bluuebunny/crossref_metadata_2025 huggingface dataset:
Band-edge photorefractive effect in semiconductors
Authors: Partovi Afshin , Kost Alan , Garmire Elsa M. , Valley George C. , Klein Marvin B.
Published: July 2002

We report here for the first time, two-beam coupling energy transfer using the Franz–Keldysh electrorefractive effect in photorefractive semiconductors. A large beam coupling gain coefficient (Γ=2.8 cm−1) exceeding the absorption coefficient (α=2.0 cm−1) is obtained in undoped semi-insulating GaAs using the new photorefractive process. The new photorefractive process differs from conventional photorefractivity in that the direction of energy transfer is dictated by the direction of an externally applied electric field. Using a moving grating technique and combining the electrorefractive grating with the conventional electro-optic grating, a very large gain Γ=16.3 cm−1 (α=3 cm−1, crystal length l=4mm) resulting in net amplification by more than a factor of 200 has been demonstrated.
AlxGa1−xAs-GaAs vertical-cavity surface-emitting laser grown on Si substrate
Authors: Deppe D. G. , Chand Naresh , van der Ziel J. P. , Zydzik G. J.
Published: July 2002

Data are presented demonstrating room-temperature operation of AlxGa1−xAs-GaAs vertical-cavity surface-emitting lasers grown on Si substrates. The device structures are grown using molecular beam epitaxy and a grown-in quarter-wave AlAs-GaAs stack is used as the n-side reflector and a nonalloyed Ag dot used as the p-side reflector/contact. Pulsed threshold currents of ∼125 mA are obtained for a 15-μm-diam device.
X-ray photoelectron spectroscopy studies of initial growth mechanism of CdTe layers grown on (100)GaAs by organometallic vapor phase epitaxy
Authors: Sone Syuji , Ekawa Mitsuru , Yasuda Kazuhito , Sugiura Yoshiyuki , Saji Manabu , Tanaka Akikazu
Published: July 2002

Variations of the GaAs surface conditions and the adsorption of the precursor elements of Cd and Te on the (100)GaAs substrate were studied by x-ray photoelectron spectroscopy at the initial stage of CdTe growth by organometallic vapor phase epitaxy. The stoichiometry of GaAs substrates was found to recover by annealing in the H2 environment (500 °C, 5 min), while the surface was initially in an As-rich condition after etching with H2SO4:H2O2:H2O (5:1:1). The preferential adsorption of Te on the GaAs surface was also observed. 〈100〉 oriented growth was obtained routinely when the GaAs surface was fully stabilized with Te after the H2 anneal under the above conditions. 〈111〉 oriented growth resulted when dimethylcadmium was first introduced after the anneal.
Epitaxial growth of n+-n GaAs metal-semiconductor field-effect transistor structures using tertiarybutylarsine
Authors: Lum R. M. , Klingert J. K. , Ren F. , Shah N. J.
Published: July 2002

We report the first demonstration of metal-semiconductor field-effect transistors (MESFETs) made from GaAs structures grown with an alkylarsine source, tertiarybutylarsine (t-BuAsH2). MESFET fabrication was performed in parallel on t-BuAsH2 and arsine-grown wafers to enable direct comparison of device characteristics. The GaAs n+-n MESFETs made with t-BuAsH2 exhibited excellent saturation and pinch-off characteristics, and diode performance comparable to arsine-grown devices. Although the peak transconductance gm was lower than that achieved with the arsine sample, the form of the gm versus gate voltage curves for the t-BuAsH2-grown devices were characteristic of well-behaved GaAs MESFETs. These initial results demonstrate the capability of t-BuAsH2 for growing electronic device structures having good carrier transport properties and effective isolation layers.
Deactivation of the photoionization excitation mechanism of rare-earth intraion emission in ZnS
Authors: Świa̧tek K. , Godlewski M.
Published: July 2002

A new, very efficient excitation mechanism of the rare-earth (RE) intraion emission was observed recently. This is a process in which the RE ion undergoes ionization and the RE intraion emission is induced when the ionized carrier is recaptured by the RE. The capture process proceeds via an intermediate state due to RE bound exciton (BE), from which the energy is transferred to the RE excited state. In this letter we discuss mechanisms which limit the efficiency of this excitation channel. The first is the thermal dissociation of the RE BE, which reduces the efficiency of the energy transfer thus quenches the RE intraion emission. It is also indicated that an efficient energy transfer from RE Be to iron Fe2+ centers competes with the transfer of the BE energy to RE core states. Our results mean that the practical utilization of the above excitation mechanism of emission in the RE-activated ZnS devices may be hampered by the common contamination of this material with iron.
Determination of donor and acceptor densities in high-purity GaAs from photoluminescence analysis
Authors: Lu Z. H. , Hanna M. C. , Szmyd D. M. , Oh E. G. , Majerfeld A.
Published: July 2002

We report a new analysis technique to determine the acceptor density NA and the donor density ND in high-purity GaAs from 10 K photoluminescence (PL) measurements. For both n-type and p-type samples, we find that NA/ND is related to the excitonic intensity ratio rx=I(A0,X)/I(D0,X) by NA/ND=0.89rx−0.06. In addition, NA can be determined from NA=1014IA, where IA is the emission intensity of the donor-acceptor pair transition normalized to the intensity of the unresolved exciton peak. Therefore, for n-type and p-type material with an impurity density 1013–1016 cm−3, NA and ND can be determined solely from a 10 K PL measurement. The advantage of this technique lies in its nondestructive nature and its applicability to situations where Hall measurements are not possible or suitable.
Observations of quasi-particle tunneling and Josephson behavior in Y1Ba2Cu3O7−x/ native barrier/Pb thin-film junctions
Authors: Kwo J. , Fulton T. A. , Hong M. , Gammel P. L.
Published: July 2002

