this temperature, the onset of thermal instabilities of the AFM cantilever was observed.
Voltage divider based on submicron slits in a high T
c superconducting film and two bicrystal grain boundaries
Authors: Kaplunenko V. K. , Ivanov Z. G. , Stepantsov E. A. , Claeson T. , Wikborg E.
Published: July 2002

Experiments on a model of rapid single flux quantum (RSFQ) flip-flop cell, based on high-Tc (HTS) Josephson junctions show that it can operate as a voltage divider at frequency up to 400 GHz. The junctions were formed in YBaCuO film, deposited on novel Y–ZrO2 bicrystals with two asymmetric 32° grain boundaries, about 10 μm apart, and allow a new design of RSFQ logic based on a single HTS layer. Small inductances (≂10 pH) were made as narrow, submicron size slits. The junction widths were between 4 and 10 μm and for ten junctions located close to the tested circuits, the linear critical current densities at T=4.4 K were 10.7 μA/μm±50% for one grain boundary and 8.3 μA/μm±50% for the other one. IcRn was about 1 mV±50%. A current density of half the expected value meant that the test circuit did not act as an ideal flip–flop down to the lowest frequency. As a voltage divider it gave a half value division up to 0.82 mV at T=4.4 K and to 0.4 mV at 30 K.
Influence of the process environment on the thermal intermixing of GaAs/AlAs/AlGaAs double barrier quantum wells
Authors: Kupka R. K. , Chen Y.
Published: July 2002

The influence of the process environment on thermally induced intermixing of GaAs/AlAs/AlGaAs double barrier quantum wells is investigated by effecting different low energy reactive ion etch (RIE) surface treatments and then depositing PECVD/sputtered cap layers prior to annealing. We observe photoluminescence blue shifts, small red shifts and total intermixing, dependent on the actual combination of cap material and RIE treatment. The results indicate that the RIE surface damage is an important factor for the onset of the intermixing process and that PECVD processes may require a few monolayers thick surface disordering in order to trigger off the quantum well intermixing, even using SiO2 cap layers.
Determination of the excess surface arsenic concentration in the metalorganic molecular beam epitaxial growth of GaAs using TMGa and As2
Authors: Kaneko T. , Joyce B. A. , Naji O. , Jones T. S.
Published: July 2002

We report an in situ dynamic method for the determination of the surface concentration of adsorbed arsenic in the metalorganic molecular beam epitaxial growth of GaAs from trimethylgallium (TMGa) and As2. Changes in the reflection high electron energy diffraction intensity oscillation period are observed under arsenic limited conditions, in which the decomposition of TMGa is strongly mediated by adsorbed arsenic. The period of the oscillations at the onset of growth indicates a time dependence in which the period gradually increases from the first oscillation to a steady state value after the growth of several monolayers. The results are interpreted as being due to an initial excess arsenic coverage which acts as a secondary source in this growth regime.
Optimization of compositional grading in Zn(Se,Te) graded ohmic contacts to p-type ZnSe
Authors: Fan Y.
Published: July 2002

The energy barrier at heterojunctions can be minimized by introducing a parabolic grading potential. For ternary materials with a strong bowing effect in band gap energy, such as Zn(Se,Te), the composition must vary nonparabolically in order to have the required parabolic grading potential. In this letter, a new compositional grading style for Zn(Se,Te) graded band gap ohmic contacts to p-type ZnSe has been obtained through a simplified analytical model where both the bowing factor in ternary band gap energy and the change in electron affinity χ with alloy composition are included. To show the effectiveness of the proposed grading style, it is compared with both parabolic and linear grading in numerical calculations. The specific contact resistance of the proposed optimized compositional grading is about two orders of magnitude lower than that of a linear grading over a grading distance of 170 to 310 Å, and about one order of magnitude lower than that of a parabolic grading at a grading distance less than 220 Å.
Novel laser ion sources
Authors: Fournier P. , Haseroth H. , Kugler H. , Lisi N. , Scrivens R. , Rodriguez F. Varela , Di Lazzaro P. , Flora F. , Duesterer S. , Sauerbrey R. , Schillinger H. , Theobald W. , Veisz L. , Tisch J. W. G. , Smith R. A.
Published: July 2002

