bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrared optical switch by the use of optically excited free carriers in semiconductors
Authors: Saito Mitsunori , Inoue Takeshi
Published: July 2002

Infrared light switching was achieved by using the infrared absorption due to the optically excited carriers in a semiconductor. A Nd:YAG laser pulse was used as a gate signal for excitation. A Si wafer was chosen as a switching element, since its band-gap energy coincides with the photon energy of the Nd:YAG laser. The intensity of incandescent infrared radiation was controlled by inducing free-carrier absorption in the Si wafer.
Characteristics of fritting contacts utilized for micromachined wafer probe cards
Authors: Itoh Toshihiro , Suga Tadatomo , Engelmann Gunter , Wolf Jürgen , Ehrmann Oswin , Reichl Herbert
Published: July 2002

We have investigated the relationship between contact forces and the fritting which should be utilized for making contact to integrated circuit pads in micromachined wafer probe cards. Micromachined probe cards are requisite to tests of higher pad-density and smaller pad-pitch chips with high-speed signals above 1 GHz. In addition, if a microactuator is integrated into each probe, we can realize a novel probe card in which contacts can directly be switched on and off. The critical problem of the micromachined probe cards, however, is that each micromachined probe cannot generate or endure the force required to break the oxide on an Al pad surface mechanically. To overcome the problem, we planned to reduce the required contact force by putting the fritting effect to good use. For that, in this article, we have measured forces required for making contact to Al pads and contact resistance when utilizing the fritting process for the break of the pad surface oxide. Furthermore, we have also measured the force necessary to disconnect the contact made by the fritting, since it is important to design the probe dimensions and integrated actuators. As a result, we have found that the fritting makes it possible to get low resistance contact to Al pads without applying external forces. Moreover, the contact resistance and the adhesion force, when using electroplated Au bumps as the contact probe, were found to be one-third of those when using electroplated Ni bumps, respectively. This indicates that the Au bump is more suitable for the probe of micromachined probe cards.
Growth and giant magnetoresistance properties of La-deficient LaxMnO3−δ (0.67≤x≤1) films
Authors: Gupta A. , McGuire T. R. , Duncombe P. R. , Rupp M. , Sun J. Z. , Gallagher W. J. , Xiao Gang
Published: July 2002

Epitaxial thin films of lanthanum-deficient LaxMnO3−δ (0.67≤x≤1) have been grown on (100) SrTiO3 substrates by pulsed laser deposition. The as-deposited films exhibit a ferromagnetic transition at temperatures ranging from 115 to 240 K, with the transition temperature (Tc) increasing with higher La deficiency. A sharp drop in resistivity and negative magnetoresistance is observed close to Tc, a behavior similar to that observed in divalent substituted La1−xMxMnO3−δ (M=Ba, Sr, Ca, Pb) films. Postannealing the films in O2 reduces the resistivity and raises the Tc to values close to room temperature. A magnetoresistance value of 130% (Δρ/ρH) has been obtained at 300 K at 4 T for a post-annealed film with x=0.75.
Third-order nonlinearity of semiconductor doped glasses at frequencies below band gap
Authors: Banfi G. P. , Degiorgio V. , Fortusini D. , Tan H. M.
Published: July 2002

We measured, through nearly degenerate three-wave mixing, the third-order nonlinear optical susceptibility of semiconductor doped glasses at frequencies below band gap. With typical 1.5%–5% volume fractions, the CdTe and CdS1−xSex nanocrystals give a contribution to the nonlinearity of the composite that is comparable to that of the glass matrix. The magnitude and the band-gap dependence of the nonlinear susceptibility of the nanocrystals are similar to those of the bulk semiconductors.
Fabrication of bi-periodic sinusoidal structures on silicon
Authors: Tanaka So , Umbach Christopher C. , Blakely Jack M.
Published: July 2002

We illustrate a method for fabricating bi-periodic quasi-sinusoidal structures that are difficult to produce by conventional microfabrication techniques. By annealing bi-periodic square wave structures in ultra-high vacuum (UHV), the surface profile becomes quasi-sinusoidal due to mass transport processes. The method is illustrated by the creation of 4 and 6 μm period structures on Si(001) through annealing at 1100 °C in UHV. Surface profiles are measured by atomic force microscopy (AFM). The structures produced may have applications in electronic or optical devices.
Nanometer-scale surface modifications of YBa2Cu3O7−δ thin films using a scanning tunneling microscope
Authors: Bertsche G. , Clauss W. , Kern D. P.
Published: July 2002