Low-leakage, thin-film planar tunnel junctions made of Y1Ba2Cu3O7−x/ native barrier/Pb were fabricated. The Y1Ba2Cu3O7−x films were prepared by in situ molecular beam epitaxy aided with an activated oxygen source. The as-grown, smooth superconducting perovskite film surface exhibits quasi-particle tunneling characteristics very similar to the etched bulk single-crystal data. The results in agreement are a linear dependence of the normal-state conductance on voltage, a gap-like structure at ∼20 mV, asymmetric modulations up to 50 mV, and a finite zero-bias conductance at low temperature. Junctions of lower resistance show, at temperatures below Tc of Pb, the development of a supercurrent at zero bias and associated hysteretic subgap structure, with a typical IcR ∼0.5 mV. Josephson-like behavior occurred in response to applied magnetic field and microwaves.
Carbon tetrachloride doped AlxGa1−xAs grown by metalorganic chemical vapor deposition
Authors: Cunningham B. T. , Baker J. E. , Stillman G. E.
Published: July 2002

A dilute mixture of CCl4 in H2 has recently been shown to be a suitable carbon doping source for obtaining p-type GaAs grown by metalorganic chemical vapor deposition (MOCVD) with carbon acceptor concentrations in excess of 1×1019 cm−3. To understand the effect of growth parameters on carbon incorporation in CCl4-doped AlxGa1−xAs, carbon acceptor concentration was studied as a function of Al composition, growth temperature, growth rate, and CCl4 flow rate using electrochemical capacitance-voltage profiling. The carbon incorporation as a function of Al composition, growth temperature, and CCl4 flow rate was also measured by secondary-ion mass spectroscopy. All layers were grown by low-pressure MOCVD using TMGa and TMAl as column III precursors, and 100% AsH3 as the column V source. Increased Al composition reduced the dependence of carbon concentration on the growth temperature. Reduced growth rate, which resulted in substantially decreased carbon acceptor concentrations in GaAs, had an insignificant effect on the carrier concentration of Al0.4Ga0.6As. A linear relationship between hole concentration and CCl4 flow rate in AlxGa1−xAs for 0.0≤x≤0.8 was observed. These results are interpreted to indicate that adsorption and desorption of CCly (y≤3) on the AlxGa1−xAs surface during crystal growth plays an important role in the carbon incorporation mechanism.
Migration-enhanced epitaxy growth and characterization of high quality ZnSe/GaAs superlattices
Authors: Ramesh S. , Kobayashi N. , Horikoshi Y.
Published: July 2002

We report the growth of high quality ZnSe/GaAs superlattices by migration-enhanced epitaxy (MEE) and their characterization using x-ray diffraction, electron microscopy, and photoluminescence. A streaky reflection high-energy electron diffraction (RHEED) pattern and strong, persistent RHEED oscillation during the MEE growth of the superlattices indicate a smooth growing surface. The sharp satellite peaks observed clearly in the double-crystal x-ray diffraction rocking curve of a 21-period ZnSe/GaAssuperlattice confirm the excellent crystalline and interfacial quality of the superlattice. Cross-section high-resolution electron microscopy indicates coherent lattice arrangement and abrupt interface at ZnSe-on-GaAs as well as GaAs-on-ZnSe heterointerfaces. Photoluminescence spectra from a superlattice with a periodicity of 40 nm show a strong peak at a wavelength of 829.5 nm with a linewidth of 6.3 meV at a temperature of 10 K; we believe this to be the first observation of photoluminescence from any multilayer structure involving GaAs-on-ZnSe growth.
Microsecond carrier lifetimes in strained silicon-germanium alloys grown by rapid thermal chemical vapor deposition
Authors: Schwartz P. V. , Sturm J. C.
Published: July 2002

We report generation lifetimes of over 1 μs in Si0.82Ge0.18 epitaxial strained layers grown by rapid thermal chemical vapor deposition on silicon substrates. By using a pulsed metal-oxide-semiconductor capacitor technique, we were able to probe the minority-carrier properties of a layer of Si0.82Ge0.18 sandwiched between two epitaxial layers of silicon. We also show that the band gap and the intrinsic carrier concentration are important when relating experimental results to the generation lifetime τg.
ac conductance measurements on radiation-damaged (100) Si/SiO2 interface after defect transformation
Authors: Vishnubhotla Lakshmanna , Chen Wenliang , Ma T. P.
Published: July 2002

We report experimental results of multifrequency ac conductance measurements on radiation-damaged (100) Si-SiO2 interface after the interfacial defect transformation process has taken place. Previous quasi-static capacitance-voltage measurements on such samples have identified two distinct interface trap peaks, one above midgap and the other below midgap. The presence of such a double-peak interface trap distribution has been confirmed by our ac conductance measurements, and excellent quantitative agreement with the capacitance measurements has been achieved. In addition, the ac conductance measurements showed a strong energy dependence of the capture cross sections of interface traps for both electrons (in the upper half of the band gap) and holes (in the lower half of the band gap). The capture cross sections vary systematically by more than an order of magnitude in an energy range of about 200 meV in either half of the band gap, suggesting that there may be a broad distribution of different defect configurations and/or their surrounding environments at the interface after radiation damage. The energy dispersion of the interface trap time constants and surface potential fluctuations are also obtained and used for accurate extraction of the interface trap density from these data.
Pressure-dependent transition in the mechanism of remote plasma SiNx deposition
Authors: Meikle Scott , Hatanaka Yoshinori
Published: July 2002

