bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of As overpressure during annealing on the nonuniformity of activation efficiency in Si-implanted GaAs layer
Authors: Sato Takashi , Tajima Michio , Ishida Koichi
Published: July 2002

The effects of As overpressure during annealing on nonuniformity, i.e., the variations of sheet carrier concentration and Hall mobility across wafers, have been studied for Si-implanted undoped semi-insulating GaAs. The nonuniformity was found to be suppressed by annealing under high As overpressure. It is suggested that the high As overpressure suppresses the As vacancy generation during annealing which may induce the nonuniformity due to the local variation of compensation ratio of [SiAs]/[SiGa] around a dislocation.
2.2 μm GaInAsSb/AlGaAsSb injection lasers with low threshold current density
Authors: Caneau C. , Zyskind J. L. , Sulhoff J. W. , Glover T. E. , Centanni J. , Burrus C. A. , Dentai A. G. , Pollack M. A.
Published: July 2002

Double heterostructure 2.2 μm wavelength lasers were fabricated from Ga0.84In0.16As0.15Sb0.85/ AlxGa1−xAs0.04Sb0.96 wafers grown by liquid phase epitaxy. These structures were grown with Al-rich confinement layers (x=0.4) for optical confinement and thin intermediate cladding layers (x=0.34) to relieve the strain resulting from the lattice mismatch of the Al-rich confinement layers with respect to the substrate and the active layer. A threshold current density as low as 1.7 kA/cm2 was obtained at room temperature.
Ellipsometric analysis of built-in electric fields in semiconductor heterostructures
Authors: Snyder Paul G. , Oh Jae E. , Woollam John A. , Owens R. E.
Published: July 2002

Measurements of Franz–Keldysh (FK) effects in GaAs-AlxGa1−xAs heterostructures by variable angle of incidence spectroscopic ellipsometry are reported. The measured FK effects are due to the built-in band bending at the surface and heterointerfaces, with no externally applied bias or modulating voltage. The polarity of the FK line shape indicates whether the field is uniform or nonuniform (inhomogeneous) in the direction along the growth axis, while the linewidth is determined by the peak field strength. This information can be used to characterize the doping concentration in the AlxGa1−xAs layer.
Electron mobility in p-GaAs by time of flight
Authors: Ahrenkiel R. K. , Dunlavy D. J. , Greenberg D. , Schlupmann J. , Hamaker H. C. , MacMillan H. F.
Published: July 2002

The minority-carrier mobility of electrons in metalorganic chemical vapor deposition grown p-GaAs has been measured by a diffusion time-of-flight technique. Doping levels of 1×1017 and 2×1018 cm−3 were investigated. The measured mobilities were about 2900 and 1300 cm2/V s, respectively. The minority-carrier mobilities are lower than the expected majority-carrier mobilities at the same doping levels. The lower mobility is caused by heavy-hole scattering.
Characterization of interface defects in GaAs-GaAlAs superlattices
Authors: Mauger A. , Feng S. L. , Bourgoin J. C.
Published: July 2002

Capacitance transients in a 20-20 Å GaAs-Ga0.7 Al0.3As n-type (3×1016 cm−3) Si-doped superlattice have been performed. In the temperature range 50–100 K, a logarithmic time decay is observed. We attribute this time decay to the existence of a continuum of deep levels in the fundamental gap of the superlattice, related to electron traps located at the GaAs-GaAlAs interfaces.
Nonlinear excitonic optical absorption in GaInAs/InP quantum wells
Authors: Fox A. M. , Maciel A. C. , Shorthose M. G. , Ryan J. F. , Scott M. D. , Davies J. I. , Riffat J. R.
Published: July 2002

We have studied the saturation of optical absorption in GaInAs/InP quantum wells at room temperature. Using optical excitation from a tunable cw Co:MgF2 laser, we find a saturation intensity of 70 W cm−2 when exciting resonantly at the n=1 heavy-hole exciton, and we deduce values of the nonlinear absorption and refraction coefficients −60 cm W−1 and −0.3 cm2 kW−1, respectively. The saturation intensity in the quantum well is significantly lower than in bulk GaInAs, and also in GaAs quantum wells.
Effect of active layer thickness on differential quantum efficiency of 1.3 and 1.55 μm InGaAsP injection lasers
Authors: Cheng W. H. , Su C. B. , Renner D.
Published: July 2002

The dependence of differential quantum efficiency (ηd) on active layer thickness (d) for 1.3 and 1.55 μm InGaAsP buried crescent (BC) injection lasers has been measured. A comparison of the results shows that ηd for 1.55 μm lasers increases more rapidly with decreasing d than ηd for 1.3 μm lasers. The significantly different dependence of ηd on d in BC lasers suggests that the optical absorption in the active region of InGaAsP lasers is strongly wavelength dependent. This gives the important practical conclusion that the ηd for 1.55 μm lasers can be significantly improved by reducing d, whereas the ηd for 1.3 μm lasers can only be slightly improved by reducing d. As a result of ηd vs d investigation, we have obtained high performance 1.3 and 1.55 μm BC lasers which exhibit threshold currents as low as 9 mA at 25 °C, high-temperature operation (up to 100 °C), and ηd over 65% (1.3 μm) and 45% (1.55 μm).
Noncontact measurement of thermal conductivity of epoxy bonds by pulsed photothermal radiometry
Authors: Leung W. P. , Tam A. C.
Published: July 2002

Pulsed photothermal radiometry relies on the flash heating of a surface and the detection of the time dependence of the surface cooling by infrared radiometry. We show that this is useful as a nondestructive and noncontact technique to quantify the thermal conductivity of a subsurface epoxy bond underneath an opaque surface. The degree of curing of the epoxy can also be monitored.
Dimer arsenic source using a high efficiency catalytic cracking oven for molecular beam epitaxy
Authors: Garcia J. C. , Barski A. , Contour J. P. , Massies J.
Published: July 2002