Development in the field of high-power laser systems with repetition rates of several Hz and energies of few joules is highly active and opening, giving new possibilities for the design of laser ions sources. Preliminary investigations on the use of four different laser and target configurations are presented: (1) A small CO2 laser (100 mJ, 10.6 μm) focused onto a polyethylene target to produce C ions at 1 Hz repetition rate (CERN). (2) An excimer XeCl laser (6 J, 308 nm) focused onto solid targets (Frascati). (3) A femtosecond Ti: sapphire laser (250 mJ, 800 nm) directed onto a solid targets (Jena). (4) A picosecond Nd: yttrium–aluminum–garnet (0.3 J, 532 nm) focused into a dense medium of atomic clusters and onto solid targets (London). The preliminary experimental results and the most promising schemes will be discussed with respect to the scaling of the production of high numbers of highly charged ions. Different lasers are compared in terms of current density at 1 m distance for each charge state.
Scanning tunneling microscopy study of CdTe(001)
Authors: Seehofer L. , Falkenberg G. , Johnson R. L. , Etgens V. H. , Tatarenko S. , Brun D. , Daudin B.
Published: July 2002

Atomic resolution scanning tunneling microscope images of the (2×1) and c(2×2) reconstructions of CdTe(001) are presented. Both reconstructions can be described by a structural model in which the surface is terminated with 0.5 ML twofold-coordinated Cd atoms. Step edges, domain boundaries and various types of point defects are characterized. The measurements indicate that the surface atoms are highly mobile at room temperature.
Doppler-free modulation transfer spectroscopy of rubidium 52S1/2–62P1/2 transitions using a frequency-doubled diode laser blue-light source
Authors: Ito Nobuhiko
Published: July 2002

A frequency-doubled diode laser spectrometer in the blue-wavelength region with a continuous-frequency tuning range of up to ∼7 GHz has been developed, comprising a powerful fundamental light source based on a master/slave laser combination coupled with a KNbO3 crystal for second-harmonic generation in a power-enhancement cavity. It has been employed for the observation of saturated absorption spectra in 52S1/2–62P1/2 hyperfine transitions of atomic rubidium with the aid of a modulation transfer spectroscopic technique, and also the frequency stabilization to one of them has been demonstrated, suggesting the possibility of the establishment of a precision optical frequency marker in the blue range. A relative stability of better than 2.8×10−10 is estimated based on the locked error signal in the preliminary stabilization. With a further study of the frequency instability and uncertainty, in addition to supplementary improvements of apparatus, one may expect a realization of a stabilized laser in the blue-spectral domain. Details of the developed blue-light source are described and the demonstrated results of the rubidium spectroscopy are also described and discussed.
Method for simultaneously reducing the misalignment offset and separating the Hall voltage from the off-diagonal piezoresistive voltage in Hall effect and piezoresistive devices based on silicon
Authors: Mani R. G. , Klitzing K. von , Jost F. , Marx K. , Lindenkreuz S. , Trah H. P.
Published: July 2002

A novel misalignment offset reduction technique is extended in order to separate a piezoresistive voltage, from the Hall voltage, in doubly connected Hall elements based on silicon. In a special configuration, this method exploits directional averaging using biaxial current injection from four electrically separate current sources in order to cancel in situ the stress-generated off-diagonal piezoresistive voltage across the Hall voltage contacts. Measurements suggest field-equivalent offsets below 1 mT in (001) surface n-Si devices with current injection in the [110] direction.
Phonon mode and electronic bottleneck associated with the nonradiative relaxation in Ni2+-doped MgO
Authors: Demos S. G. , Han B. Y. , Alfano R. R.
Published: July 2002

The up-converted hot luminescence technique was utilized to obtain information on the nonradiative relaxation in Ni2+:MgO. The experiment shows the presence of an electronic bottleneck associated with the 3T1(t4e4) electronic state which was determined to be at 2.973±0.003 eV (23978±25 cm−1). The spectral profiles indicate that the 395±15 cm−1 phonon mode is involved in the nonradiative relaxation of the impurity Ni2+ ions.
Noise-enhanced optical heterodyning in an all-optical bistable system
Authors: Dykman M. I. , Golubev G. P. , Kaufman I. Kh. , Luchinsky D. G. , McClintock P. V. E. , Zhukov E. A.
Published: July 2002

A new form of frequency-selective all-optical heterodyning at the difference frequency of two modulated laser beams, related to stochastic resonance, is reported. A noise-induced enhancement of the heterodyne signal and of the signal-to-noise ratio has been predicted and observed in an all-optical bistable system for the beams at different wavelengths.
Second-harmonic generation of blue light in bulk periodically poled LiTaO3
Authors: Zhu Shi-ning , Zhu Yong-yuan , Yang Zhen-ju , Wang Hai-feng , Zhang Zhi-yong , Hong Jing-fen , Ge Chuan-zhen , Ming Nai-ben
Published: July 2002