Optical emission properties in remote plasma deposition of SiNx from an N2 electron cyclotron resonance microwave plasma and SiH4 have been investigated over a pressure range of 0.002–0.3 Torr. The deposition process divides into three pressure regions. For p≳0.1 Torr, film deposition results from gas phase reactions between active nitrogen and SiH4. For p<0.02 Torr SiH4 travels back to the plasma and deposition results from radical species created in the N2/SiH4 mixture. At intermediate pressures deposition was negligible because neither process could substantially break down SiH4.
Hydrogen desorption on various H-terminated Si(100) surfaces due to electron beam irradiation: Experiments and modeling
Authors: Hsu T. , Lin S. , Anthony B. , Qian R. , Irby J. , Kinosky D. , Mahajan A. , Banerjee S. , Tasch A. , Marcus H.
Published: July 2002

Hydrogen desorption from (2×1) and (3×1) H-terminated Si(100) surfaces due to irradiation by electron beams with 2–5 keV beam energies has been investigated both experimentally and theoretically. Auger electron spectroscopy (AES) has been employed to monitor Si, O, and C signals periodically with continuous irradiation of an electron beam on H-terminated Si(100) surfaces. An incubation phenomenon is observed in the time evolution profiles of the Si, O, and C AES signals for all H-terminated Si(100) surfaces. The incubation period is believed to be associated with the time required for desorption of hydrogen from the H-terminated Si surface as a result of electron beam irradiation. Among (2×1) and (3×1) H-terminated Si(100) surfaces, the (3×1) surface is found to have greater hydrogen coverage than (2×1) surface. The hydrogen desorption cross section is found to range from 4×10−19 to 8×10−18 cm2 and decrease with increasing beam energy in the 2–5 keV range.
Role of dispersion in limiting pulse width in fiber lasers
Authors: Dennis Michael L. , Duling Irl N.
Published: July 2002

Generation of spectral sidebands is characteristic of ultrashort soliton all-fiber lasers. We relate the generation of these sidebands to a loss mechanism dependent on the pulse duration and total cavity dispersion. We confirm experimentally using an erbium figure eight laser that the minimum pulse duration is therefore limited by the total dispersion. Short cavities constructed from low-dispersion components allow shorter pulses to be obtained, and we demonstrate pulse durations as short as 160 fs.
Fermi level dependence of the ambipolar diffusion length in amorphous silicon thin film transistors
Authors: Schwarz R. , Wang F. , Reissner M.
Published: July 2002

The steady-state photocarrier grating technique was applied to measure the ambipolar diffusion length (Lamb) in a-Si:H thin film transistors. With this device we studied the Fermi level dependence of Lamb through the variation of the gate voltage. The experimental results show that Lamb decreases whereas the photoconductivity increases with positive gate voltage. We simulated the Fermi level dependence of electron and hole mobility-lifetime products and found good agreement with the measured diffusion length.
Bismuth induced enhancement of the second-harmonic generation efficiency in bismuth-substituted yttrium iron garnet films
Authors: Petrocelli G. , Martellucci S. , Richetta M.
Published: July 2002

The second-harmonic response of pure and bismuth-substituted yttrium iron garnet films has been measured. A strong enhancement of the second-harmonic generation (SHG) efficiency, related to the incorporation of bismuth, has been observed.
Green upconversion continuous wave Er3+:LiYF4 laser at room temperature
Authors: Heine F. , Heumann E. , Danger T. , Schweizer T. , Huber G. , Chai B.
Published: July 2002

We report room-temperature upconversion pumped continuous wave laser emission of 1% Er3+:LiYF4 at 551 nm excited by a Ti:sapphire laser at 810 nm. Output powers of up to 40 mW with output coupling of 6.6% have been obtained by using nearly concentric resonator design.
Tokamak ion temperature and poloidal field diagnostics using 3-MeV protons
Authors: Heidbrink W. W. , Strachan J. D.
Published: July 2002

The 3-MeV protons created by d(d, p)t fusion reactions in a moderately sized tokamak leave the plasma on trajectories determined by the position of their birth and by the poloidal magnetic field. Pitch-angle resolution of the escaping 3-MeV protons can separately resolve the spatial distribution of the d(d, p)t fusion reactions and the poloidal field distribution inside the tokamak. These diagnostic techniques have been demonstrated on PLT with an array of collimated surface barrier detectors.
Confinement and bistability in a tapered hemispherically lensed optical fiber trap
Authors: Lyons E. R. , Sonek G. J.
Published: July 2002

We describe the operation and performance of a dual fiber optical trap created using tapered lensed optical fibers pigtailed to 1300 nm laser diodes. Single-mode fibers, having mode field diameters of ∼9.5 μm, and separated by up to 350 μm, are used to demonstrate dielectric particle confinement over two orders of magnitude in fiber trapping power ratio. Axial and transverse trap efficiencies, as well as the existence of bistable trapping positions, are predicted and experimentally confirmed. The use of fiber lenses results in the creation of an optical trap that provides strong transverse optical confinement and which is nearly insensitive to the fiber polarization state.
Photoluminescence of infrared-sensing materials using an FTIR spectrometer
Authors: Reisinger A. R. , Roberts R. N. , Chinn S. R. , Myers II T. H.
Published: July 2002