The cracking efficiency of a catalytic double-oven arsenic dimer source has been studied by modulated beam mass spectrometry and the catalytic activity of several materials investigated through the cracking of the tetramer molecules produced from crystalline arsenic with a pyrolytic boron nitride (pBN) oven. The catalytic activity decreases as follows: Pt, Pt-Rh>Re>Ta>Mo, W-Re>graphite>pBN. Platinum and its alloys with rhodium react with arsenic above 500 °C giving definite compounds and therefore cannot be used as a catalyst. A 95% conversion efficiency is obtained with rhenium around 700 °C, a temperature which is more than 300 °C lower than the temperature required with graphite for an equivalent efficiency. Such a decrease of the operating temperature of the cracker cell is of great practical interest because of the reduction of the arsenic flux contamination by outgassing impurities.
Chirp and cw linewidth measurements of integrated external cavity lasers
Authors: Dutta N. K. , Cella T. , Piccirilli A. B. , Brown R. L. , Green C.
Published: July 2002

Single-frequency integrated external cavity distributed Bragg reflector lasers emitting near 1.5 μm have been fabricated. The chirp and cw linewidth measurements of these devices show that lasers with long external cavity exhibit lower chirp and lower cw linewidth compared to lasers with short external cavity. The experimental data are explained by using theoretical calculations.
Diffusion of reactive ion beam etched polymers
Authors: Tead Stanley F. , Vanderlinde William E. , Ruoff Arthur L. , Kramer Edward J.
Published: July 2002

The diffusion of deuterated polystyrene (d-PS) in a polystyrene matrix was used to probe the damage to the polymer surface caused by reactive ion beam etching (RIBE). Diffusion was seen to be hindered in a d-PS film treated by RIBE, an immobilization apparently due to crosslinking of the surface monolayer of the polymer sample.
Ytterbium monolayer diffusion barriers at Hg1−xCdxTe/Al junctions
Authors: Franciosi A. , Raisanen A. , Wall A. , Chang S. , Philip P. , Troullier N. , Peterman D. J.
Published: July 2002

Single layers of Yb at the Hg1−xCdxTe(110) interface prevent Al-Te reaction and dramatically increase the Hg concentration at the interface. Synchrotron radiation photoemission studies of the interface as a function of Al deposition show a two orders of magnitude increase in the Hg/Te core intensity ratio as a result of the interlayer-induced change in atomic interdiffusion. Calculations of thermodynamic parameters following a semiempirical alloying model suggest that other rare earths should also act as effective diffusion barriers at mercury-cadmium-telluride/reactive metal junctions.
Effects of processing conditions on negative bias temperature instability in metal-oxide-semiconductor structures
Authors: Lu D. , Ruggles G. A. , Wortman J. J.
Published: July 2002

Radiation from electron beam metallization has been found to result in damage to metal-oxide-semiconductor structures, which, when not properly annealed out, can lead to severe negative bias temperature instability (NBTI), even at room temperature. Similarly, rapid thermal annealing (RTA) after oxidation can result in significant room-temperature instability. The post-metal-annealing behavior of this RTA-induced instability depends on the high-temperature annealing ambient (Ar vs O2). Processing in O2 can also lead to significant NBTI; however, following such steps by an Ar anneal at the same temperature can negate the effect, and in some cases, lead to an overall reduction in NBTI. An electrical current flows during the stressing measurement and can be correlated with the development of oxide positive charge. This finding supports a model for charge buildup during NBT stressing in which holes from the Si substrate become trapped in intrinsic hole traps located within the oxide.
Novel high voltage transistor fabricated using the i
n s
i
t
u junctions in a Si-TaSi2 eutectic composite
Authors: Ditchek B. M. , Middleton T. R. , Rossoni P. G. , Yacobi B. G.
Published: July 2002

Transistor action has been observed for the first time in a Si-TaSi2 eutectic composite. These devices, utilizing the in situ cylindrical Schottky junctions between the Si matrix and the TaSi2 rod phase, have characteristics typical of a metal-semiconductor field-effect transistor (MESFET). However, unlike a conventional planar device like a MESFET, eutectic transistors are resistant to avalanche breakdown. A device is demonstrated that blocks 600 V, a value that is three times larger than would be expected for a planar device of the same carrier concentration.
Generation of an anomalous hole trap in GaAs by As overpressure annealing
Authors: Plano M. A. , Plano W. E. , Haase M. A. , Bose S. S. , Holonyak N. , Stillman G. E.
Published: July 2002

Deep levels in high-purity n-type molecular beam epitaxy (MBE) GaAs and in undoped n-type metalorganic chemical vapor deposition (MOCVD) GaAs samples annealed with various As overpressures were investigated using constant capacitance deep level transient spectroscopy on evaporated Au Schottky barrier diodes. Anomalous hole traps, which could be measured because of a surface effect, were observed in all annealed samples. EL2 traps were created in the MBE material by the annealing, while the concentration of EL2 in the annealed MOCVD material was about the same as that before annealing. The effect of annealing on the other electron traps in these samples is also studied and reported.
Optical properties of very narrow GaInAs/InP quantum wells grown by low-pressure metalorganic vapor phase epitaxy
Authors: Grützmacher D. , Wolter K. , Jürgensen H. , Balk P. , Bulle Lieuwma C. W. T.
Published: July 2002

Single and multiple GaInAs/InP quantum well structures with well widths of 0.5–20 nm have been grown by metalorganic vapor phase epitaxy at 20 mbar using TMG, TMI, PH3, and AsH3. Low-temperature (2 K) photoluminescence and transmission electron microscopy were used to characterize the structures obtained for various growth conditions. For the optimized deposition process the interfacial roughness amounts to less than one monolayer. The narrowest exciton linewidth obtained for a 20-nm well was 2.2 meV; the largest energy shift was observed for a 0.5-nm well and amounted to 528 meV.
Microstructures of YBa2Cu3O7−x superconducting thin films grown on a SrTiO3(100) substrate
Authors: Chen C. H. , Kwo J. , Hong M.
Published: July 2002

