bluuebunny/crossref_metadata_2025 huggingface dataset:
Photocurrent multiplication in ion implanted lateral In0.2Ga0.8As/GaAs strained-layer superlattice photodetectors
Authors: Bulman G. E. , Myers D. R. , Zipperian T. E. , Dawson L. R. , Wiczer J. J. , Biefeld R. M.
Published: July 2002

We demonstrate the first Be+-implanted lateral In0.2Ga0.8As/GaAs strained-layer superlattice (SLS) avalanche photodetector. This planar device exhibits low capacitance (<1 pF), low dark current (less than 6 nA at −60 V), and large active area (100 μm × 250 μm at −50 V). In addition, an uncoated external quantum efficiency of 50% at −50 V between 775 and 880 nm is observed with response continuing to 1.03 μm. Detailed photocurrent measurements confirm the occurrence of photocurrent multiplication and indicate that the ionization coefficient of electrons is larger than that of holes for transport in the plane of the SLS. This work demonstrates the advantages of exploiting the preferred transport direction that occurs in superlattice structures through the use of ion implantation.
Efficient narrow spectral output in the blue-green region from an injection-controlled electron-beam excited XeF (C→A) laser
Authors: Marowsky G. , Nishida N. , Stiegler H. , Tittel F. K. , Wilson W. L. , Zhu Y. , Nighan William L.
Published: July 2002

Efficient, narrow spectral output has been achieved by injection control of an electron-beam excited XeF (C→A) laser medium using a 482-nm dye laser pulse having a spectral width of 0.01 nm. The energy density and intrinsic efficiency characteristic of the amplified output beam were 3 J/l and approximately 2.5%, respectively, and the spectral width was on the order of that of the injected pulse.
Measurement of picosecond semiconductor laser pulse duration with internally generated second harmonic emission
Authors: Chen Y. C. , Liu J. M.
Published: July 2002

We demonstrate a simple practical technique for the measurement of the duration of picosecond semiconductor laser pulses using the internally generated second harmonic emission accompanying the laser output. The pulse duration is determined by the ratio of the conversion efficiencies of the second harmonic emission generated by the picosecond pulses and by the cw emission or other pulsed emission whose duration can be accurately measured. The pulse duration thus measured is in close agreement with that determined by the nonlinear intensity autocorrelation technique.
Copper conductors by reduction of copper (I) complex in a host polymer
Authors: Auerbach A.
Published: July 2002

A technique for generating fine copper conductors using a laser is described. The laser radiation is focused onto a Cu(CH3CN)4BF4-doped poly (N-vinylpyrrolidone) polymer thin film, and scanned across the polymer surface to produce a highly conductive copper run. The measured resistance is ony a factor of 5 above the calculated value for a copper conductor of comparable dimensions.
Frequency limit of double-barrier resonant-tunneling oscillators
Authors: Luryi Serge
Published: July 2002

The speed of operation of negative differential resistance (NDR) devices based on resonant tunneling in a double-barrier quantum-well structure is considered. It is shown that the intrinsic RC delay of a single barrier limits the frequency of active oscillations to fmax =1/(2πτ), where τ=εα−1(λ/c)exp(4πd/λ) with λ being the de Broglie wavelength of the tunneling electron, d the barrier thickness, ε the dielectric permittivity, c the speed of light, and α≊1/137 the fine-structure constant. The relevance of this estimate to recent experimental results is discussed. An alternative mechanism for the NDR is proposed—not involving resonant tunneling. It should be observable in various single-barrier structures in which tunneling occurs into a two-dimensional system of states. In a double-barrier structure, specially designed experiments are required to distinguish this effect from resonant tunneling.
Raman scattering from phonon-plasmon modes in Ga1−xAlxAs
Authors: Becker R. J. , Luehrmann P. F. , Langer D. W.
Published: July 2002

The relation for the frequencies of phonon-plasmon modes in two-component solutions is presented. Raman spectra from over 80 samples of Ga1−xAlxAs over a wide range of x values and varying electron concentrations have been fitted to predicted curves using this relationship. Data were taken at both room and nitrogen temperature on samples grown by both metalorganic chemical vapor deposition and molecular beam epitaxy.
Rapid thermal annealing of interface states in aluminum gate metal-oxide-silicon capacitors
Authors: Reed Michael L. , Fishbein Bruce , Plummer James D.
Published: July 2002

Rapid thermal processing has been used to study the annealing of interface states in aluminum gate metal-oxide-silicon capacitors. Midgap Dit values of 1012 cm−2 eV−1, as measured by the combined high-frequency/quasistatic capacitance-voltage method, drop to less than 2×1010 cm−2 eV−1 after 2 min at 300 °C and after only 5 s at 380 °C. This compares with a standard anneal time of 30 min at 450 °C commonly used in conventional furnace annealing. The temporal dependence of Dit suggests that interface state annealing is not limited by transport of neutral hydrogen through the oxide. Room-temperature annealing is insignificant over a trial interval of several months.
Interdigital transducer method of acoustic beam steering for bulk wave acousto-optic Bragg cells
Authors: Palmieri L. , Socino G. , Verona E.
Published: July 2002

A LiNbO3 acousto-optic Bragg cell is described, which uses an interdigital transducer deposited on the x surface of the crystal so as to excite longitudinal bulk acoustic waves along a direction almost normal to the surface itself. By a proper choice of the periodicity of the transducer electrodes, a frequency-dependent steering of the acoustic beam can be obtained, which allows the Bragg conditions to be satisfied over a wide range of frequencies. Results are reported relative to a cell operating in the frequency range 1.4–2.1 GHz, with a diffraction efficiency of approximately 2% per watt of acoustic power.
Effect of dopant on reaction between polycrystalline silicon and thin-film rhodium
Authors: Psaras P. A. , Thompson R. D. , Tu K. N.
Published: July 2002