Semiconductor materials commonly used in mid- and long-wavelength infrared sensing applications generally have a low radiative efficiency, and the ambient blackbody radiation spectrum often dominates and interferes with the identification of the photoluminescence peak. Two techniques designed to facilitate the extraction of the spontaneous radiative spectrum are discussed and compared.
Time resolution limits for two-color pump/probe spectroscopy
Authors: Alavi D. S. , Waldeck D. H.
Published: July 2002

The characteristics of various cw mode-locked dye lasers were thoroughly explored, and the characteristics of a pump/probe apparatus employing two such lasers pumped by a single mode-locked pump laser. First, it was observed that the rf source used to drive the acousto-optic mode-locking element in the pump laser, while not affecting the average power or obtainable pulse width, had a profound effect on the noise characteristics of the pump laser, which are in turn passed along to the dye lasers. For a driver based on a tunable frequency synthesizer, pulse jitter on the order of 300 ps was observed, which represents ≊2.5% of the total interpulse time interval. This was deemed unacceptably high, and use of this mode-locker driver was discontinued. For a driver with a fixed frequency reference source (an oscillating quartz crystal), pulse jitter on the order of 5 ps was measured, a great improvement over the other driver. Next we studied various dye laser configurations pumped by this pump laser. We found that for time resolution of several picoseconds, a pair of synchronously pumped dye lasers employing birefringent filter tuning elements was an excellent source of independently tunable pulse trains, and that these pulse trains were well synchronized with each other despite the absolute timing jitter in the pump pulse train. To obtain better temporal resolution with independently tunable lasers was more problematic, however. A hybridly mode-locked laser (employing a saturable absorber within a synchronously pumped dye laser) can produce shorter pulses (≤100 fs has been reported), but the lack of temporal synchronization between two of these lasers limits the resolution of a two wavelength experiment to several picoseconds. An attempt was made to reduce this relative timing jitter by crossing two independently tunable hybrid lasers through a common intracavity saturable absorber, thereby temporally locking the pulse trains together. This mechanism failed to work well on time scales longer than a millisecond.
Nitrogen and fluorine ion implantation in InxGa1−xN
Authors: Zolper J. C. , Pearton S. J. , Abernathy C. R. , Vartuli C. B.
Published: July 2002

Implantation of N+ ions in n-type InxGa1−xN (0.37≤x≤1.0) produces maximum increases in sheet resistance of 50–100 times upon annealing in the range of 400–600 °C. The dominant deep state introduced by implantation and annealing have ionization energies of ∼0.35–0.39 eV and therefore are relatively high in the band gap of the InGaN. There was no evidence for chemical deep levels associated with the implanted N+ or F+. The implant isolation behavior of n-type InGaN appears analogous to that of InP and InGaAs.
Ideal hydrogen termination of Si(001) surface by wet-chemical preparation
Authors: Morita Yukinori , Tokumoto Hiroshi
Published: July 2002

A nearly ideal Si(001) surface was prepared by a wet-chemical method with a solution of HF:HCl=1:19 (pH<1). The surface was examined by scanning tunneling microscopy and was found to be covered by the uniform dihydride phase. Its successful preparation was the direct consequence of the following facts: The suppression of the (111) facet formation due to a low concentration of OH ions in the etchant solution and the stabilization of the surfaces structure due to the formation of the ordered steps.
Calibration and test of the tangential phase contrast imaging diagnostic on CDX-U
Authors: Lo E. , Nazikian R. , Stutman D. , Choe W. , Kaita R.
Published: July 2002

A novel CO2 laser-based tangential imaging diagnostic was completely calibrated and the system was tested on CDX-U plasmas. It was shown that localized, two-dimensional images of the plasma electron density fluctuations in the tokamak core can be obtained from a tangential imaging beam [E. Lo, J. Wright, and R. Nazikian, Rev. Sci. Instrum. 66, 1180 (1995)]. A variation of the Zernike phase mirror is used. Test measurements made on sound waves verify that the system will image phase variations placed at the object plane. An absolute sensitivity of ≈1×107 cm−3/Hz was determined. A series of plasma measurements made on CDX-U is described. Results show that the plasma fluctuations are two to three orders of magnitude above the noise floor of the diagnostic. It is also verified that a high-pass cut-off wave number (from ≈1 to 8 cm−1) for the fluctuations can be selected by translating the phase mirror. The density fluctuation k spectrum is measured and found to peak at ≈1–3 cm−1.
A high-resolution scanning Kelvin probe microscope for contact potential measurements on the 100 nm scale
Authors: Nabhan W. , Equer B. , Broniatowski A. , De Rosny G.
Published: July 2002

The article describes the principles and current performance of a scanning Kelvin probe microscope for contact potential measurements with a lateral resolution on the 100 nm scale and a sensitivity in the millivolt range. Preliminary results are presented regarding the variation of the surface potential across charged grain boundaries in polycrystalline silicon.
Teeth mobility measurement by laser Doppler vibrometer
Authors: Castellini P. , Scalise L.
Published: July 2002

In this work a new procedure for the assessment of teeth mobility, based on a noncontact sensor (laser Doppler vibrometer), is presented. The procedure is based on the manual application of impulses applied with an instrumented hammer on the tooth crown. The ratio of the maximum displacement of the tooth to the peak of the input force has been considered as the mobility degree index. A basic mechanical model of the hammer-tooth impact based on the data extracted from literature has been realized. Results produced with the model mentioned above have been compared with those measured with in vitro and in vivo tests using the proposed procedure. Good agreement between data from the mechanical model and in vitro conditions has been found. Tests carried out in in vitro and in vivo conditions have demonstrated the validity of the procedure proposed. Measurement results compared to those found in the relevant literature show the validity of the procedure proposed.
Effects of column III alkyl sources on deep levels in GaN grown by organometallic vapor phase epitaxy
Authors: Lee W. I. , Huang T. C. , Guo J. D. , Feng M. S.
Published: July 2002