We report microstructures of superconducting YBa2Cu3O7−x thin films epitaxially grown on a SrTiO3(100) substrate. Transmission electron microscopy studies reveal epitaxial, highly ordered grains ∼5000 Å in size for both types of films with either the a or c axis perpendicular to the film. Defects due to out-of-phase boundaries are observed in the former case. The high density of twin boundaries found in the latter case seems to correlate with the higher Jc observed in thin-film samples.
Heterojunction acoustic charge transport devices on GaAs
Authors: Tanski William J. , Merritt Sears W. , Sacks Robert N. , Cullen Donald E. , Branciforte Emilio J. , Carroll Roger D. , Eschrich Timothy C.
Published: July 2002

We have demonstrated a new type of buried-channel acoustic charge transport device in which charge is transported in an (Al,Ga)As/GaAs/(Al,Ga)As heterojunction channel. Traveling-wave potential wells, associated with a surface acoustic wave (SAW) propagating on the 〈100〉 surface of a GaAs crystal, transport electrons at the SAW velocity by means of a large-signal acoustoelectric interaction. Heterojunction acoustic charge transport (HACT) delay lines have been fabricated, and the transport of charge demonstrated. Charge packets with up to 16×106 electrons/cm were measured in a delay 1.4 μs long. The HACT device is much simpler, the transport channel is more reliably produced (by molecular beam epitaxy or metalorganic chemical vapor deposition), and the device has potential for higher dynamic range when compared to the previously developed acoustic charge transport technology. This new device type is useful for the implementation of high-speed monolithic signal processors.
Control of TM waves by strong TE nonlinear guided waves
Authors: Shen T. P. , Stegeman G. I. , Maradudin A. A.
Published: July 2002

We show that a TM-polarized wave can propagate along the interface between a linear and a nonlinear dielectric medium, provided that a high-power TE-polarized wave is also present. On the basis of the threshold powers for the two waves, we predict that 100% modulation of the TM wave by the strong beam is possible.
Thin-film YBaCuO superconductors formed by Cu/BaO/Y2O3 layer structures
Authors: Chang Chin-An , Tsuei C. C. , Chi C. C. , McGuire T. R.
Published: July 2002

Thin-film superconductors of the YBaCuO family have been formed using layer structures of Cu, BaO, and Y2O3 deposited on different substrates by electron beam evaporation. Using a thickness ratio of Cu, BaO, and Y2O3 layers off that of the stoichiometric YBa2Cu3O7 phase, superconducting films were formed on MgO and SrTiO3 substrates, with a Tc for zero resistance exceeding 77 K. The films were found to be composed of the superconducting YBa2Cu3O7 phase and the green Y2BaCuO7 phase, with the excessive Cu repelled to the surface in an oxidized form. The studies also provide information on the dependence of the superconducting transition on the annealing temperatures and the substrates used.
Cross coupled cavity semiconductor laser
Authors: Salzman J. , Osinski J. S. , Bhat R. , Cummings K. , Harriott L.
Published: July 2002

A novel monolithic coupling scheme in which three or four active waveguides interact interferometrically to form a multicavity semiconductor laser is demonstrated. The coupling between perpendicular waveguides is obtained by an integrated beamsplitter. Frequency selection, tunability, and single mode operation are demonstrated.
Thermal equilibrium process in undoped hydrogenated amorphous silicon and silicon-carbon alloy films
Authors: Xu Xixiang , Morimoto Akiharu , Kumeda Minoru , Shimizu Tatsuo
Published: July 2002

We find the thermally induced defects in undoped hydrogenated amorphous silicon carbon (a-Si1−xCx:H) by electron spin resonance and conductivity measurements. The equilibrium temperature (Te) determined from the annealing temperature-dependent density of dangling bonds after fast cooling decreases with increasing C content, from 190 °C for hydrogenated amorphous silicon to 150 °C for a-Si0.82C0.18:H. Possible microscopic mechanisms for the observed thermally induced defects in a-Si1−xCx:H alloys are discussed.
Radiation and hot-electron effects on SiO2/Si interfaces with oxides grown in O2 containing small amounts of trichloroethane
Authors: Wang Yu , Nishioka Yasushiro , Ma T. P. , Barker R. C.
Published: July 2002

The hardness of SiO2/Si structures grown in O2+TCA (1,1,1,-trichloroethane) to damages caused by ionizing radiation and hot-electron injection has been found to depend strongly on the amount of TCA introduced. Using minute amounts (much smaller than conventionally used) of TCA, we have been able to achieve a dramatic improvement of the hardness. When excess amounts of TCA are used, however, the hardness degrades. In addition, the use of TCA also causes a significant change in the gate size dependence of the radiation or hot-electron-induced interface traps. These results will be explained in terms of the effects of Cl on the interfacial strain near the SiO2/Si transition region.
Determination of the valence-band offset at a HgTe/CdTe heterojunction by intervalence subband spectroscopy
Authors: Yang Z. , Furdyna J. K.
Published: July 2002

The relation between the value of the valence-band offset Vp at a HgTe/CdTe heterojunction and the energy difference ΔE between the first heavy hole subband and the first light hole subband at the Brillouin zone center of a HgTe/CdTe superlattice is examined theoretically. The theoretical values of ΔE are compared with those obtained from far-infrared magneto-optical measurements for two HgTe/CdTe superlattices. A single value of Vp =63±5 meV is found which fits the experimental values of ΔE for both superlattices.
Enhanced adhesion at oxide/oxide interfaces by ion beam stitching
Authors: Musket R. G. , Thomas I. M. , Wilder J. G.
Published: July 2002