High phosphorus doping (6.0 × 1020 atom/cm3) of polycrystalline silicon has been found to change the first rhodium silicide formation from the monosilicide (RhSi) to a metal-rich silicide (Rh2Si) which has twice the conductivity of the former. The methods of specimen analysis and characterization utilized in this study are Rutherford backscattering spectrometry, Seemann Bohlin x-ray diffraction, sheet resistance, and resistivity measurement via four-point probe.
Current-voltage relation in a time-dependent diode
Authors: Kadish Abraham , Peter William , Jones Michael E.
Published: July 2002

A simple time-dependent relation between the current and voltage pulse in a one-dimensional diode has been obtained. The relation is applicable to diodes in which the voltage or current pulse changes appreciably during the beam transit time across the diode gap. A simple application of the results to eliminating current transients in ion diodes is presented.
Preparation of highly photosensitive hydrogenated amorphous Si-Ge alloys using a triode plasma reactor
Authors: Matsuda A. , Yagii K. , Koyama M. , Toyama M. , Imanishi Y. , Ikuchi N. , Tanka K.
Published: July 2002

Highly photoconductive hydrogenated amorphous Si-Ge alloys have been prepared from a SiH4/GeH4 gas mixture using a triode glow-discharge reactor. High photoconductivity (Δσp≂10−4 Ω−1 cm−1) under the AM1 (100 mW/cm2) illumination and low dark conductivity (σd=10−8–10−9 Ω−1 cm−1) have been obtained for the optical gap in the range between 1.5 and 1.7 eV.
New nondestructive method for the investigation of insulator-semiconductor structures
Authors: Bouillier G. , Alquie C. , Dreyfus G.
Published: July 2002

The pressure wave propagation (PWP) method, originally designed to study the electrical properties of polymers, is used to determine the distribution of charges in metal-insulator-silicon and metal-oxide-silicon structures. This method is nondestructive and fast. The electric field distribution in a sample is derived from the signal induced by the propagation of a pressure wave with a short rise time in the structure. As a new application of this method, we demonstrate that it is possible to observe the evolution of the surface states of silicon during ambient oxidation and to reveal the charge distribution in a metal-glass-silicon structure.
Chemical vapor deposition of boron-doped hydrogenated amorphous silicon
Authors: Ellis F. B. , Delahoy A. E.
Published: July 2002

Deposition conditions and film properties for a variety of boron-doped hydrogenated amorphous silicon films and silicon-carbon films produced by chemical vapor deposition (CVD) are discussed. Deposition gases include monosilane, disilane, trisilane, and acetylene. Two types of optically wide band-gap p layers are obtained. One of these window p layers (without carbon) has been extensively tested in photovoltaic devices. Remarkably, this p layer can be deposited between about 200 to 300 °C. A typical open circuit voltage in an all CVD p-i-n device is 0.70–0.72 V, and in a hybrid device where the i and n layers are deposited by glow discharge, 0.8–0.83 V.
Indium doping of HgCdTe layers during growth by molecular beam epitaxy
Authors: Boukerche M. , Reno J. , Sou I. K. , Hsu C. , Faurie J. P.
Published: July 2002

Successful n-type doping of HgCdTe layers with indium during growth by molecular beam epitaxy is reported for the first time. The indium concentration is found to increase with the In flux. The doping level reaches around 1018 cm−3, which is nearly two orders of magnitude more than what can be achieved by stoichiometry deviation in as-grown samples. In the range studied, the electrical efficiency is substantial and decreases with increasing In atomic concentration. The experimental measurements are in agreement with a model suggesting that indium, which is not singly ionized, precipitates as In2Te3 and that the native acceptor defect concentration remains approximately constant. n-type doping is shown to be possible for a wide range of alloy concentrations. Good electron mobilities in the alloy are presented for the doping concentrations studied. This technique will greatly enhance the potential of molecular beam epitaxy for device applications of this material.
Millimeter wave absorption and refraction in tungsten bronze ferroelectrics
Authors: Bobbs Bradley , Matloubian Mehran , Fetterman Harold R. , Neurgaonkar Ratnakar R. , Cory Warren K.
Published: July 2002

Tungsten bronze ferroelectric crystals SrxBa1−x Nb2O6 and Ba2−xSrxK1−yNay Nb5O15 have been investigated for potential electro-optic uses at millimeter wave carrier frequencies between 55 and 110 GHz. Indices of absorption and refraction at temperatures between 20 and 300 K were derived from measurements of transmittance spectra containing Fabry–Perot fringes produced by crystal surface reflections. The anisotropies observed in both indices at these frequencies are huge compared with those at optical frequencies. Absorption in both materials decreases markedly upon cooling, but only for incident waves polarized along the crystal polar axis.
Influence of sputtering damage on chemical interactions at Cr-SiO2 interfaces
Authors: Cros A. , Schrott A. G. , Thompson R. D. , Tu K. N.
Published: July 2002