Two different kinds of n-type GaN films were prepared by organometallic vapor phase epitaxy, one by using trimethylgallium (TMGa) and another by using triethylgallium (TEGa) as the alkyl source. Schottky diodes with well-behaved current–voltage and capacitance–voltage characteristics were fabricated. Deep-level transient spectroscopy studies were performed on these samples. Three distinct deep levels, labeled E1, E2, and E3, were measured in the film grown with TMGa, with an activation energy of 0.14, 0.49, and 1.63±0.3 eV, respectively. Only one level, E3, was observed in the film prepared with TEGa.
High electro-optic side-chain polymer by vapor deposition polymerization
Authors: Roberts C. C. , Yang G.-R. , Cocoziello A. , Zhao Y.-P. , Wnek G. , Lu T.-M.
Published: July 2002

In this letter, we report high electro-optic methylene di(phenylene isocyanate) (MDI)/DR19 side-chain polymer film polymerization by vapor deposition polymerization. For samples deposited at substrate temperatures from 10 to 30 °C, the electro-optic (EO) coefficient, r33, was measured to be 5 pm/V after poling. A lifetime of about one week was obtained. The highest EO effect observed were films deposited at −40 °C and polymerized after poling. The EO coefficient of these samples is about 24 pm/V while the lifetime is only about 30 min. The effect of substrate temperature, the ratio of monomers, and the poling temperature on the nonlinearity of the films are studied.
High-resolution imaging using near-field scanning optical microscopy and shear force feedback in water
Authors: Moyer Patrick J. , Kämmer Stefan B.
Published: July 2002

Near-field scanning optical microscopy using shear force feedback in water is demonstrated with better than 50 nm lateral spatial resolution. Results also indicate the potential to measure topographic features with higher force sensitivity than contact and noncontact atomic force microscopy (AFM) measurements. Soft surface contaminants are clearly imaged using shear force feedback while the simultaneously obtained optical contrast is due to the higher optical density surface features lying beneath the surface contaminants. AFM measurements do not appear to show the same sensitivity to surface features leading to the conclusion that shear force feedback may be better for imaging soft (i.e., low mechanical compliance) samples, such as biological specimens, in liquid.
Friction force microscopy study of a cleaved ferroelectric surface: Time and temperature dependence of the contrast, evidence of domain structure branching
Authors: Correia A. , Massanell J. , García N. , Levanyuk A. P. , Zlatkin A. , Przeslawski J.
Published: July 2002

A freshly cleaved surface of ferroelectric triglycine sulfate (TGS) has been studied by scanning friction force microscopy. Well-defined areas, exhibiting a strong contrast difference, have been found just after cleavage in the ferroelectric phase. These regions correspond to different ferroelectric domains. ‘‘Interactions’’ between small regions and large negative domains are also observed leading us to conclude that these small features could be interpreted as nuclei of negative domains inside the positive one, evidence of the domain structure branching at the submicron scale.
In situ Si flux cleaning technique for producing atomically flat Si(100) surfaces at low temperature
Authors: Wilk G. D. , Wei Yi , Edwards Hal , Wallace R. M.
Published: July 2002

We have developed a method for removing oxides and producing atomically flat Si(100) surfaces with single atomic height steps using a Si flux cleaning technique. By introducing a Si flux in the range 1.0–1.5 Å/s at the onset of an SiO2 thermal desorption step as low as 780 °C, scanning tunneling microscopy (STM) and atomic force microscopy images reveal smooth surfaces with atomically flat terraces with an rms roughness of 0.5 Å and single-step heights of 1.4 Å. STM reveals that A- and B-type steps are present across the entire area of the scanned surface. Desorption of the surface oxide layer with Si fluxes below this range results in rougher surfaces with pits ∼50 Å deep and 1000 Å across. For Si fluxes above this range, no pits are seen but atomic steps are not visible on the surface.
The structure, magnetostriction, and anisotropy compensation of (Tb1−xPrx)(Fe0.4Co0.6)1.9 alloys
Authors: Guo Z. J. , Wang B. W. , Zhang Z. D. , Zhao X. G. , Jin X. M. , Liu W. , Xiao Q. F.
Published: July 2002

The structure, magnetostriction, and anisotropy compensation of (Tb1−xPrx)(Fe0.4Co0.6)1.9 (0⩽x⩽1) alloys were investigated. It was found that the matrix of (Tb1−xPrx)(Fe0.4Co0.6)1.9 alloys is a (Tb,Pr)(Fe,Co)2 phase with a MgCu2-type cubic Laves structure, and a second phase of small amount is a (Tb,Pr)(Fe,Co)3 phase when x⩽0.2 and x=0.5 and 1.0. In the range 0.2<x⩽0.4, the amount of the (Tb,Pr)(Fe,Co)3 phase with a PuNi3-type structure increases with increasing x and becomes the main phase when x=0.4. When 0.5<x⩽0.6, the (Tb,Pr)(Fe,Co)3 phase increases sharply, whereas the 0.6<x⩽0.8, it decreases with increasing x. The spontaneous magnetostriction λ111 of the alloys exhibits a peak near x=0.8 (λ111=2370×10−6). The magnetostriction |λ∥−λ⊥| at room temperature was also examined.
Isoelectronic In-doping effect in GaN films grown by metalorganic chemical vapor deposition
Authors: Shu C. K. , Ou J. , Lin H. C. , Chen W. K. , Lee M. C.
Published: July 2002