The adhesion of antireflective, porous SiO2 coatings on fused silica substrates has been dramatically improved by irradiation with 200 keV helium ions. The approximately 220-nm-thick coatings consisted of layers of spherical, 20-nm-diam silica particles. Although the as-deposited coatings had only low adhesion, coatings irradiated with doses greater than 1014 He/cm2 passed both adhesive-tape and abrasive-resistance tests. Since these coatings are used on optical elements for 1064 nm high-power lasers, the effect of the helium irradiation on optical transmission was determined.
Reversible laser chemically induced phase transformations in thin-film Ba2YCu3Ox superconductors
Authors: Rothschild M. , Sedlacek J. H. C. , Black J. G. , Ehrlich D. J.
Published: July 2002

Phase transformations of a thin film of Ba2YCu3Ox were induced with a focused laser beam in chemical ambients. The transformations, involving superconductive and nonsuperconductive phases, are achieved rapidly and with a high degree of spatial control. They are fully reversible, and the appropriate processing parameters have been studied. These effects are interpreted within present models, which relate the superconducting properties of Ba2YCu3Ox to its oxygen content and crystalline structure.
Stable, single-(array)-mode operation from phase-locked, interferometric arrays of index-guided AlGaAs/GaAs diode lasers
Authors: Botez D. , Mawst L. , Hayashida P. , Roth T. J. , Anderson E.
Published: July 2002

Single-array-mode (L=8) oscillation is achieved with virtually diffraction-limited beams to three times above lasing threshold, and with beamwidths 1.5 times the diffraction limit at 4.7 times above lasing threshold and 130 mW output power per uncoated emitting facet. The arrays are 10/11-element wide-waveguide-interferometric (WWI) AlGaAs/GaAs lasers grown by liquid-phase epitaxy over channeled substrates. Selection of the L=8 mode is a consequence of two mode-dependent loss mechanisms: transverse antiguiding between elements and radiation losses in the interferometer. Near-field pattern analysis indicates that the (high-order) array mode is self-stabilizing with increasing drive above threshold. Simple means of achieving single-lobe operation are discussed.
Diffuse scattering and superstructure reflections from In1−y(AlxGa1−x)yP
Authors: Yasuami S. , Nozaki C. , Ohba Y.
Published: July 2002

The structure of In1−y (Alx Ga1−x )y P semiconductor alloy systems was studied by electron diffraction. Varied distribution of diffuse scattering and superstructure reflections was observed. The pertinent ordered structure is dependent on growth parameters, but not on the alloy systems. The diffuse scattering could be attributed to mean square atomic displacements due to the coexistence of different column III atoms in the same sublattice.
Oscillations in the optical response of (001)GaAs and AlGaAs surfaces during crystal growth by molecular beam epitaxy
Authors: Harbison J. P. , Aspnes D. E. , Studna A. A. , Florez L. T. , Kelly M. K.
Published: July 2002

Upon initiation of growth by molecular beam epitaxy, reflectance-difference (RD) signals for (001)GaAs and AlAs surfaces exhibit a cyclic component that is periodic with (001) atomic bilayer coverage and that follows either surface structure or surface chemistry (coverage), depending on measurement wavelength. These RD oscillations may be phase shifted with respect to their reflection high-energy electron diffraction counterparts, depending on deposition conditions.
Reduction of interface hydrogen content by partially ionized beam deposition technique
Authors: Yapsir A. S. , Lu T.-M. , Lanford W. A.
Published: July 2002

Hydrogen content at the Al/Si interface of samples with Al films deposited by conventional means and by a partially ionized beam (PIB) was measured using 1H(15N,αγ)12C nuclear resonance reaction. We observed that samples with PIB-deposited Al film exhibited significantly lower hydrogen concentration at the Al/Si in interface than that of the sample deposited by conventional means. The results demonstrated the ability of the PIB technique to perform interface self-cleaning, at least for hydrogen contaminant.
Intrinsic efficiency comparison in various low-pressure XeF laser mixtures pumped at high excitation rates and with short-pulse electron beam pumping
Authors: Nishida Naoto , Tittel Frank K. , Kumagai Hiroshi , Lee Young-Woo , Obara Minoru
Published: July 2002

A comparison of XeF(B→X) laser efficiency is reported using four different gas mixtures at room temperature, i.e., Ar/Xe/F2 , Ar/Xe/NF3 , Ne/Xe/F2 , and Ne/Xe/NF3 . All mixtures were pumped at the same high excitation rate of 1.1 MW/cm3 with a 65 ns electron beam current pulse. The respective pressure for the Ne-based mixtures was 1170 Torr and for the Ar-based mixtures was 711 Torr. The measured intrinsic efficiency of all four mixtures was ∼2%, which is comparable to those reported in high-pressure mixtures pumped at low excitation rates with long electron beam pumping.
Superconductivity in the Bi-Sr-Ca-Cu-O compounds with noble metal additions
Authors: Jin S. , Sherwood R. C. , Tiefel T. H. , Kammlott G. W. , Fastnacht R. A. , Davis M. E. , Zahurak S. M.
Published: July 2002

The Bi-Sr-Ca-Cu-O superconductors have been doped with various noble metals and their superconducting properties have been investigated. The resistivity and magnetic susceptibility measurements on sintered Bi4Sr3Ca2Cu4O16+x containing 20% by weight of Au, Ag, or Pt-group metals indicate that Au and the Pt-group metals significantly suppress or eliminate the superconducting transition in Bi-Sr-Ca-Cu-O. Only Ag is found to be benign, maintaining both the 115 and 85 K transitions in the compound. This nonpoisoning behavior of silver is of significant technical importance because of the need for a proper stabilizing normal metal for composite superconductor wire, nonreactive crucible materials for melt processing or crystal growth, and suitable nonpoisonous substrates or barriers for thin- or thick-film superconducting devices.
Rapid thermal annealing of high T
c superconducting thin films formed by metalorganic deposition
Authors: Mantese J. V. , Hamdi A. H. , Micheli A. L. , Chen Y. L. , Wong C. A. , Johnson J. L. , Karmarkar M. M. , Padmanabhan K. R.
Published: July 2002