Systematic modifications on the nanometer scale of YBa2Cu3O7−δ (YBCO) epitaxial, thin films have been achieved by using a scanning tunneling microscope in air at room temperature. Working with tunneling parameters slightly above those used for imaging results in irreversible, nanometer-sized surface modifications. The surface topography of our YBCO films showed characteristic growth spirals, of which one revealed a remarkable ‘‘S’’-shaped top end. This unusual growth behavior indicates possibly a distortion caused by a line defect in the second to last turn of the spiral. We succeeded in cutting through this structure by producing a 2.5 nm wide groove across it. This corresponds to the controlled removal of only a few unit cells of YBCO. As an alternative modification technique, bias voltage pulses were applied, leading to the formation of 3 nm wide craters.
Effects of biaxial strain on exciton resonance energies of hexagonal GaN heteroepitaxial layers
Authors: Chichibu S. , Shikanai A. , Azuhata T. , Sota T. , Kuramata A. , Horino K. , Nakamura S.
Published: July 2002

Exciton resonance energies of hexagonal (h-) GaN(0001) epilayers were determined by a combination of high-resolution modulated photoreflectance methods. The results were analyzed thoretically using the Luttinger-Kohn type Hamiltonian for the valence bands under the in-plain biaxial stress, and we obtained the shear deformation potential constants and energy gap in unstrained crystal. Occurrence of the anticrossing of B and C valence bands in tensile biaxially strained h-GaN was suggested.
Miniaturization of p -Ge lasers: Progress toward continuous wave operation
Authors: Bründermann E. , Linhart A. M. , Röser H. P. , Dubon O. D. , Hansen W. L. , Haller E. E.
Published: July 2002

We have observed intervalence band laser action in Al-doped germanium crystals with volumes as small as 0.03 cm3, one order of magnitude smaller than previously studied p-Ge laser crystals. A duty cycle of 1.5×10−4 was achieved. We show evidence that crystal heating plays a key role in the duration of the emission pulse and have measured a maximum pulse length of 11 μs for a 0.43 cm3, Ga-doped germanium crystal. Such progress in the understanding and performance of p-Ge hot hole lasers marks a significant step toward continuous wave operation.
175 K continuous wave operation of InAsSb/InAlAsSb quantum-well diode lasers emitting at 3.5 μm
Authors: Choi H. K. , Turner G. W. , Manfra M. J. , Connors M. K.
Published: July 2002

Multiple quantum-well diode lasers incorporating compressively strained InAs0.935Sb0.065 wells and tensile-strained In0.15Al0.85As0.9Sb0.1 barriers are reported. These lasers, grown on InAs substrates by molecular beam epitaxy, have emission wavelengths between 3.2 and 3.55 μm. Broad-stripe lasers have exhibited pulsed threshold current density as low as 30 A/cm2 at 80 K and the characteristic temperatures between 30 and 40 K. The maximum pulsed operating temperature is 225 K. Ridge-waveguide lasers have cw threshold current of 12 mA at 100 K, and the maximum cw operating temperature is 175 K.
Ultrafast reflectivity changes in photoexcited GaAs Schottky diodes
Authors: Fischler W. , Buchberger P. , Höpfel R. A. , Zandler G.
Published: July 2002

The transient reflectivity of GaAs Schottky diodes is measured by femtosecond time resolved pump-probe experiments. The measured reflectivity for photon energies near the band gap reveals transient quasioscillatory behavior with frequencies up to 5.5 THz. The changes of the reflectivity are due to extremely fast changes of the carrier density within the depletion layer. We interpret the observed oscillatory signal as coherent plasma oscillations. Ensemble Monte Carlo simulations for the scenario agree well with the observed plasmon frequencies.
Nanometer-sized silicon crystallites prepared by excimer laser ablation in constant pressure inert gas
Authors: Yoshida Takehito , Takeyama Shigeru , Yamada Yuka , Mutoh Katsuhiko
Published: July 2002

We report nanometer-sized silicon (Si) crystallites prepared by excimer laser ablation in constant pressure inert gas ambient. Size distribution of the Si ultrafine particles depends on the pressure of inert gas ambients. The relation between the average size and the ambient pressure can be explained by an inertia fluid model. It is verified that the size of the Si ultrafine particles is ∼3 nm and greater in diameter. Furthermore, crystallinity of the nanoscale ultrafine particles is crystalline similar to that of bulk Si.
Recrystallization of MeV Si implanted 6H-SiC
Authors: Harada Shinsuke , Ishimaru Manabu , Motooka Teruaki , Nakata Toshitake , Yoneda Tomoaki , Inoue Morio
Published: July 2002