We fabricated a periodic domain structure in a 0.5 mm thick slab of LiTaO3 crystal by applying an external pulsed field at room temperature. Using a pulse dye laser with a pulsewidth of 8 ns and a repetition rate of 10 Hz, third-order quasi-phase-matched second-harmonic blue-light generation is demonstrated by free propagation of a 862.6 nm fundamental Gaussian beam in a 4 mm long LiTaO3 slab, with a single pulse energy of 0.072 mJ and an energy conversion efficiency of 5.7%.
Atomic force microscopy imaging of transition metal layered compounds: A two-dimensional stick–slip system
Authors: Kerssemakers J. , De Hosson J. Th. M.
Published: July 2002

Various layered transition metal dichalcogenides were scanned with an optical-lever atomic force microscope (AFM). The microscopic images indicate the occurrence of strong lateral stick–slip effects. In this letter, two models are presented to describe the observations due to stick–slip, i.e., either as a static or as a dynamic phenomenon. Although both models describe correctly the observed shapes of the unit cell, details in the observed and simulated images point at dynamic nonequilibrium effects. This exact shape of the unit cell depends on cantilever stiffness, scan direction, and detector direction.
Background limited performance in p-doped GaAs/Ga0.71In0.29As0.39P0.61 quantum well infrared photodetectors
Authors: Hoff J. , Kim S. , Erdtmann M. , Williams R. , Piotrowski J. , Bigan E. , Razeghi M. , Brown G. J.
Published: July 2002

Background limited infrared photodetection has been achieved up to 100 K at normal incidence with p-type GaAs/Ga0.71In0.29As0.39P0.61 quantum well intersubband photodetectors grown by low-pressure metalorganic chemical vapor deposition. Photoresponse covers the wavelength range from 2.5 μm up to 7 μm. The device shows photovoltaic response, the cutoff wavelength increases slightly with bias, and the responsivity increases nonlinearly with bias. These effects are attributed to an asymmetric quantum well profile.
Direct jointing of Ba–Sr–Ca–Cu–O superconducting glass-ceramics by welding
Authors: Kasuga Toshihiro , Nakamura Koichi , Abe Yoshihiro , Inukai Eikichi , Nagaya Shigeo , Hirano Naoki
Published: July 2002

Superconducting glass-ceramics in the Bi–Sr–Ca–Cu–O (BSCCO) system were found to be successfully joined together by welding with an LNG-O2 flame. Two edge portions of the glass- ceramics to be joined were set in close contact to each other and melted by the high-temperature flame. After the welding, the joined portion was annealed with a soft flame, so that no crack formed in the product. The directly joined product was converted into a superconductor with Tc≂90 K by a postheating at ∼830 °C. Its critical current density Jc, was almost the same as the of the original glass-ceramic. This technique is very important for fabrication of large-sized superconducting apparatus such as current leads or magnetic shields.
Exploring the dissipative regime of superconductors for practical current-lead applications
Authors: Kunchur Milind N. , Christen D. K. , Klabunde C. E. , Salama Kamel
Published: July 2002

A pulsed-current technique was used to measure the extended I–V characteristics of a wide variety of prototype high-temperature-superconductor (HTS) leads. It was found that the average resistivity rises with J(≳Jc) more gradually than in conventional superconductors—often remaining very small compared to silver, for values of J(≫Jc) that are high enough to be practically useful. This observation, combined with the low thermal conductivity (∼50 times smaller than Ag), should extend the utility of HTS leads to the dissipative regime where J/Jc≫1.
Self-pulsating lasers with quantum well saturable absorber
Authors: Hoskens R. C. P. , van de Roer T. G. , van der Poel C. J. , Ambrosius H. P. M.
Published: July 2002

Self-pulsations are induced in broad area diode lasers by including an extra layer functioning as a saturable absorber. The absorption of this layer and its coupling to the photon field can be tuned via the layer thickness and its distance to the active layer. In the case of an Al0.13GaAs bulk active layer a GaAs absorber of 4 nm thickness at a distance of 0.2 μm induces stable oscillations with frequencies of 300–700 MHz in a current range just above the threshold.
2.7-μm InGaAsSb/AlGaAsSb laser diodes with continuous-wave operation up to −39 °C
Authors: Garbuzov D. Z. , Martinelli R. U. , Menna R. J. , York P. K. , Lee H. , Narayan S. Y. , Connolly J. C.
Published: July 2002

We have demonstrated continuous wave operation of 2.7-μm InGaAsSb/AlGaAsSb multiquantum-well diode lasers up to a temperature of 234 K (−39 °C). These devices were grown by molecular-beam-epitaxy. They have a tendency to operate in a dominant single mode over well-defined temperature and current intervals. A comparison of spontaneous emission spectra shows that above threshold, the quasi-Fermi level is pinned and that most of the carriers are injected into nonlasing states. This effect leads to a rapid decrease of differential efficiency with increasing temperature.
Optical study of good quality InGaP/GaAs quantum wells: Influence of the indium content around the lattice-matched composition
Authors: Martínez-Pastor Juan , González Luisa , Roussignol Philippe
Published: July 2002