Thin-film superconductors of Y-Ba-Cu and Yb-Ba-Cu have been formed by the nonvacuum method of metalorganic deposition (MOD). The films produced in this manner were homogeneous and free of voids and cracks over large dimensions. A two-step rapid thermal annealing of the MOD films, in oxygen, at 850 °C for 60 s followed by a second annealing at 920 °C for 30 s enhanced grain growth in the films and reduced the effects of substrate interaction. Preferred epitaxial grain growth, in the high Tc films, with the c axis both perpendicular and parallel to the substrate surface, occurred on 〈100〉 SrTiO3. Both the Y-Ba-Cu and Yb-Ba-Cu films showed superconducting onset temperatures above 90 K and zero resistance at 86 K.
Metalorganic molecular beam epitaxy of γ-Al2O3 films on Si at low growth temperatures
Authors: Sawada Kazuaki , Ishida Makoto , Nakamura Tetsuro , Ohtake Norio
Published: July 2002

Heteroepitaxial growth of γ-Al2O3 films was performed successfully onto (100) and (111) Si substrates at low substrate temperatures between 720 and 800 °C by metalorganic molecular beam epitaxy using N2 bubbled Al(CH3)3 and N2O. Using in situ reflection high-energy electron diffraction, the orientation relationships between epitaxial γ-Al2O3 films and Si substrates were found to be (100) γ-Al2O3//(100) Si with [11̄0] γ-Al2O3//[11̄0] Si and (111) γ-Al2O3//(111) Si with [11̄2̄] γ-Al2O3//[11̄2̄] Si. The stoichiometry of the grown films was found similar to that of Al2O3 from Auger signals.
Critical current densities in single-crystal Bi2.2Sr2Ca0.8Cu2O8+δ
Authors: van Dover R. B. , Schneemeyer L. F. , Gyorgy E. M. , Waszczak J. V.
Published: July 2002

We report transport and magnetic measurements of critical current densities in single crystals of high Tc superconductors in the Bi-Sr-Ca-Cu-O system. Transport measurements of Jc were made using a pulse method, while a vibrating sample magnetometer was used to measure M-H loops of oriented crystals in fields up to 15 kOe. Values for transport Jc in zero applied field are higher than those of Ba2YCu3O7 single crystals at comparable temperatures. Also we observe a strong magnetic field dependence for Jc at temperatures above ∼20 K.
Cubic zirconia as a species permeable coating for zinc diffusion in gallium arsenide
Authors: Bisberg J. E. , Dabkowski F. P. , Chin A. K.
Published: July 2002

Diffusion of zinc into GaAs through an yttria-stabilized cubic zirconia (YSZ) passivation layer has been demonstrated with an open-tube diffusion method. Pure zinc or GaAs/Zn2As3 sources produced high quality planar p-n junctions. The YSZ layer protects the GaAs surface from excessive loss of arsenic, yet is permeable to zinc, allowing its diffusion into the semiconductor. The YSZ films, deposited by electron beam evaporation, were typically 2000 Å thick. Zinc diffusion coefficients (DT) at 650 °C in the YSZ passivated GaAs ranged from 3.6×10−10 cm2/min for the GaAs/Zn2As3 source to 1.9×10−9 cm2/min for the pure zinc source. Doping concentrations for both YSZ passivated and uncapped samples were approximately 5×1019 cm−3.
Diffusion of ion-implanted Sn and Sb in heavily doped n-type silicon
Authors: Andersen P. E. , Larsen A. Nylandsted , Tidemand-Petersson P. , Weyer G.
Published: July 2002

The diffusion of ion-implanted Sn and Sb in silicon single crystals during rapid thermal annealing at 1000 and 1050 °C has been studied as a function of P donor concentration. For extrinsic/intrinsic carrier concentration ratios (n/ni )≥20, almost identical, extremely large diffusion coefficients are found for both impurities. The diffusion coefficients exhibit approximately an (n/ni )4 dependence. The onset of this dependence is accompanied by the appearance of a new defect-complex type containing Sb or Sn, respectively.
Low-temperature heteroepitaxial growth of InSb on CdTe by metalorganic chemical vapor deposition
Authors: Chen J. C. , Bush P. , Chen W. K. , Liu Pao-Lo
Published: July 2002

We report the low-temperature metalorganic chemical vapor deposition of InSb on (001)CdTe. This low-temperature process was carried out by a precracking technique. Epitaxial growth with a substrate temperature as low as 185 °C can be obtained using a simple two-stage heater. The deposited films were examined by double-crystal x-ray diffraction, scanning electron microscope, and energy dispersive analysis of x ray. The films grown at 240 °C are stoichiometric, single crystal, and of specular surface morphology.
Intersubband absorption in a modulation-doped Ga0.47In0.53As/Al0.48In0.52As multiple quantum well structure
Authors: Lobentanzer H. , König W. , Stolz W. , Ploog K. , Elsaesser T. , Bäuerle R. J.
Published: July 2002

We observe strong intersubband absorption at a wavelength of 8.4 μm in an n-type modulation-doped Ga0.47In0.53As/Al0.48In0.52As multiple quantum well structure with a well thickness of 8.2 nm. An oscillator strength of the corresponding dipole transition of 21.0 is measured. The dependence of the intersubband absorption on the polarization of the incident light and on lattice temperature is investigated in detail.
New approach to the atomic layer epitaxy of GaAs using a fast gas stream
Authors: Ozeki M. , Mochizuki K. , Ohtsuka N. , Kodama K.
Published: July 2002