The interaction of SiO2 surfaces with ultrathin layers of 4–16 Å of Cr evaporated in ultrahigh vacuum has been studied by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy. The surfaces were treated either by Ar sputtering, sputtering, and simultaneous annealing, or by annealing in O2. A room-temperature reaction occurs on sputtered samples and produces a new XPS peak at a binding energy 4.9 eV lower than that of oxidized Si 2p and a shoulder in the O 1s line. These effects are less pronounced in sputtered-annealed samples and insignificant in nonsputtered ones. Our results suggest the presence of silicon in a Cr-rich environment which is at a maximum concentration away from the SiO2-Cr interface, following a buffer region richer in oxygen.
Shallow junction formation by preamorphization with tin implantation
Authors: Delfino M. , Sadana D. K. , Morgan A. E.
Published: July 2002

Amorphizing n-type 〈100〉 silicon by tin implantation prior to implanting 10 keV boron and then annealing for 30 min at 800 °C results in a 0.22-μm-deep p+/n junction. The implanted tin prevents boron channeling, enhances the quality of the solid phase epitaxial regrowth of the silicon, and shows no measurable diffusion. A discontinuous band of dislocation loops, 20–30 nm in diameter, with a density below 1010 cm−2 remains at the original amorphous-crystalline interface after annealing. Junctions are nearly ideal and are characterized at −5 V reverse bias by an areal leakage of −5 nA cm−2 and a peripheral leakage less than −0.1 fA μm−1.
Electroluminescence and photoluminescence in sputtered ZnS:TbFx thin films
Authors: Okamoto Kenji , Watanabe Kazuhiro
Published: July 2002

A study has been made on the electroluminescence (EL) and photoluminescence (PL) spectra of sputtered ZnS:TbFx thin films. The brightness and EL spectra are strongly dependent on x. The highest brightness has been obtained at x=1. From a comparison of EL and PL of standard samples, we conclude that, in the case of x=1, Tb exists as a complex center composed of Tb substituted for Zn and interstitial F and this center gives high EL efficiency. In the case of x=3, on the other hand, TbF3 is doped as a molecular center in the ZnS film exhibiting inefficient EL.
Phase-locked controlled filament laser
Authors: Salzman J. , Larsson A. , Yariv A.
Published: July 2002

A broad area semiconductor laser with induced self-focusing in the form of a phase-locked array of filaments is demonstrated. The multifilamentary laser has a single lobed and nearly diffraction limited far-field pattern, for injection currents up to I≂1.85Ith.
Output pulse termination of a self-sustained excimer laser
Authors: Osborne M. R. , Coutts J. , Hutchinson M. H. R. , Webb C. E.
Published: July 2002

Theoretical predictions of the nature of the premature termination of the laser output of a long-pulse discharge-pumped rare-gas-halide laser are tested on a large scale x-ray preionized XeCl system. Results are presented which support the theory that the termination is caused by local depletion of the halogen donor molecule leading to discharge instabilities. Predictions of methods of extending the laser output pulse duration have been made and experimentally demonstrated.
Figure of merit for magneto-optical media based on the dielectric tensor
Authors: Mansuripur M.
Published: July 2002

A figure of merit is derived for the magneto-optical media used in erasable optical data storage. It is shown that the ratio of the useful signal generated by the polar Kerr effect to the incident laser power is always below the figure of merit, but the figure can be approached if the magneto-optic medium is embedded in a proper multilayer structure.
Speed and effectiveness of windowless GaAs étalons as optical logic gates
Authors: Lee Y. H. , Gibbs H. M. , Jewell J. L. , Duffy J. F. , Venkatesan T. , Gossard A. C. , Wiegmann W. , English J. H.
Published: July 2002

The effectiveness of surface recombination in speeding up the relaxation of a GaAs étalon is reported. Various thickness (1.5, 0.5, 0.3, 0.13 μm) bulk GaAs windowless samples were fabricated and tested as optical logic gates. Times for complete recovery of transmission lie between 400 and 30 ps. The response shows a roughly linear increase in speed as the sample thickness decreases, consistent with surface recombination being the dominant relaxation mechanism. A very fast cycle time of 70 ps is demonstrated using a 0.3-μm-thick windowless GaAs étalon as an all-optical logic device. Proton-bombarded samples show slower recovery and poorer contrast, and they require more gating energy.
Single crystal, epitaxial multilayers of AlAs, GaAs, and AlxGa1−xAs for use as optical interferometric elements
Authors: Gourley P. L. , Drummond T. J.
Published: July 2002

We propose using single crystal multilayers of AlAs, GaAs, and AlxGa1−xAs for use as several different kinds of optical interference elements which include high reflectors, transmission filters, and Fabry–Perot cavities. We have grown many of these structures by molecular beam epitaxy and measured their optical characteristics. We find the characteristics of these structures to be very useful for a number of applications for integrated optical devices which we also propose.
Surface-emitting second order distributed Bragg reflector laser with dynamic wavelength stabilization and far-field angle of 0.25°
Authors: Evans G. A. , Hammer J. M. , Carlson N. W. , Elia F. R. , James E. A. , Kirk J. B.
Published: July 2002

Operation of a dynamically stabilized surface-emitting AlGaAs channeled-substrate-planar large-optical-cavity distributed Bragg reflector laser with a 0.25°×8° far-field pattern is reported. With 50% duty cycle and 50 ns–2 ms pulse length operation at room temperature, thresholds as low as 150 mA were obtained. No side modes were observed to 40 dB below the principal wavelength intensity under 100% modulation depth operation.
High-efficiency, high-energy operation of an intense electron beam initiated HF chemical laser using a mixture containing a heavy fluoride MoF6
Authors: Inagaki Hirohito , Suda Akira , Obara Minoru
Published: July 2002