High structural and optical quality InxGa1−xP/GaAs quantum wells, with x from 0.51 to 0.45, have been successfully grown by atomic layer molecular beam epitaxy. In that compositional range, an important blue shift of the quantum well luminescence lines is observed, which is explained by an increase of the conduction band gap offset from compressive to tensile strain conditions. The luminescence intensity decreases with temperature above 20–30 K, which is attributed to impurities located at the interfaces and inside the quantum wells. The influence of the In content on the oscillator strength of the optical transitions is also evaluated.
Effect of the oxide-desorption temperature on the substrate–epilayer interface charge in organometallic vapor-phase epitaxy of GaAs
Authors: Kim T. S. , Files L. A. , Magel L. K.
Published: July 2002

We found that the use of relatively low temperatures during oxide desorption in organometallic vapor-phase epitaxy of GaAs reduced the epilayer–substrate interface charge. No differences in Hall mobility were found between the samples grown with 550 and 650 °C desorption. O, C, Si, and S were the dominant impurities at the interface. Si and S, which are shallow donors, induce the interface-charge accumulation. Low-temperature desorption leaves oxygen at the surface, suppressing the activation of shallow dopants. A high level (≳1×1012/cm2) of Cl was detected on every GaAs substrate we measured, but its effect on epitaxy is not clear.
Auger electron spectroscopy of the hydrogen terminated chemical vapor deposited diamond surface
Authors: Krainsky I. L. , Mearini G. T. , Asnin V. M. , Sun H. , Foygel M. , Petukhov A. G.
Published: July 2002

A simultaneous study of Auger spectra and secondary electron emission from the chemical vapor deposited diamond films under extended electron beam exposure is presented. Up to a 1.2 eV increase in energy of the carbon Auger peak accompanied by the decrease of the total secondary electron yield has been found. Exposure to hydrogen has resulted in recovery of the both Auger peak position and secondary yield. First-principles electronic structure calculations have shown that the effect is due to a change in the surface upper-valence band local density of states of the diamond crystal which is dependent on the extent of hydrogen coverage of the surface.
Effect of hydrostatic pressure in lead indium niobate-based complex perovskite systems
Authors: Yasuda Naohiko , Ohwa Hidehiro , Mizuno Takuya , Iwata Makoto , Ishibashi Yoshihiro
Published: July 2002

Dielectric properties of (1−x)Pb(In1/2Nb1/2)O3−xPbZrO3(Zr-PIN) and (1−x)Pb(In1/2Nb1/2) O3−xPb(Yb1/2Nb1/2)O3(Yb-PIN) mixed crystal systems were investigated under hydrostatic pressures up to 0.7 GPa. There are striking pressure effects in these systems. In Zr-PIN (x=0.1) the temperature Tm, indicating the maximum of the dielectric constant, εrm′, decreases at the rate of dTm/dp|p=0=−54 °C/GPa, and the εrm′ decreases with increasing pressure, while in Zr-PIN (x=0.34) the Tm decreases at the rate of dTm/dp|p=0=−100 °C/GPa, and the εrm′ decreases and the maximum value of the tan δ, denoted as tan δm, increases with increasing pressure. For Yb-PIN (x=0.16) the Tm increases at the rate of dTm/dp=155 °C/GPa, both the εrm′ and the tan δm decrease with increasing pressure.
Nonresonant electron tunneling in cluster superlattice of tellurium in zeolite
Authors: Tang Zi Kang , Wang Xiang-Rong
Published: July 2002

We study the current-voltage (I–V) characteristics of tellurium (Te) cluster superlattice in Linde type-A (LTA) zeolite. We observed both the hysteresis current loop and the current peak. The current loop and the current peak, however, are history dependent. Under the same experimental conditions, we find that the current peak position moves to the higher voltage when the sample temperature is increased. We clarify that both the current loop and peak are transient phenomena. The observed current peak is a phenomenon of the pseudonegative differential resistance. We explain the observed I–V characteristics as the results of the structure change of Te clusters inside zeolite cages under an external electric field. The current peak can be understood as the effect of the finite transient time during which Te clusters change from one structure into another under an applied voltage.
Magnetic properties of melt-textured YBa2Cu3O7 prepared in a solar furnace
Authors: Reich S. , Godin T.
Published: July 2002