The isoelectronic In-doping effect in GaN films grown by metalorganic chemical vapor deposition was investigated by using Raman scattering, scanning electron microscopy (SEM), and x-ray and photoluminescence (PL) measurements. In our study, the phonon spectra of films remain sharp without alloy formation after incorporation of small amounts of In atoms. The SEM pictures of the sample surface reveal greatly reduced nanopits indicating better surface flatness that is also supported by the multiple interference effect in the PL signals. More importantly, isoelectronic doping has caused the linewidth at 15 K of the near-band-edge emission of GaN to decrease sharply to 10 meV or less, reflecting improved optical property.
Magnetoresistance of a two-dimensional electron gas due to a single magnetic barrier and its use for nanomagnetometry
Authors: Kubrak V. , Rahman F. , Gallagher B. L. , Main P. C. , Henini M. , Marrows C. H. , Howson M. A.
Published: July 2002

We investigate the longitudinal resistance of a semiconductor near-surface two-dimensional electron gas (2DEG) subjected to a magnetic barrier induced by the stray field from a single sub-micron ferromagnetic line on the surface of the device. The amplitude of the magnetic barrier is controlled by the application of an external magnetic field in the plane of the 2DEG. We show that this type of magnetoresistance can be used to deduce properties of the ferromagnetic line, so that our hybrid ferromagnet-semiconductor structure acts as a nanomagnetometer.
Electron field emission from surface treated tetrahedral amorphous carbon films
Authors: Shi X. , Cheah L. K. , Tay B. K. , Silva S. R. P.
Published: July 2002

The electron field emission properties of tetrahedral amorphous carbon thin films deposited using a filtered cathodic vacuum arc system have improved as a result of surface treatment with H, O, and Ar ions. The limiting factor of the emission process does not appear to be only the front surface of the films. The improvement in the emission after ion beam treatment appears to be independent of the ions used. The surface which has been analyzed using ultraviolet photospectroscopy, reflected electron energy loss spectroscopy, and scanning tunneling microscopy shows evidence of the creation of segregated sp2 and sp3 rich regions of less than 20 nm in dimension. An extension to the space charge-induced band bending model including a multistep emission process that occurs in this mixed phase material is proposed.
Nonlinear optical absorption and temporal response of arsenic- and oxygen-implanted GaAs
Authors: Lederer M. J. , Luther-Davies B. , Tan H. H. , Jagadish C. , Haiml M. , Siegner U. , Keller U.
Published: July 2002

We have measured the nonlinear optical absorption of arsenic and oxygen implanted epitaxial GaAs for a range of ion doses and annealing temperatures. The response time, τA, and a parameter, Mmax, which characterizes the performance of the structures as modulators, are both reduced by implantation, and correspondingly the nonbleachable losses are increased. We show that similar combinations of (τA, Mmax) can be achieved using either ion species and various combinations of dose and annealing temperatures. Furthermore, the data were all located on a well-defined curve in the (τA, Mmax) plane, provided amorphization, which occurs at high implant doses, was avoided. We deduce that there exists a limit to the modulation if a specific response time is required.
Electroluminescence of europium silicate thin film on silicon
Authors: Qi Jifa , Matsumoto Takahiro , Tanaka Masanori , Masumoto Yasuaki
Published: July 2002

We report an electroluminescent device fabricated by a europium silicate layer on a silicon substrate. The device exhibits uniform-intense white color electroluminescence with an external quantum efficiency about 0.1% at room temperature, a low operating threshold voltage (about 6 V) and a fast response to the modulation signal at the frequency of 1 MHz.
Fountain pen nanochemistry: Atomic force control of chrome etching
Authors: Lewis Aaron , Kheifetz Yuri , Shambrodt Efim , Radko Anna , Khatchatryan Edward , Sukenik Chaim
Published: July 2002

In this report we demonstrate a general method for affecting chemical reactions with a high degree of spatial control that has potentially wide applicability in science and technology. Our technique is based on complexing the delivery of liquid or gaseous materials through a cantilevered micropipette with an atomic force microscope that is totally integrated into a conventional optical microscope. Controlled etching of chrome is demonstrated without detectable effects on the underlying glass substrate. This simple combination allows for the nanometric spatial control of the whole world of chemical reactions in defined regions of surfaces. Applications of the technique in critical areas such as mask repair are likely.
Field emission of different oriented carbon nanotubes
Authors: Chen Yan , Shaw David T. , Guo Liping
Published: July 2002

Field emission data from aligned high-density carbon nanotubes (CNTs) with orientations parallel, 45°, and perpendicular to the substrate have been obtained. The large-area uniformly distributed CNTs were synthesized on smooth nickel substrates via dc plasma-assisted hot filament chemical vapor deposition. CNTs with diameters in the range of 100–200 nm were employed in this study. The different orientations were obtained by changing the angle between the substrate and the electrical field direction. The growth mechanism for the alignment and orientation control of CNTs has been discussed. The CNTs oriented parallel to the substrate have a lower onset applied field than those oriented perpendicular to the substrate. This result indicates that electrons can emit from the body of the CNT, which means that the CNT can be used as a linear emitter. The small radius of the tube wall and the existence of defects are suggested as the reasons for the emission of electrons from the body of the tubes.
Imaging of acoustic fields in bulk acoustic-wave thin-film resonators
Authors: Safar H. , Kleiman R. N. , Barber B. P. , Gammel P. L. , Pastalan J. , Huggins H. , Fetter L. , Miller R.
Published: July 2002