High-efficiency and high-energy operation of HF chemical lasers was simultaneously realized using a low pressure mixture of H2/F2/O2/MoF6 initiated by an intense electron beam. The addition of small amounts of MoF6 gas to the mixture of H2/F2/O2 resulted in an increase in laser output energy and electrical and chemical efficiencies. A HF laser energy obtained from a mixture containing 17.9 Torr of MoF6 was 3.7 kJ (136 J/l) with a pulse width of 190 ns full width at half-maximum. The corresponding electrical efficiency and chemical efficiency are 189% and 15.2%, respectively, with a low pressure (362.4 Torr) mixture of H2/F2/O2/MoF6=30/242/72.5/17.9 Torr.
Propagation of plasmons across layers of GaAs-Ga1−xAlxAs superlattices
Authors: Kirillov D. , Webb C. , Eckstein J.
Published: July 2002

The transformation from quasi-two-dimensional to three-dimensional behavior of the free-electron gas in GaAs-Ga1−xAlxAs multiple quantum well structures was observed. This transformation corresponds to the establishment of well defined three-dimensional plasmon modes measured by Raman scattering, which propagate across layers of an n-type doped superlattice when the layer thicknesses are reduced to 20 Å or less. This type of behavior is consistent with the Kronig–Penney type model describing coupling between the wells and formation of energy minibands in a superlattice.
Enhanced grain growth of phosphorus-doped polycrystalline silicon by titanium silicide formation
Authors: Chou T. C. , Wong C. Y. , Tu K. N.
Published: July 2002

The influence of silicide reaction on the grain growth behavior of phosphorus-doped polycrystalline silicon has been studied. Intrinsic and phosphorus-doped polycrystalline silicon, with and without evaporated titanium overlayer, were prepared by low pressure chemical vapor deposition on thermally grown SiO2 on silicon wafer. These samples were then annealed in He ambient at 700 °C for various time. The Rutherford backscattering spectrum showed that TiSi2 formed completely after 30 min anneal. Cross-sectional transmission electron microscopy showed that Kirkendall voids exist at the interface between TiSi2 and polycrystalline silicon, indicating that silicon is the predominant diffusing species. Enhanced grain growth as a result of silicide reaction was observed in phosphorus-doped polycrystalline silicon. The enhanced grain growth of polycrystalline silicon is suggested to be due to the redistribution of dopant, analogous to the alloying and dealloying processes in diffusion induced grain boundary migration.
Photoinduced surface morphology improvement and preferential orientation enhancement in film deposition of evaporated ZnS
Authors: Yokoyama Hiroyuki
Published: July 2002

Surface photoionization durign the deposition of evaporated ZnS resulted in surface morphology improvement and preferential orientation enhancement of films deposited on quartz and sapphire substrates. Such film quality improvement was induced with very low intensity light as long as photon energy was greater than the band-gap energy of deposited ZnS. These features of the photoinduced effect suggest that enhancement of surface adatom rearrangement occurs through a certain nonthermal process resulting from an excited carrier. Such a nonthermal effect is essentially important for lowering the epitaxial growth temperature.
New omnipresent electron paramagnetic resonance signal in as-grown semi-insulating liquid encapsulation Czochralski GaAs
Authors: Kaufmann U. , Baeumler M. , Windscheif J. , Wilkening W.
Published: July 2002

Electron paramagnetic resonance studies on as-grown semi-insulating liquid encapsulation Czochralski (LEC) GaAs at 35 GHz have revealed a new resonance labeled FR3. It is consistently present in LEC material but usually unobservable in Bridgman samples, thus strongly indicating that the defect involved contains boron. The center has trigonal symmetry and is electrically active. Its spectrum indicates a d 9 or p5 one-hole configuration. We tentatively identify FR3 with a Ga antisite complex, Ga−As -B0Ga.
Ordering of oxide precipitates in oxygen implanted silicon
Authors: van Ommen A. H. , Koek B. H. , Viegers M. P. A.
Published: July 2002

An order network of oxide precipitates has been found in the monocrystalline superficial silicon layer of a silicon-on-insulator structure obtained by high-dose oxygen implantation. The network consists of oxide precipitates 2 nm in size spaced about 5 nm apart. Electron diffraction patterns indicate that ordering occurs both parallel and perpendicular to the surface along the 〈100〉 directions of the silicon lattice. The precipitate network has a cubic symmetry.
X-ray study of misfit strain relaxation in lattice-mismatched heterojunctions
Authors: Kamigaki K. , Sakashita H. , Kato H. , Nakayama M. , Sano N. , Terauchi H.
Published: July 2002

High-resolution x-ray diffraction measurements have been carried out in AlxGa1−xAs and InxGa1−xAs grown by the molecular beam epitaxy method on (001) GaAs substrates. The thin epitaxial layers in these lattice-mismatched semiconductor single heterojunctions are uniformly distorted and there is an elastic limit for large x. The epitaxial layer is affected by a thick substrate even over the elastic limit, i.e., the epitaxial layer still shows a strained state beyond the elastic limit. The relationship between the misfit strain and the lattice distortion is discussed.
Instabilities and all-optical phase-controlled switching in a nonlinear directional coherent coupler
Authors: Wabnitz S. , Wright E. M. , Seaton C. T. , Stegeman G. I.
Published: July 2002