A new growth method has been developed for the atomic layer epitaxy of GaAs. The gas system was based on a conventional metalorganic vapor phase epitaxial system but the decomposition of methylgallium was suppressed in the stagnant layer by using a fast pulsed gas stream from a jet nozzle. The method enabled us to grow high purity epitaxial layers with a clear self-limiting mechanism even at 560 °C. The variations in the growth rate with respect to various growth parameters were explained by the rate equations based on the selective adsorption of methylgallium on surface As atoms. The decomposition rate of methylgallium on the surface had an activation energy of 42 kcal/mole from 440 to 560 °C.
Low-temperature native oxide removal from silicon using nitrogen trifluoride prior to low-temperature silicon epitaxy
Authors: Burns G. P.
Published: July 2002

The native oxide of silicon has been shown to be removed in situ within a radiantly heated epitaxial reactor by passing a gas mixture of nitrogen trifluoride and hydrogen over a silicon substrate at a temperature of 580 °C. After oxide removal, silicon epitaxy has been grown from silane at temperatures between 750 and 850 °C forming abrupt dopant profiles at the substrate/layer interface of arsenic-, boron-, and antimony-doped substrates. It has been demonstrated that this epitaxial process may be adapted to a closely spaced, vertically stacked substrate geometry with the consequent large increase in reactor loading.
Narrow far fields from extended-window broad-area lasers
Authors: Lang Robert J. , Forouhar Siamak , Cser Jim , Katz Joseph , Gavrilovic Paul , Williams Jeannie , Stutius Wolfgang , Chin Aland
Published: July 2002

Broad-area lasers are fabricated with a long (80–100 μm), nonabsorbing window at each end. The window is shown to dramatically improve the spatial mode properties, stabilizing and smoothing the near field, and reducing the far field from 5°–15° to as low as 2°. This improvement comes at the expense of an increase in threshold current and reduction of quantum efficiency.
Tumor attenuation by combined heparan sulfate and polyamine depletion
Authors: Belting Mattias , Borsig Lubor , Fuster Mark M. , Brown Jillian R. , Persson Lo , Fransson Lars-Åke , Esko Jeffrey D.
Published: July 2002

Cells depend on polyamines for growth and their depletion represents a strategy for the treatment of cancer. Polyamines assemblede novothrough a pathway sensitive to the inhibitor, α-difluoromethylornithine (DFMO). However, the presence of cell-surface heparan sulfate proteoglycans may provide a salvage pathway for uptake of circulating polyamines, thereby sparing cells from the cytostatic effect of DFMO. Here we show that genetic or pharmacologic manipulation of proteoglycan synthesis in the presence of DFMO inhibits cell proliferationin vitroandin vivo. In cell culture, mutant cells lacking heparan sulfate were more sensitive to the growth inhibitory effects of DFMO than wild-type cells or mutant cells transfected with the cDNA for the missing biosynthetic enzyme. Moreover, extracellular polyamines did not restore growth of mutant cells, but completely reversed the inhibitory effect of DFMO in wild-type cells. In a mouse model of experimental metastasis, DFMO provided in the water supply also dramatically diminished seeding and growth of tumor foci in the lungs by heparan sulfate-deficient mutant cells compared with the controls. Wild-type cells also formed tumors less efficiently in mice fed both DFMO and a xylose-based inhibitor of heparan sulfate proteoglycan assembly. The effect seemed to be specific for heparan sulfate, because a different xyloside known to affect only chondroitin sulfate did not inhibit tumor growth. Hence, combined inhibition of heparan sulfate assembly and polyamine synthesis may represent an additional strategy for cancer therapy.
Mammalian thioredoxin reductase: Oxidation of the C-terminal cysteine/selenocysteine active site forms a thioselenide, and replacement of selenium with sulfur markedly reduces catalytic activity
Authors: Lee Seung-Rock , Bar-Noy Shoshana , Kwon Jaeyul , Levine Rodney L. , Stadtman Thressa C. , Rhee Sue Goo
Published: July 2002

Mammalian cytosolic thioredoxin reductase (TrxR) has a redox
center, consisting of Cys
59
/Cys
64
adjacent to
the flavin ring of FAD and another center consisting of
Cys
497
/selenocysteine (SeCys)
498
near the C
terminus. We now show that the C-terminal
Cys
497
-SH/SeCys
498
-Se
−
of
NADPH-reduced enzyme, after anaerobic dialysis, was converted to a
thioselenide on incubation with excess oxidized Trx (TrxS
2
)
or H
2
O
2
. The
Cys
59
-SH/Cys
64
-SH pair also was oxidized to a
disulfide. At lower concentrations of TrxS
2
, the
Cys
59
-SH/Cys
64
-SH center was still converted
to a disulfide, presumably by reduction of the thioselenide to
Cys
497
-SH/SeCys
498
-Se
−
. Specific
alkylation of SeCys
498
completely blocked the
TrxS
2
-induced oxidation of
Cys
59
-SH/Cys
64
-SH, and the alkylated enzyme
had negligible NADPH-disulfide oxidoreductase activity. The effect of
replacing SeCys
498
with Cys was determined by using a
mutant form of human placental TrxR1 expressed in
Escherichia
coli
. The NADPH-disulfide oxidoreductase activity of the
purified Cys
497
/Cys
498
mutant enzyme was 6%
or 11% of that of wild-type rat liver TrxR1 with
5,5′-dithio
bis
(2-nitrobenzoic acid) or
TrxS
2
, respectively, as substrate. Disulfide formation
induced by excess TrxS
2
in the mutant form was 12% of that
of the wild type. Thus, SeCys has a critical redox function during the
catalytic cycle, which is performed poorly by Cys.
Crystal structure of cytochrome P450 14α-sterol demethylase (CYP51) from
Mycobacterium tuberculosis
in complex with azole inhibitors
Authors: Podust Larissa M. , Poulos Thomas L. , Waterman Michael R.
Published: July 2002