Magnetic properties of bulk melt-textured YBa2Cu3O7 prepared in a solar furnace were investigated. The material obtained demonstrates high critical current density of the order of 105 A/cm2 in the field of a 1 T at 50 K. The effective activation energy for flux creep was calculated using Maley’s approach and fits the universal relation: Ueff=U0G(T)(J/Jc)−n, where n=3/2, which agrees with the result predicted by the collective flux creep theory.
Validation of a scattering state model for liquid crystal polymer composites
Authors: Bosc D. , Trubert C. , Vinouze B. , Guilbert M.
Published: July 2002

With a view to making highly efficient light shutters, the object of this study is to know the best structure of the liquid crystal polymer composite yielding a high extinction ratio. The main feature of this letter shows that experimental results are consistent with anomalous diffraction approach. For our demonstration, we made polymer dispersed liquid crystal (PDLC) cells having different droplet radius (a), ranging from 0.5 to 1 μm. In addition, a strong wavelength sensitivity with these radii is measured in the visible region. Beyond one micron, droplets yield higher transmission as regards both the wavelength and the optical anisotropy of the liquid crystal. These results bring forward that the most efficient PDLC films must follow the optimization relation: a(Δn)/λ=0.3, where λ is the wavelength and Δn the liquid crystal anisotropy.
Surface modification of silicon (111) by annealing at high temperature in hydrogen
Authors: Zhong Lei , Takeda Ryuji , Izunome Koji , Matsushita Yoshiaki , Aiba Yoshiro , Matsushita Junichi , Yoshikawa Jun , Hayashi Kenro , Shirai Hiroshi , Saito Hiroyoki
Published: July 2002

A vicinal silicon (111) surface exhibits well defined single steps after being annealed at 1200 °C in hydrogen, which is in sharp contrast with step bunches featuring the surface annealed in argon. As a temporary explanation for its ability to unzip the step bunches, we suggest that hydrogen destroys the faulted triangles of a [112̄] step, eliminates this kind of step, and eventually leaves the single [1̄1̄2] steps alone behind on the surface.
Use of annealed low-temperature grown GaAs as a selective photoetch-stop layer
Authors: Chen E. H. , McInturff D. T. , Chin T. P. , Melloch M. R. , Woodall J. M.
Published: July 2002

We demonstrate annealed low-temperature grown (LTG) GaAs to be a highly effective etch-stop layer while photoetching n-type normal-growth-temperature GaAs. During this process, the etch rate is controlled by the transport of photogenerated carriers to the semiconductor/electrolyte interface. Because of the very short minority carrier lifetime in LTG-GaAs, only a very small portion of photogenerated carriers can reach the semiconductor surface to complete the electrolytic decomposition reaction. Therefore, the etch rate of LTG-GaAs is reduced considerably. In our studies, the etch rate selectivity can be as high as 800. Furthermore, we demonstrate that doping concentration, presence of an ohmic contact, photon energy, and pH value of the etching solution can be used to control the etch rate of n-GaAs during the photoetching process.
Transient photovoltaic properties in Al/tin-phthalocyanine/indium–tin–oxide sandwich cell
Authors: Pan Yong-Le , Chen Ling-Bing , Wang Yan , Zhao You-Yuan , Li Fu-Ming , Wagiki A. , Yamashita M. , Tako T.
Published: July 2002

The intensity dependence of the transient photovoltaic response in the nanosecond region and the spectral dependence of the continuous wave (cw) photovoltaic response in Al/α-SnPc/ITO sandwich cell have been reported. The cw photovoltaic action spectrum is similar to the absorption spectrum. The magnitude, polarity, and response time of the transient photovoltage are found to be dependent on the intensity and wavelength of the incident light, which offers a potential application in the area of light-controlled nonlinear optoelectronic detectors.
Birefringence and tilted modes in ordered GaInP/AlGaInP waveguides and lasers
Authors: Moritz A. , Wirth R. , Geng C. , Scholz F. , Hangleiter A.
Published: July 2002

We have studied the influence of chemical ordering on the properties of GaInP/AlGaInP optical waveguide structures and lasers. We have observed mode-conversion between transverse electric (TE) and transverse magnetic (TM) modes within typically 60 μm. We show that an ordering-induced birefringence with the optical axis tilted with respect to the growth direction is responsible for the coupling of the TE and TM modes. A new linear combination of TE and TM modes, the ‘‘super modes,’’ can propagate in the waveguide without change. We show that these super modes govern the polarization of lasers made from ordered material.
Voltage-tunable two-color detection by interband and intersubband transitions in a p-i-n-i-n structure
Authors: Schneider H. , Schönbein C. , Bihlmann G.
Published: July 2002