The instability in the evolution of a nonlinear wave along a directional coupler is investigated with the beam propagation method, both for self-focusing and for self-defocusing materials. A new device is proposed for switching an intense light beam at the device output by changing the relative phase of a weak input signal.
N2 excited state absorption in XeF lasers
Authors: Mandl A. , Hyman H. A.
Published: July 2002

The mechanism by which NF3 fueled XeF laser mixtures degrade has been identified. The degradation is due to the formation of trace amounts of N2 which charge exchange with electron beam excited Ne+2 to form N+2X2Σ+g (v″=3,4). The N+2 X2 Σ+g (v″=3,4) absorbs selectively at the 353-nm band but not at the 351-nm band. This has been confirmed experimentally by direct observation of the laser output spectrum which shows a significant decrease of the 353-nm laser band with almost no decrease of the 351-nm bands when N2 is added to the laser mixture. A Stern–Volmer analysis of the decreased XeF* emission due to the addition of N2 to the laser mixture has been performed and the charge exchange rate between N2 and Ne+2 of 9×10−10 cm3/s deduced. The decrease of XeF laser output with N2 addition has been measured for both F2 and NF3 fuels. The results are in good agreement with model calculations using measured and literature values for the reactions.
Gas-sensitive electrical conduction observed in a locally discontinuous Au thin film
Authors: Yasuda Tetsuji , Komiyama Hiroshi , Tanaka Kazunobu
Published: July 2002

Gas-sensitive electrical conduction is reported in a gold thin film with a locally discontinuous structure. Two novel features were found. Firstly, irregular current pulsation was induced under a stream of helium gas. Secondly, the pulsation was suppressed by adsorption of gas molecules such as oxygen, carbon monoxide, and hydrogen. These phenomena are tentatively interpreted in terms of atomic rearrangement of the gold surface.
Improved polycrystalline silicon sheet resistance by rapid thermal annealing prior and subsequent to ion implantation
Authors: Wilson S. R. , Gregory R. B. , Paulson W. M. , Krause S. J. , Leavitt J. A. , McIntyre L. C. , Seerveld J. L. , Stoss P.
Published: July 2002

Polycrystalline Si films on oxidized Si wafers have been subjected to a rapid thermal processing anneal prior to As ion implantation. After ion implantation the films are given another rapid thermal processing anneal to activate the As. The preimplant anneal causes the as-deposited grain size to increase by ∼ a factor of 10. These films have a 20–30% lower sheet resistance than films that were post-implant annealed only. The increase in grain size by the preimplant anneal reduces the grain boundary area and therefore, minimizes the amount of dopant in the grain boundary relative to the grain.
Observation of novel steplike structure in the photocurrent and dark current of a superlattice: Charge collection by successive depletion of quantum wells
Authors: Allam J. , Capasso F. , Panish M. B. , Hutchinson A. L.
Published: July 2002

We have observed novel manifestations of superlattice properties in a Ga0.47In0.53As/InP quantum well photodiode structure grown by gas source molecular beam epitaxy, evident as multiple steps in the reverse bias photocurrent and dark current. This striking effect is explained in terms of depletion of successive quantum wells as the reverse bias field is increased, while the diffusion of carriers generated in the undepleted region is suppressed by localization within the wells. As the electric field punches through into each well the carriers photogenerated and thermally generated in that well are collected by the field, while carriers generated in the undepleted wells remain localized and cannot contribute to the current. This causes a steplike increase in the photocurrent and the dark current.
Junction leakage in titanium self-aligned silicide devices
Authors: Amano Jun , Nauka K. , Scott M. P. , Turner J. E. , Tsai Rick
Published: July 2002

Successful utilization of a titanium self-aligned silicide (salicide) process for reproducible device fabrication with high yield requires junction leakage due to the silicide process to be minimized. The microstructure and microchemistry of titanium salicide shallow junction diodes were studied and correlated with junction leakage. The direct correlation between junction leakage and junction structure was established by using several analytical techniques. The main cause of large leakage current was found to be a loss of p+/n junction under the titanium silicide layer and formation of titanium silicide/n-silicon Schottky barrier contact at the perimeter of the diodes. Process parameters for low leakage titanium silicide/p+/n diode fabrication were also established.
Long cavity ridge waveguide AlGaAs/GaAs distributed feedback lasers for spectral linewidth reduction
Authors: Kojima Keisuke , Noda Susumu , Tai Shuichi , Kyuma Kazuo , Hamanaki Koichi , Nakayama Takashi
Published: July 2002

Ridge waveguide AlGaAs/GaAs distributed feedback lasers with the cavity length of 820 μm were fabricated. The differential quantum efficiency was as high as 28%, and the power-dependent linewidth was more than an order of magnitude smaller than that of the 290 μm laser, which agrees with theory. The narrowest linewidth was 12 MHz at 6.2 mW.
High quality zinc sulfide epitaxial layers grown on (100) silicon by molecular beam epitaxy
Authors: Yokoyama Meiso , Kashiro Ko-ichi , Ohta Shin-ichi
Published: July 2002

ZnS films were successfully grown by molecular beam epitaxy on (100) Si substrates. These high quality films, grown at a substrate temperature of 340 °C with a molecular beam flux ratio of Zn to S of unity, exhibited high crystallographic quality and revealed a flat surface. Secondary ion mass spectroscopic analysis showed that the film contained little impurity. In photoluminescence, a broad peak was observed at 2.6 eV at room temperature while an additional sharp peak at 3.6 eV appeared at 77 K. The latter is considered to be band-to-band emission and the former may be due to the Cu-blue emission.
Surface structure of GaAs with adsorbed Te
Authors: Feldman R. D. , Austin R. F.
Published: July 2002