By using an atomic-force-microscope-based technique, we image the vibration of high-frequency, bulk-mode, thin-film resonators. Our experimental technique is capable of monitoring the vibration of these devices over a broad frequency range, from 1 MHz to beyond 10 GHz, allowing us to obtain quantitative measurements of the piezoelectric properties of thin-film materials in that frequency range. This technique allows us to map the complex vibration modes of a new generation of high-frequency bulk piezoelectric resonators, revealing the presence of vibration patterns of very different characteristic lengths.
Semiconductor wafer bonding via liquid capillarity
Authors: Liau Z. L.
Published: July 2002

Liquid surface tension has been used to pull different semiconductor wafers to very close contact and strong bonding. Bonded wafers, such as GaAs/GaP, were heat treated without pressure application to achieve wafer fusion. The bonding process has been analyzed, and criteria for surface tension, wafer flatness, and elasticity have been derived.
Finite-source dye-diffusion thermal transfer for doping and color integration of organic light-emitting devices
Authors: Wu Chung-chih , Yang Cheng-chung , Chang Hsin-hua , Chen Chieh-Wei , Lee Cheng-Chung
Published: July 2002

An effective process of performing controllable doping of polymer films in organic light-emitting devices is reported. In this approach, a film to be doped is brought into direct contact with a dye-dispersed polymer donor film to permit direct dye-diffusion thermal transfer. Theoretical and experimental studies indicate that this doping process can be modeled by Fick’s diffusion theory and that a desired dopant distribution may be obtained in a single transfer step by adjusting the diffusion conditions. Doped-polymer light-emitting devices made by this process exhibited the same device characteristics as those by the conventional blending process. Along with patterned color donor plates, we also demonstrated multicolor devices of arbitrary patterns over large areas with a single thermal transfer step.
Detection of localized hot electrons in low-pressure large-area microwave discharges
Authors: Terebessy Tibor , Kando Masashi , Kudela Jozef
Published: July 2002

A localized hot-electron region was observed in low-pressure (<3 mTorr) large-area microwave discharges. The region appears in the vicinity of the waveguiding plasma–dielectric interface in the place of critical plasma density. The existence of localized hot electrons is explained on the basis of transit time heating in the resonantly enhanced electric field. The phenomenon provides experimental evidence that the plasma resonance region plays an active role in heating mechanism in low-pressure microwave discharges.
Scaling reduction of the perturbative triples correction (T) to coupled cluster theory via Laplace transform formalism
Authors: Constans Pere , Ayala Philippe Y. , Scuseria Gustavo E.
Published: July 2002

A reformulation of the perturbative triples correction to coupled cluster singles and doubles (CCSD) based on the numerical Laplace transform of the energy denominator is presented. Rearranged equations reduce the O(N7) canonical scaling to O(N6), where N is a size measure of the electronic system. Two to three quadrature points is adequate for chemical predictions. The Laplace ansatz permits simple, noniterative expressions in noncanonical orbital representations. Furthermore, substituting canonical by generalized CCSD natural orbitals, the Laplace ansatz exhibits scaling close to O(N5), while retaining accuracy and providing crossover with respect to canonical triples for small size systems. A developing atomic orbital formulation is also introduced.
Normal-distribution-function-shaped Josephson tunnel junctions
Authors: Kikuchi Katsuya , Myoren Hiroaki , Iizuka Takeshi , Takada Susumu
Published: July 2002

We have investigated the magnetic field dependence of the Josephson tunnel junctions whose shape is defined with a normal-distribution function. It was revealed that the normal-distribution-shaped Josephson tunnel junctions show an exponential decay and no side lobes of the Josephson current as the external magnetic field is increased. This behavior was confirmed experimentally by using the Nb/AlOx/Nb tunnel junctions with a Josephson critical current density of Jc=50 A/cm2, and an area of 10 000 μm2 corresponding to the “small” junction region. Normal-distribution junctions should have a potential for wide applications to photon detection since the external magnetic field to suppress the Josephson current can be lowered to less than one tenth that of other junctions reported.
Normal modes of a Si(100) double-paddle oscillator
Authors: Spiel Christoph L. , Pohl R. O. , Zehnder Alan T.
Published: July 2002

Six low-frequency eigenmodes of a double-paddle oscillator have been measured and have been identified with a finite-element model. The internal friction Q−1 of these modes has been measured in the range of 4–80 K. Only one of the oscillator’s modes has a Q−1<3×10−8 below 40 K, which is furthermore very reproducible. All other modes have a higher internal friction which is not as reproducible and also sometimes changes after thermal cycling. It is shown that the internal friction of the different modes is related to the restoring force needed to hold the oscillator in place. The finite-element model is used to predict the damping of the different modes.
A brilliant infrared light source at BESSY
Authors: Peatman W. B. , Schade U.
Published: July 2002

The optical design of a brilliant synchrotron source of infrared radiation for structural and time resolved studies in the biological and material sciences is described. With this infrared source, BESSY will open its doors to a new community of scientists and to new research possibilities.
A spectroscopic determination of the bond length of the LiOLi molecule: Strong ionic bonding
Authors: Bellert D. , Breckenridge W. H.
Published: July 2002