We report on a novel class of voltage-tunable two-color detectors, based on a quantum-well intersubband photodetector (QWIP) and a p-i-n photodiode in a p-i-n-i-n configuration. In this approach, the two constituting detector structures can be optimized independently because of their different conductance characteristics. The detection concept is demonstrated for the case of an 8-10μm GaAs/AlGaAs QWIP and a 850 nm p-i-n diode containing In0.08Ga0.92As quantum wells.
ab-plane anisotropy of the critical currents in twinned YBa2Cu3O7−δ superconductors
Authors: Safar H. , Foltyn S. , Kung H. , Maley M. P. , Willis J. O. , Arendt P. , Wu X. D.
Published: July 2002

We show that the in-plane critical current density of heavily twinned YBa2Cu3O7−δ thick films is anisotropic, depending on the relative orientation between the transport electrical current and the twin boundary mosaic. We demonstrate that the critical current density in an applied magnetic field can be substantially increased by producing conductors with twin boundaries tilted 45° with respect to the electrical current flow. We show that these findings can be readily incorporated to improve the critical currents of biaxially textured YBa2Cu3O7−δ thick films deposited on metallic substrates.
Parabolic quantum wells of diluted magnetic semiconductor Cd1−xMnxTe
Authors: Wojtowicz T. , Kutrowski M. , Surma M. , Kopalko K. , Karczewski G. , Kossut J. , Godlewski M. , Kossacki P. , The Khoi Nguyen
Published: July 2002

We report on the growth by molecular beam epitaxy of II-VI semiconductor parabolic quantum wells. Cd1−xMnxTe diluted magnetic semiconductor was chosen for constructing these structures because of its extreme sensitivity to the presence of a magnetic field. Photoluminescence excitation spectra revealed several series of energetically equidistant peaks due to interband optical transitions. The energy separation in a given series scales proportionally to the inverse width of the quantum well as expected for a parabolic confining potential. Analysis of the spectra made possible a model-independent determination of the valence band offset in CdTe/Cd1−xMnxTe.
Mark observation of TbFeCo recorded on land/groove substrate by spin-polarized scanning electron microscopy
Authors: Kohashi Teruo , Matsuyama Hideo , Haginoya Chiseki , Koike Kazuyuki , Miyamoto Harukazu , Ushiyama Junko
Published: July 2002

Using spin-polarized scanning electron microscopy, the laser power dependence of recorded mark shapes and sizes is studied for the TbFeCo, magneto-optical recording medium, prepared on a land/groove-shaped substrate. The bit width which increases with laser power is suppressed to the land or groove width during higher power, showing that the land/groove border acts as a barrier to bit expansion. These results confirm the advantages of a land/groove shaped medium for high density recording.
The effect of stress on the resistivity of submicron aluminum lines
Authors: Beverly N. L. , Alers G. B. , Prybyla J. A.
Published: July 2002

The stress coefficient of resistivity (piezoresistance coefficient) of passivated submicron Al-Cu lines was determined using a wafer-bending technique. This coefficient is significant for the interpretation of early resistance changes in reliability lifetime measurements. At room temperature the effect of stress on the resistivity of the lines is negligible. From 110 to 200 °C the piezoresistance coefficient is roughly constant at (Δρ/ρ)/Δσ∼1.2×10−5 MPa−1. Resistance changes associated with stress relaxation in the vicinity of a stress-induced void will be discussed.
Shallow donor neutralization in CdTe:In by atomic hydrogen
Authors: Gurumurthy Suma , Bhat H. L. , Sundersheshu B. , Bagai R. K. , Kumar Vikram
Published: July 2002

Passivation effects in In doped n-CdTe upon exposure to rf hydrogen plasma are studied by electrical and photoluminescence measurements and were found to be maximum at about 150 °C. Depth profiling by capacitance-voltage measurements show passivation of approximately an order of magnitude at 150 °C and 50% at 170 °C. No visual surface damage is seen. Reverse bias annealing experiments show that atomic hydrogen drifts in a charged state in n-CdTe, with reactivation kinetics of the donors different from the other well studied semiconductors. Manifestation of the donor passivation in photoluminescence is seen by the reduction of the donor bound luminescence.
Direct scanning tunneling microscopy observation of non-unit-cell growth of YBa2Cu3O7−δ thin films
Authors: Haage T. , Jiang Q. D. , Cardona M. , Habermeier H.-U. , Zegenhagen J.
Published: July 2002