The surface structures that result from the adsorption of Te on (100) GaAs have been shown to affect the orientation of CdTe films on GaAs. Two structures are described here. A low-temperature (6×1) surface leads to (100) film growth. At 580 °C, a new surface results which is characterized by ordering along directions 60° from [011̄] and [01̄1], and leads to (111) growth of CdTe. Both surface structure and the interaction of the group II element with the surface are believed to be important in determining the orientation of the film.
Structural stability and selectivity of thin epitaxial semiconductors
Authors: Mbaye A. A. , Zunger Alex , Wood D. M.
Published: July 2002

It is shown how the availability of structural degrees of freedom in various ternary AnB4−nC4 adamantine semiconductors can lead to their energetic stabilization when grown epitaxially, and how the substrate strain can preferentially stabilize one structure over another even when the two are equally stable (or unstable) in bulk form.
Elastic strains in CdTe-GaAs heterostructures grown by metalorganic chemical vapor deposition
Authors: Olego D. J. , Petruzzello J. , Ghandhi S. K. , Taskar N. R. , Bhat I. B.
Published: July 2002

The elastic response associated with the lattice mismatch in (100)CdTe∥(100)GaAs heterostructures was investigated by performing photoluminescence measurements as a function of CdTe layer thicknesses. The heterostructures were grown by metalorganic chemical vapor deposition. Estimates of strains, stresses, and lattice constants were obtained from shifts in near-band-edge photoluminescence features. Biaxial compressive strains are present in CdTe layers thinner than 1 μm. The magnitudes of the strains are larger than those expected from equilibrium models and from transmission electron microscopy results. With increasing CdTe layer thicknesses above 0.1 μm biaxial tensile strains affect the GaAs surfaces.
Selective epitaxial growth of gallium arsenide by molecular beam epitaxy
Authors: Okamoto A. , Ohata K.
Published: July 2002

GaAs was selectively grown on GaAs substrates patterned with SiO2 by conventional molecular beam epitaxy (MBE). No growth was found on SiO2, except for small GaAs particles, when the substrate temperature was above 700 °C under 1.2×10−5 Torr arsenic pressure. With the increase in the substrate temperature, the selectivity was better while the growth rate decreased. At 775 °C, no growth occurred, even on GaAs. Selective epitaxial growth of GaAs by MBE is promising for application to device fabrications.
Low-temperature process for the preparation of high T
c superconducting thin films
Authors: Adachi Hideaki , Hirochi Kumiko , Setsune Kentaro , Kitabatake Makoto , Wasa Kiyotaka
Published: July 2002

We have established the low-temperature process for the preparation of high Tc superconducting films by rf magnetron sputtering. The films were deposited at a temperature (650 °C) lower than the tetragonal-orthorhombic transition point with sufficient crystallizing and oxidizing conditions. The as-deposited Er-Ba-Cu-O film on MgO exhibited a sharp superconductive transition with zero resistance at 86 K. This process prevented diffusion at the film and substrate interface and reduced the porous structure in the films.
I
n s
i
t
u resistance of Y1Ba2Cu3Ox films during anneal
Authors: Davidson A. , Palevski A. , Brady M. J. , Laibowitz R. B. , Koch R. , Scheuermann M. , Chi C. C.
Published: July 2002

Measurements of electrical resistance of thin films of the high-temperature superconductor Y1Ba2Cu3Ox have been carried out between 100 and 800 °C, and in the range between roughly 0.1 and 1 atm of ambient oxygen. Results are reported for both quick and slow heating, and also for sudden changes in oxygen partial pressure. At constant temperature we observe that the response to the oxygen partial pressure is both quick and reversible. We show evidence that quick heating is preferable, and we also observe features in the resistance similar to those reported in bulk samples and interpreted as an order–disorder transition of oxygen in the one-dimensional Cu-O chains.
Anomalous rf magnetoresistance in copper at 4 K
Authors: Rogers J. T. , De Panfilis S. , Melissinos A. C. , Moskowitz B. E. , Semertzidis Y. K. , Wuensch W. U. , Halama H. J. , Prodell A. G. , Fowler W. B. , Nezrick F. A.
Published: July 2002

We have measured the effect of a magnetic field on the surface resistance of polycrystalline Cu at f=1.2 GHz and at 4.4 K; under these conditions the surface resistance is well into the anomalous skin effect regime but has not reached its limiting value. We find that the transverse and longitudinal magnetoresistance are an order of magnitude smaller than the dc magnetoresistance and depend quadratically on the field. At low fields we observe a decrease in surface resistance with increasing field which can be interpreted as a size effect of the rf surface current, but is also typical of superconductors.
High-power phase-locked InGaAs strained-layer quantum well heterostructure periodic laser array
Authors: Baillargeon J. N. , York P. K. , Zmudzinski C. A. , Fernández G. E. , Beernink K. J. , Coleman J. J.
Published: July 2002

Data are presented on high-power strained-layer InGaAs quantum well heterostructure laser arrays. These devices are periodic nonplanar arrays, formed by a single metalorganic chemical vapor deposition growth over a selectively etched corrugated GaAs substrate. The corrugation serves to provide both stripe definition and index guiding while suppressing lateral lasing perpendicular to the stripes. Maximum pulsed optical powers of 2.5 W/facet (width=1600 μm, length=440 μm) for an emission wavelength of 1.03 μm have been obtained from uncoated devices having threshold current densities in the range 290–600 A/cm2. Far-field radiation patterns indicate that the arrays are phase locked.
Controlled disassembling of self-assembling systems: Toward artificial molecular-level devices and machines
Authors: Balzani Vincenzo , Credi Alberto , Venturi Margherita
Published: July 2002