Cytochrome P450 14α-sterol demethylases (CYP51) are essential
enzymes in sterol biosynthesis in eukaryotes. CYP51 removes the
14α-methyl group from sterol precursors such as lanosterol,
obtusifoliol, dihydrolanosterol, and
24(28)-methylene-24,25-dihydrolanosterol. Inhibitors of CYP51 include
triazole antifungal agents fluconazole and itraconazole, drugs used in
treatment of topical and systemic mycoses. The 2.1- and 2.2-Å crystal
structures reported here for 4-phenylimidazole- and fluconazole-bound
CYP51 from
Mycobacterium tuberculosis
(MTCYP51) are the
first structures of an authentic P450 drug target. MTCYP51 exhibits the
P450 fold with the exception of two striking differences—a bent I
helix and an open conformation of BC loop—that define an active
site-access channel running along the heme plane perpendicular to the
direction observed for the substrate entry in P450BM3. Although a
channel analogous to that in P450BM3 is evident also in MTCYP51, it is
not open at the surface. The presence of two different channels, with
one being open to the surface, suggests the possibility of
conformationally regulated substrate-in/product-out openings in
CYP51. Mapping mutations identified in
Candida albicans
azole-resistant isolates indicates that azole resistance in fungi
develops in protein regions involved in orchestrating passage of CYP51
through different conformational stages along the catalytic cycle
rather than in residues directly contacting fluconazole. These new
structures provide a basis for rational design of new, more efficacious
antifungal agents as well as insight into the molecular mechanism of
P450 catalysis.
The Dewar photoproduct of thymidylyl(3′→5′)- thymidine (Dewar product) exhibits mutagenic behavior in accordance with the “A rule”
Authors: Lee Joon-Hwa , Bae Sung-Hun , Choi Byong-Seok
Published: July 2002

In contrast to the highly mutagenic pyrimidine(6–4)pyrimidone photoproduct, its Dewar valence isomer (Dewar product) has low mutagenic potential and produces a broad range of mutations [LeClerc, J. E., Borden, A. & Lawrence, C. W. (1991)
Proc. Natl. Acad. Sci. USA
88, 9685–9689]. To determine the origin of the mutagenic property of the Dewar product, we used experimental NMR restraints and molecular dynamics to determine the solution structure of a Dewar-lesion DNA decamer duplex. This DNA decamer duplex (DW/GA duplex) contains a mismatched base pair between the 3′ T residue of the Dewar lesion (T6) and an opposed G residue (G15). The 3′ T (T6) of the Dewar lesion formed stable hydrogen bonds with the opposing G15 residue. However, the helical bending and unwinding angles of the DW/GA duplex were much larger than those of a second duplex that contains the Dewar lesion and opposing A15 and A16 residues (DW/AA duplex). The DW/GA duplex showed poorer stacking interactions at the two bases of the Dewar product and at the adjacent A7⋅T14 base pair than did the DW/AA duplex. These structural features imply that no thermal stability or conformational benefit is obtained by incorporating a G instead of an A opposite the 3′ T of the Dewar lesion. These properties may thus facilitate the preferential incorporation of an A in accordance with the A rule during translesion replication and lead to the low frequency of 3′ T→C mutations observed at this site.
Single-electron reduction of the oxidized state is coupled to proton uptake via the K pathway in
Paracoccus denitrificans
cytochrome
c
oxidase
Authors: Ruitenberg Maarten , Kannt Aimo , Bamberg Ernst , Ludwig Bernd , Michel Hartmut , Fendler Klaus
Published: July 2002

The reductive part of the catalytic cycle of cytochrome
c
oxidase from
Paracoccus denitrificans
was examined by using time-resolved potential measurements on black lipid membranes. Proteoliposomes were adsorbed to the black lipid membranes and Ru
II
(2,2′-bipyridyl)
3
2+
was used as photoreductant to measure flash-induced membrane potential generation. Single-electron reduction of the oxidized wild-type cytochrome
c
oxidase reveals two phases of membrane potential generation (τ
1
≈ 20 μs and τ
2
≈ 175 μs) at pH 7.4. The fast phase is not sensitive to cyanide and is assigned to electron transfer from Cu
A
to heme
a
. The slower phase is inhibited completely by cyanide and shows a kinetic deuterium isotope effect by a factor of 2–3. Although two enzyme variants mutated in the so-called D pathway of proton transfer (D124N and E278Q) show the same time constants and relative amplitudes as the wild-type enzyme, in the K pathway variant K354M, τ
2
is increased to 900 μs. This result suggests uptake of a proton through the K pathway during the transition from the oxidized to the one-electron reduced state. After the second laser flash under anaerobic conditions, a third electrogenic phase with a time constant of ≈1 ms appears. The amplitude of this phase grows with increasing flash number. We explain this growth by injection of a second electron into the single-electron reduced enzyme. On multiple flashes, both D pathway mutants behave differently compared with the wild type and two additional slow phases of τ
3
≈ 2 ms and τ
4
≈ 15 ms are observed. These results suggest that the D pathway is involved in proton transfer coupled to the uptake of the second electron.
Targeted mutagenesis of
Lis1
disrupts cortical development and LIS1 homodimerization
Authors: Cahana Aviv , Escamez Teresa , Nowakowski Richard S. , Hayes Nancy L. , Giacobini MaiBritt , von Holst Alexander , Shmueli Orit , Sapir Tamar , McConnell Susan K. , Wurst Wolfgang , Martinez Salvador , Reiner Orly
Published: July 2002

Lissencephaly is a severe brain malformation in humans. To study
the function of the gene mutated in lissencephaly
(
LIS1
), we deleted the first coding exon from the mouse
Lis1
gene. The deletion resulted in a shorter protein
(sLIS1) that initiates from the second methionine, a unique situation
because most LIS1 mutations result in a null allele. This mutation
mimics a mutation described in one lissencephaly patient with a milder
phenotype. Homozygotes are early lethal, although heterozygotes are
viable and fertile. Most strikingly, the morphology of cortical neurons
and radial glia is aberrant in the developing cortex, and the neurons
migrate more slowly. This is the first demonstration, to our knowledge,
of a cellular abnormality in the migrating neurons after Lis1 mutation.
Moreover, cortical plate splitting and thalomocortical innervation are
also abnormal. Biochemically, the mutant protein is not capable of
dimerization, and enzymatic activity is elevated in the embryos, thus a
demonstration of the
in vivo
role of LIS1 as a subunit
of PAF-AH. This mutation allows us to determine a hierarchy of
functions that are sensitive to LIS1 dosage, thus promoting our
understanding of the role of LIS1 in the developing cortex.
Effects of anesthesia on functional activation of cerebral blood flow and metabolism
Authors: Nakao Yasuaki , Itoh Yoshiaki , Kuang Tang-Yong , Cook Michelle , Jehle Jane , Sokoloff Louis
Published: July 2002