The surface morphology of thin YBa2Cu3O7−δ films grown on LaSrAlO4 (001) and SrTiO3 (001) by pulsed laser deposition was studied by scanning tunneling microscopy (STM). STM was carried out in ultra high vacuum without prior exposure of the films to air. In the STM images, stacks of terraces which are separated by steps of various heights were observed. Some steps were shorter than the YBa2Cu3O7−δ unit cell height c≊1.2 nm. We present evidence of the fact that the YBa2Cu3O7−δ thin film surface exhibits different terminations. The microstructure revealed from the STM images implies a non-unit-cell growth mechanism.
Current conduction in quantum well infrared photodetectors under low bias operation
Authors: Wang Daniel C. , Bosman Gijs , Li Sheng S.
Published: July 2002

Device operation under low applied bias voltage has been examined for both p-type and n-type quantum well infrared photodetectors. Under low electric fields and at low temperatures, the carrier lifetime in the quantum well bound states is large compared to the intrawell carrier scattering time, allowing the carriers to thermalize in the heavily doped quantum wells. This thermalization process partially or fully decouples the statistical fluctuations in carrier transport through a barrier region from one section to the next. Therefore, the device can be thought of as M statistically independent sections in series, where M is equal to the ratio of active device length over the extended state carrier trajectory. Based on this concept and the additional assumption that the charge transport process is emission limited, the dark current was calculated for four different devices. The excellent agreement between the measured and calculated dark current–voltage characteristics confirms our model assumptions.
High-precision laser cutting of high-temperature superconductors
Authors: Schuster Th. , Kuhn H. , Raiber A. , Abeln T. , Dausinger F. , Hügel H. , Kläser M. , Müller-Vogt G.
Published: July 2002

A laser-cutting technique was used to pattern high-temperature superconducting single crystals with high precision into various shapes. The low mean power of 135 mW of the laser enables one to produce cuts of arbitrary shape without affecting the superconducting parameters. This was proven by local observations of the magnetic flux distribution using the magnetooptical Faraday effect.
Grazing incidence x-ray diffraction studies of the initial growth of Pd on MgO(001)
Authors: Fornander H. , Birch J. , Hultman L. , Petersson L.-G. , Sundgren J.-E.
Published: July 2002

Grazing incidence x-ray diffraction (GIXRD) using a standard laboratory x-ray source was used to study the structure of thin palladium films, with nominal thicknesses varying between 0.5 and 40 nm. The films were grown by electron beam evaporation on MgO(001) at 600 °C. Epitaxial [001]-oriented grown Pd was detected for all the films. For nominal film thicknesses <2.5 nm, the lateral Pd d020 plane distance was expanded as much as ∼0.8%, whereas thicker films had bulk Pd lattice spacing. The average Pd particle size, estimated from the peak broadening, was found to increase from 4 to 100 nm as the film thickness increased from 0.5 to 20 nm. For 2.5 and 10 nm thick films, four 111-oriented Pd domains rotated 90° with respect to each other were also detected.
Highly reliable chemical vapor deposited stacked oxynitride gate dielectrics fabricated by in situ rapid thermal multiprocessing
Authors: Yan J. , Han L. K. , Kwong D. L.
Published: July 2002

High-quality chemical vapor deposited (CVD) stacked oxynitride gate dielectrics have been fabricated by in situ rapid thermal multiprocessing. Bottom thin oxynitrides were formed by rapid thermal processing (RTP) of Si in nitric oxide (NO) or nitrous oxide (N2O) ambient, followed by rapid thermal chemical vapor deposition (RTCVD) of SiO2 films using SiH4 and N2O. The stacked dielectrics were then subjected to in situ rapid thermal annealing in an O2 ambient. Results show that CVD stacked oxynitride gate dielectrics have improved endurance to interface state degradation, higher charge to breakdown values, and significantly reduced defect densities compared to control thermal gate oxide.
Localization-induced blueshift in quantum wires: Direct evidence from magnetophotoluminescence measurements
Authors: Lehr G. , Härle V. , Scholz F. , Schweizer H.
Published: July 2002

For In0.53Ga0.47As/InP quantum wires fabricated from quantum well material by dry etching and epitaxial regrowth, a blueshift with decreasing wire width is found, where the blueshift is larger than expected by lateral quantization. In magnetophotoluminescence, the quantum well clearly displays a feature that is due to carrier transport between growth islands of different well widths. This feature cannot be observed for small wire structures, which means that the carrier transport is suppressed in these structures. Hence, magnetophotoluminescence measurements give new direct evidence for the fact that the excitons are more and more localized in states of higher energy as the wire width is decreased, which explains the blueshift in the optical spectra.
Spurious momentum mismatch introduced by an approximate model of acousto-optic interactions
Authors: Wu Yih-Tyng , Anderson Betty Lise
Published: July 2002