Investigations on self-assembling/induced-disassembling systems have led to the design of molecular-level devices capable of performing a variety of functions. Some of the work carried out in this field is illustrated.
The accuracy of codon recognition by polypeptide release factors
Authors: Freistroffer David V. , Kwiatkowski Marek , Buckingham Richard H. , Ehrenberg Måns
Published: July 2002

The precision with which individual termination codons in mRNA are recognized by protein release factors (RFs) has been measured and compared with the decoding of sense codons by tRNA. An
Escherichia coli
system for protein synthesis
in vitro
with purified components was used to study the accuracy of termination by RF1 and RF2 in the presence or absence of RF3. The efficiency of factor-dependent termination at all sense codons differing from any of the three stop codons by a single mutation was measured and compared with the efficiency of termination at the three stop codons. RF1 and RF2 discriminate against sense codons related to stop codons by between 3 and more than 6 orders of magnitude. This high level of accuracy is obtained without energy-driven error correction (proofreading), in contrast to codon-dependent aminoacyl-tRNA recognition by ribosomes. Two codons, UAU and UGG, stand out as hotspots for RF-dependent premature termination.
DNA microarray analysis of differential gene expression in
Borrelia burgdorferi
, the Lyme disease spirochete
Authors: Revel Andrew T. , Talaat Adel M. , Norgard Michael V.
Published: July 2002

DNA microarrays were used to survey the adaptive genetic responses of
Borrelia burgdorferi
(
Bb
) B31, the Lyme disease spirochete, when grown under conditions analogous to those found in unfed ticks (UTs), fed ticks (FTs), or during mammalian host adaptation (
Bb
in dialysis membrane chambers implanted in rats). Microarrays contained 95.4% of the predicted B31 genes, 150 (8.6%) of which were differentially regulated (changes of ≥1.8-fold) among the three growth conditions. A substantial proportion (46%) of the differentially regulated genes encoded proteins with predicted export signals (29% from predicted lipoproteins), emphasizing the importance to
Bb
of modulating its extracellular proteome. For B31 cultivated at the more restrictive UT condition, microarray data provided evidence of a bacterial stringent response and factors that restrict cell division. A large proportion of genes were responsive to the FT growth condition, wherein increased temperature and reduced pH were prominent environmental parameters. A surprising theme, supported by cluster analysis, was that many of the gene expression changes induced during the FT growth condition were transient and largely tempered as B31 adapted to the mammalian host, suggesting that once
Bb
gains entry and adapts to mammalian tissues, fewer differentially regulated genes are exploited. It therefore would seem that although widely dissimilar, the UT and dialysis membrane chamber growth conditions promote more static patterns of gene expression in
Bb
. The microarray data thus provide a basis for formulating new testable hypotheses regarding the life cycle of
Bb
and attaining a more complete understanding of many aspects of
Bb
's complex parasitic strategies.
The olfactory receptor gene repertoire in primates and mouse: Evidence for reduction of the functional fraction in primates
Authors: Rouquier Sylvie , Blancher Antoine , Giorgi Dominique
Published: July 2002

Olfactory receptors (ORs) located in the cell membrane of olfactory
sensory neurons of the nasal epithelium are responsible for odor
detection by binding specific odorant ligands. Primates are thought to
have a reduced sense of smell (microsmatic) with respect to other
mammals such as dogs or rodents. We have previously demonstrated that
over 70% of the human OR genes have become nonfunctional pseudogenes,
leading us to hypothesize that the reduced sense of smell could
correlate with the loss of functional genes. To extend these results,
we sampled the OR gene repertoire of 10 primate species, from prosimian
lemur to human, in addition to mouse. About 221 previously unidentified
primate sequences and 33 mouse sequences were analyzed. These sequences
encode ORs distributed in seven families and 56 subfamilies. Analysis
showed a high fraction (≈50% on average) of pseudogenes in
hominoids. In contrast, only ≈27% of OR genes are pseudogenes in Old
World monkeys, and New World monkeys are almost free of pseudogenes.
The prosimian branch seems to have evolved differently from the other
primates and has ≈37% pseudogene content. No pseudogenes were found
in mouse. With the exception of New World monkeys, we demonstrate that
primates have a high fraction of OR pseudogenes compared with mouse. We
hypothesize that under relaxed selective constraints, primates would
have progressively accumulated pseudogenes with the highest level seen
in hominoids. The fraction of pseudogenes in the OR gene repertoire
could parallel the evolution of the olfactory sensory function.
β-Neuregulin-1 is required for the
in vivo
development of functional Ca
2+
-activated K
+
channels in parasympathetic neurons
Authors: Cameron Jill S. , Dryer Laurence , Dryer Stuart E.
Published: July 2002