Functional brain mapping based on changes in local cerebral blood flow (lCBF) or glucose utilization (lCMR
glc
) induced by functional activation is generally carried out in animals under anesthesia, usually α-chloralose because of its lesser effects on cardiovascular, respiratory, and reflex functions. Results of studies on the role of nitric oxide (NO) in the mechanism of functional activation of lCBF have differed in unanesthetized and anesthetized animals. NO synthase inhibition markedly attenuates or eliminates the lCBF responses in anesthetized animals but not in unanesthetized animals. The present study examines in conscious rats and rats anesthetized with α-chloralose the effects of vibrissal stimulation on lCMR
glc
and lCBF in the whisker-to-barrel cortex pathway and on the effects of NO synthase inhibition with
N
G
-nitro-
l
-arginine methyl ester (
l
-NAME) on the magnitude of the responses. Anesthesia markedly reduced the lCBF and lCMR
glc
responses in the ventral posteromedial thalamic nucleus and barrel cortex but not in the spinal and principal trigeminal nuclei.
l
-NAME did not alter the lCBF responses in any of the structures of the pathway in the unanesthetized rats and also not in the trigeminal nuclei of the anesthetized rats. In the thalamus and sensory cortex of the anesthetized rats, where the lCBF responses to stimulation had already been drastically diminished by the anesthesia,
l
-NAME treatment resulted in loss of statistically significant activation of lCBF by vibrissal stimulation. These results indicate that NO does not mediate functional activation of lCBF under physiological conditions.
Scrapie prion protein accumulation by scrapie-infected neuroblastoma cells abrogated by exposure to a prion protein antibody
Authors: Enari Masato , Flechsig Eckhard , Weissmann Charles
Published: July 2002

Exposure of susceptible neuroblastoma N2a cells to mouse scrapie prions leads to infection, as evidenced by the continued presence of the scrapie form of the prion protein (PrP
Sc
) and infectivity after 300 or more cell doublings. We find that exposure to phosphatidylinositol-specific phospholipase C (PIPLC) or to the monoclonal anti-prion protein (PrP) antibody 6H4 not only prevents infection of susceptible N2a cells but also cures chronically scrapie-infected cultures, as judged by the long-term abrogation of PrP
Sc
accumulation after cessation of treatment. A nonpassaged, stationary infected culture rapidly loses PrP
Sc
when exposed to the antibody or PIPLC, indicating that the PrP
Sc
level is determined by steady state equilibrium between formation and degradation, and that depletion of the cellular form of PrP can interrupt the propagation of PrP
Sc
. These findings encourage the belief that passive immunization may provide a therapeutic approach to prion disease.
Recognition of the 300-kDa mannose 6-phosphate receptor cytoplasmic domain by 47-kDa tail-interacting protein
Authors: Orsel Joke G. , Sincock Paul M. , Krise Jeffrey P. , Pfeffer Suzanne R.
Published: July 2002

Tail-interacting 47-kDa protein (TIP47) binds the cytoplasmic domains of the cation-dependent (CD) and cation-independent (CI) mannose 6-phosphate receptors (MPRs) and is required for their transport from endosomes to the Golgi complex. TIP47 recognizes a phenylalanine-tryptophan signal in the CD-MPR. We show here that TIP47 interaction with the 163-residue CI-MPR cytoplasmic domain is highly conformation dependent and requires CI-MPR residues that are proximal to the membrane. CI-MPR cytoplasmic domain residues 1–47 are dispensable, whereas residues 48–74 are essential for high-affinity binding. However, residues 48–74 are not sufficient for high-affinity binding; residues 75–163 alone display weak affinity for TIP47, yet they contribute to the presentation of residues 48–74 in the intact protein. Independent competition binding experiments confirm that TIP47 interacts with the membrane-proximal portion of the CI-MPR cytoplasmic domain. TIP47 binding is competed by the binding of the AP-2 clathrin adaptor at (and near) residues 24–29 but not by AP-1 binding at (and near) residues 160–161. Finally, TIP47 appears to recognize a putative loop generated by the sequence PPAPRPG and other hydrophobic residues in the membrane-proximal domain. Although crystallography will be needed to define the precise interaction interface, these data provide an initial structural basis for TIP47–CI-MPR association.
A trans-acting RNA as a control switch in
Escherichia coli
: DsrA modulates function by forming alternative structures
Authors: Lease Richard A. , Belfort Marlene
Published: July 2002

DsrA is an 87-nucleotide regulatory RNA of
Escherichia coli
that acts in trans by RNA–RNA interactions with two different mRNAs,
hns
and
rpo
S. DsrA has opposite effects on these transcriptional regulators. H-NS levels decrease, whereas RpoS (σ
s
) levels increase. Here we show that DsrA enhances
hns
mRNA turnover yet stabilizes
rpo
S mRNA, either directly or via effects on translation. Computational and RNA footprinting approaches led to a refined structure for DsrA, and a model in which DsrA interacts with the
hns
mRNA start and stop codon regions to form a coaxial stack. Analogous bipartite interactions exist in eukaryotes, albeit with different regulatory consequences. In contrast, DsrA base pairs in discrete fashion with the
rpo
S RNA translational operator. Thus, different structural configurations for DsrA lead to opposite regulatory consequences for target RNAs.