In acousto-optic interactions, the concept of momentum mismatch introduced by some books is misleading and nonexistent because the mismatch itself is caused by the approximate model used to explain the interactions. The so-called mismatch is very small and should not be handled with an approximate model. We use an exact model to satisfy Bragg condition, conservation of energy, and conservation of momentum. The difference between the exact model and the approximate model is actually what causes the mismatch.
Dosimetric properties of KCl:Eu2+ under α, β, γ, x ray, and ultraviolet irradiation
Authors: Meléndrez R. , Pérez-Salas R. , Pashchenko L. P. , Aceves R. , Piters T. M. , Barboza-Flores M.
Published: July 2002

The thermoluminescence (TL) characteristics of KCl:Eu2+ subjected to α, β, γ, x ray, and ultraviolet (UV) radiation (200–300 nm), as well as the dose response was investigated. The TL glow curve structure is very similar for any type of irradiation used with differences only in the peak intensities; it has good reproducibility. Also, the dose response is linear for all types of irradiations. These results allowed us to conclude that the KCl:Eu2+ crystals may be used as a dosimeter for ionizing and nonionizing radiation
Optical matrix for clock distribution and synchronous operation in two-dimensional array devices
Authors: Lee K. S. , Shu C.
Published: July 2002

A scheme to generate an optical matrix from a mode-locked Nd:YAG laser has been theoretically explored and experimentally demonstrated. The matrix consists of highly synchronized and sequentially delayed optical pulses suitable for use with two-dimensional array optoelectronic devices and clock distribution system. The output pulses have the same state of polarization and no timing jitter is produced among the elements. Encoded outputs have been generated from the matrix using a set of photomasks. This technique can be applied to high-speed optical parallel processing.
Computed light scattering cross sections of oxide particles in silicon
Authors: Laczik Z. , Booker G. R.
Published: July 2002

We have used the discrete-dipole approximation (DDA) method, implemented as a modified version of the DDSCAT program of Draine and Flatau, to calculate 3D light scattering intensity diagrams for individual oxide particles present in Czochralski silicon wafers. The particles were either octahedra or plates with normalized sizes x=0.01–5. The signals that would be measured for light scattered through 90° or 180° were determined from the diagrams, these angles corresponding to the cases of light scattering tomography (LST) and reflection confocal (RC) scanning infrared microscopy (SIRM), respectively. The results show that as x increases, the signal (particle image contrast) increases ∝x6 for x<∼1, but increased more slowly and in an irregular manner for x≳∼1. The signal also depends markedly on the particle shape and orientation. These findings demonstrate the difficulty of deducing quantitative data, e.g., individual particle sizes, from SIRM image contrasts. However, they also indicate the type of measurements that need to be made and provide a basis for the quantitative interpretation of the experimental results.
Se species in metalorganic molecular beam epitaxy of ZnSe
Authors: Qiu Y. , El-Emawy A. A. , Osinsky A. , Littlefield E. , Temkin H.
Published: July 2002

Growth of ZnSe on GaAs by metalorganic molecular beam epitaxy with dimethylselenide and metal Zn sources is investigated. We show that the decomposition of dimethylselenide is pyrolithic and achieve cracking efficiencies as high as 90% at a cracker temperature of 1200 °C. With an efficient cracker high growth rates of ZnSe are obtained, limited only by the flux of dimethylselenide. Mass spectroscopic investigations demonstrate that atomic Se is the dominant group VI species. The high sticking probability of Zn to atomic Se results in metalorganic molecular beam epitaxy with high growth efficiency, exceeding that of conventional molecular beam epitaxy.
Organochloride chemically enhanced focused ion beam micromachining of permalloy
Authors: Thaus D. M. , Stark T. J. , Griffis D. P. , Russell P. E.
Published: July 2002

In order to reduce the potentially deleterious corrosion effects associated with gases such as Cl2, I2, and XeF2, three noncorrosive organochlorides [pentachloroethane (C2HCl5), carbon tetrachloride (CCl4), and tetrachloroethene (C2Cl4)], have been investigated as more ideal chlorine sources for chemically enhanced focused ion beam micromachining (CE-FIBM) of Permalloy (Ni–Fe alloy). C2Cl4 increased the sputter yield of Permalloy by a factor of 2 while reducing micromachining generated topography from several micrometers (FIBM) to less than 100 Å (CE-FIBM). Analysis of material removal rate versus pattern refresh time indicated that material removal occurred via a chemically assisted process.
Microcrystallography using atomic force microscopy
Authors: Helbig H. F. , Beasock J. V. , Ramseyer G. O. , Walsh L. H.
Published: July 2002

Scanning microscopy can be used to obtain crystallographic information about suboptical structures. By the same token, the x-y-z length scales of scanning microscopy can be brought into mutual consistency by measuring the angles between facets of crystallites of known habit.