The development of functional Ca
2+
-activated
K
+
channels (K
Ca
) in chick ciliary ganglion
(CG) neurons requires interactions with afferent preganglionic nerve
terminals. Here we show that the essential preganglionic
differentiation factor is an isoform of β-neuregulin-1.
β-Neuregulin-1 transcripts are expressed in the midbrain
preganglionic Edinger–Westphal nucleus at developmental stages that
coincide with or precede the normal onset of macroscopic
K
Ca
in CG neurons. Injection of β-neuregulin-1 peptide
into the brains of developing embryos evoked a robust stimulation of
functional K
Ca
channels at stages before the normal
appearance of these channels in CG neurons developing
in
vivo
. Conversely, injection of a neutralizing antiserum
specific for β-neuregulin-1 inhibited the development of
K
Ca
channels in CG neurons. Low concentrations of
β-neuregulin-1 evoked a robust increase in whole-cell K
Ca
in CG neurons cocultured with iris target tissues. By contrast,
culturing CG neurons with iris cells or low concentrations of
β-neuregulin-1 by themselves was insufficient to stimulate
K
Ca
. These data suggest that the preganglionic factor
required for the development of K
Ca
in ciliary ganglion
neurons is an isoform of β-neuregulin-1, and that this factor acts in
concert with target-derived trophic molecules to regulate the
differentiation of excitability.
Dechlorination of chloroacetanilide herbicides by thiosulfate salts
Authors: Gan Jianying , Wang Qiquan , Yates Scott R. , Koskinen William C. , Jury William A.
Published: July 2002

Halogenated organic compounds (XOCs) are among the most widely used synthetic chemicals. Many XOCs are recalcitrant to natural degradation and have become prominent environmental contaminants. One group of such XOCs are the heavily used chloroacetanilide herbicides. We have found that chloroacetanilide herbicides are rapidly dechlorinated in water, sand, and soil by thiosulfate salts under ambient conditions. Structural and kinetics analysis suggests that the reaction occurred by S
N
2 nucleophilic substitution, in which the chlorine was replaced by thiosulfate and the herbicide was detoxified. Laboratory studies showed that this reaction could be used for removing residues of chloroacetanilide herbicides in water, soil, and sand. Our findings also suggest that some other XOCs may be subject to this reaction. Because common thiosulfate salts are innocuous products (e.g., fertilizers) and the reaction selectively detoxifies XOCs at low thiosulfate levels, this discovery may lead to a new way for safe removal of certain XOCs from the environment.
Regulation of Kv1 subunit expression in oligodendrocyte progenitor cells and their role in G
1
/S phase progression of the cell cycle
Authors: Chittajallu R. , Chen Y. , Wang H. , Yuan X. , Ghiani C. A. , Heckman T. , McBain C. J. , Gallo V.
Published: July 2002

Proliferative oligodendrocyte progenitor cells (OPs) express large, delayed outward-rectifying K
+
currents (I
K
), whereas nondividing immature and mature oligodendrocytes display much smaller I
K
. Here, we show that up-regulation of I
K
occurs in G
1
phase of the cell cycle in purified cultured OPs and is the result of an RNA synthesis-dependent, selective increase of the K
+
channel subunit proteins Kv1.3 and Kv1.5. In oligodendrocyte cells acutely isolated from developing rat brain, a decrease of cyclin D expression is observed as these cells mature along their lineage. This is accompanied by a decrease in Kv1.3 and Kv1.5 subunit expression, suggesting a role for these subunits in the proliferative potential of OPs
in situ
. I
K
expressed in OPs in subventricular zone and developing white matter in acutely isolated slice preparations were selectively blocked by antagonists of Kv1.3, illustrating the functional presence of this subunit
in situ
. Interestingly, Kv1.3 block inhibited S-phase entry of both purified OPs in culture and in tissue slice cultures. Thus, we employ both
in vitro
and
in situ
experimental approaches to show that (
i
) RNA-dependent synthesis of Kv1.3 and Kv1.5 subunit proteins occurs in G
1
phase of the OP cell cycle and is responsible for the observed increase in I
K
, and (
ii
) currents through Kv1.3-containing channels play a crucial role in G
1
/S transition of proliferating OPs.
Linking development with generation of novelty in mammalian teeth
Authors: Jernvall Jukka
Published: July 2002

The evolution of mammalian teeth is characterized by the frequent
and convergent evolution of new cusps. The evolution of new cusps can
be linked to tooth development via population-level variation. This
allows testing whether development increases the capacity to evolve, or
evolvability, by facilitating and even directing morphological change.
In a population sample of living seals, variation in cusp number of
individual teeth is from three to five cusps, the variably present
cusps being the shortest ones that also develop last. By factoring in
recent evidence on development, I show that the variation in cusp
number can be explained by a patterning cascade mode of cusp
development that cumulatively increases and directs height variation in
short cusps. The biased variation in seal tooth cusps supports the
recognition of teeth as highly evolvable because only small
developmental changes are needed to produce large changes in size and
number of small cusps. This evolvability of tooth cusps may have
facilitated the fast and independent acquisition of new cusps in
mammalian evolution. In phylogenetic studies, small cusps may be
unreliable as phylogenetic signals. Population level variation can be a
powerful tool in testing and generating hypotheses in developmental
evolution studies.
Network of coupled promoting motions in enzyme catalysis
Authors: Agarwal Pratul K. , Billeter Salomon R. , Rajagopalan P. T. Ravi , Benkovic Stephen J. , Hammes-Schiffer Sharon
Published: July 2002

A network of coupled promoting motions in the enzyme dihydrofolate reductase is identified and characterized. The present identification is based on genomic analysis for sequence conservation, kinetic measurements of multiple mutations, and mixed quantum/classical molecular dynamics simulations of hydride transfer. The motions in this network span time scales of femtoseconds to milliseconds and are found on the exterior of the enzyme as well as in the active site. This type of network has broad implications for an expanded role of the protein fold in catalysis as well as ancillaries such as the engineering of altered protein function and the action of drugs distal to the active site.