The short bond length of the linear ground state of the 7Li16O7Li molecule has been accurately determined to be 1.606±0.008 Å by analysis of the rotational structure of several bands assigned to transitions from the jet-cooled 1Σ+ vibronic ground state to a bent 1B1 excited state. This value is in good agreement with, but more accurate than, other experimental and ab initio bond-length estimates for the prototypical ionically bound triatomic molecule Li+O−2Li+.
Molecular, metabolic, and genetic control: An introduction
Authors: Tyson John J. , Mackey Michael C.
Published: July 2002

The living cell is a miniature, self-reproducing, biochemical machine. Like all machines, it has a power supply, a set of working components that carry out its necessary tasks, and control systems that ensure the proper coordination of these tasks. In this Special Issue, we focus on the molecular regulatory systems that control cell metabolism, gene expression, environmental responses, development, and reproduction. As for the control systems in human-engineered machines, these regulatory networks can be described by nonlinear dynamical equations, for example, ordinary differential equations, reaction-diffusion equations, stochastic differential equations, or cellular automata. The articles collected here illustrate (i) a range of theoretical problems presented by modern concepts of cellular regulation, (ii) some strategies for converting molecular mechanisms into dynamical systems, (iii) some useful mathematical tools for analyzing and simulating these systems, and (iv) the sort of results that derive from serious interplay between theory and experiment.
Real-time x-ray microbeam characterization of electromigration effects in Al(Cu) wires
Authors: Wang P.-C. , Noyan I. C. , Kaldor S. K. , Jordan-Sweet J. L. , Liniger E. G. , Hu C.-K.
Published: July 2002

We report real-time, in situ x-ray microbeam measurements of electromigration-induced Cu redistribution, and the concurrent local stress variation in Al(Cu) wires. The data, which were obtained by combining x-ray microtopography with energy-dispersive fluorescence analysis, encompass both the early and late stages of electromigration as well as the postrelaxation stage at high temperature with the current turned off. We observe that both Cu concentration and stress values show unexpected local variations that may reflect the effect of local configuration such as film–substrate interface integrity or microstructure.
Three-dimensional structure and decay properties of vortices in shallow fluid layers
Authors: Satijn M. P. , Cense A. W. , Verzicco R. , Clercx H. J. H. , van Heijst G. J. F.
Published: July 2002

Recently, several laboratory experiments on vortex dynamics and quasi-two-dimensional turbulence have been performed in thin (stratified) fluid layers. Commonly, it is tacitly assumed that vertical motions, giving rise to a three-dimensional character of the flow, are inhibited by the limited vertical dimension. However, shallow water flows, which are vertically bounded by a no-slip bottom and a free surface, necessarily possess a three-dimensional structure due to the shear in the vertical direction. This shear may lead to significant secondary circulations. In this paper, the three-dimensional (3D) structure and the decay properties of vortices in shallow layers of fluid, both homogeneous and stratified, have been studied in detail by 3D direct numerical simulations. The quasi-two-dimensionality of these flows is an important issue if one is interested in a comparison of experiments of this type with purely two-dimensional theoretical models. The influence of several flow parameters, like the depth of the fluid and the Reynolds number, has been investigated. In general, it can be concluded that the flow loses its two-dimensional character for larger fluid depth and larger Reynolds number. Furthermore, it is possible to construct a regime diagram that allows the assessment of the parameter regime, where the flow can be considered as quasi-two-dimensional. It is found that the presence of secondary circulations within a planar vortex flow results in a deformation of the radial profile of axial vorticity. In the limiting case of quasi-two-dimensional flow, the vorticity profiles can be scaled according to a simple diffusion model. In a two-layer stratified system, the decay is reduced and three-dimensional motions are significantly inhibited compared to the corresponding flows in a homogeneous layer.
Generalized proximity effect model in superconducting bi- and trilayer films
Authors: Brammertz G. , Poelaert A. , Golubov A. A. , Verhoeve P. , Peacock A. , Rogalla H.
Published: July 2002

This article presents a general model for calculating the density of states and the Cooper pair potential in proximity-coupled superconducting bi- and trilayer films. It is valid for any kind of bilayer S1-S2, whatever the quality of the materials S1 and S2, the quality of the S1-S2 interface, and the layer thicknesses. The trilayer model is valid for a thin S3 layer, whereas the other two layers have arbitrary thicknesses. Although the equations of the dirty limit are used, it is argued that the model stays valid in clean bi-and trilayer films. The typical example of superconducting tunnel junctions is used to show that existing models, which apply to very thin or very thick layers or to perfectly transparent S1-S2 interfaces, are too restrictive to apply to an arbitrary bilayer. The new model is applied to practical junctions, with layer thicknesses intermediate between the “thick” and the “thin” approximation.
Vibrational relaxation of CO in ultracold He3 collisions
Authors: Zhu C. , Balakrishnan N. , Dalgarno A.
Published: July 2002

We present results of quantum mechanical scattering calculations on the vibrational relaxation of CO induced by collisions with He3 at ultracold temperatures and compare them with the corresponding results for the He–CO4 system. The low energy cross sections are controlled by shape resonances supported by the van der Waals well. For a thermal rotational population the cross sections for the quenching of the v=1 vibrational level are larger for He3 than for He4 collisions. Because of the absence of a Feshbach resonance, the cross sections at low energies for the quenching of the j=1 rotational level are much smaller than those for He–CO3. Very good agreement is found for the rate coefficients for the vibrational relaxation of the v=1 level with the measurement data that are available at temperatures above 35 K. There are small discrepancies between the calculated rate coefficients with earlier calculations for the relaxation of the v=1 vibrational level, similar to those found for He–CO4 collisions, but the discrepancies tend to be larger with decrease of temperature.