Microstructures of recrystallized layers in 8 MeV Si3+ ion implanted 6-H-SiC (0001) wafers have been characterized by means of transmission electron microscopy. Epitaxial recrystallization of buried amorphous layers was observed at annealing temperature as low as 1000 °C. Layer-by-layer epitaxy of 6H-SiC initially occurred and it was changed to columnar growth when layer-by-layer growth exceeded 100 nm in thickness. From the microdiffraction analysis, it was found that the columnar regions are defected 6H-SiC with crystal orientations different from the substrate. In addition to 6H-SiC, epitaxial 3C-SiC was also confirmed in the recrystallized layer. Based on these results, we have proposed a structure model of the recrystallized layer in which stacking faults in the columnar regions are induced by mismatched connections between the columnar and layered 6H-SiC regions.
Spatially indirect radiative recombination of carriers localized in Si1−x−yGexCy/Si1−yCy double quantum well structure on Si substrates
Authors: Brunner K. , Winter W. , Eberl K.
Published: July 2002

Closely spaced pseudomorphic Si1−x−yGexCy and Si1−yCy quantum well (QW) layers grown by solid-source molecular beam epitaxy on Si substrates are studied by photoluminescence (PL) spectroscopy at low temperature. In thin Si0.84Ge0.16/Si1−yCy double QWs, a no-phonon PL line of enhanced intensity and a weaker Si-like TO-phonon replica line are observed at lower energy, compared to reference structures with isolated Si0.84Ge0.16 and Si1−yCy QWs. These PL lines are attributed to spatially indirect (type II) transitions of electrons and holes confined to the coupled Si1−yCy and Si1−xGex layers, respectively. The PL intensity depends strongly on layer width. It increases exponentially for decreasing QW widths and it decreases when thin Si layers are deposited in between the QWs. This behavior is well described considering the wave function overlap of carriers which strongly influences the rate of indirect optical transitions.
Biexciton luminescence from cubic ZnS epitaxial layers
Authors: Yamada Yoichi , Yamamoto Takaho , Nakamura Seiji , Taguchi Tsunemasa , Sasaki Fumio , Kobayashi Shunsuke , Tani Toshiro
Published: July 2002

Excitonic luminescence from high-quality ZnS epitaxial layers has been studied under high-density excitation. The first experimental evidence for biexciton formation in ZnS was obtained. The binding energy of the biexciton was estimated to be 9 meV, which was the largest value among II–VI compound semiconductors with zinc blende modification.
High gain-bandwidth-product silicon heterointerface photodetector
Authors: Hawkins Aaron R. , Wu Weishu , Abraham Patrick , Streubel Klaus , Bowers John E.
Published: July 2002

We report the fabrication of a near-infrared avalanche photodetector with a gain-bandwidth product of over 300 GHz. The detector uses a Si multiplication layer and an InGaAs absorption layer. A 3 dB bandwidth of over 9 GHz was measured for current gains as high as 35. Photocurrent measurements using 1.3 μm light indicate a quantum efficiency for the detector of 0.60, near the limit expected based on the absorber thickness.
Influence of thermal annealing on the photoluminescence from pseudomorphic Si1−yCy epilayers on Si
Authors: Penn C. , Zerlauth S. , Stangl J. , Bauer G. , Brunthaler G. , Schäffler F.
Published: July 2002

Near band edge photoluminescence (PL) is observed from about 100-nm-thick pseudomorphic Si1−yCy epilayers, which were grown by molecular beam epitaxy on a Si substrate. Different pieces of one wafer were annealed at temperatures between 500 and 800 °C, and it is shown that annealing leads to increased PL intensities and reduced linewidths. The smallest achieved full width at half maximum was 8.6 meV. We also observe a blueshift of the PL lines after annealing, which does not correspond to the minor changes in the amount of substitutional carbon deduced from x-ray measurements. Temperature dependent PL measurements suggest that the recombination involves electrons localized at alloy fluctuations.
Far-infrared free-hole absorption in epitaxial silicon films for homojunction detectors
Authors: Perera A. G. U. , Shen W. Z. , Mallard W. C. , Tanner M. O. , Wang K. L.
Published: July 2002

We report on the investigation of free-carrier absorption characteristics for epitaxially grown p-type silicon thin films in the far-infrared region (50–200 μm), where Si homojunction interfacial workfunction internal photoemission (HIWIP) detectors are employed. Five Si thin films were grown by molecular beam epitaxy on different silicon substrates over a range of carrier concentrations, and the experimental absorption data were compared with calculated results. The free-hole absorption is found to be almost independent of the measured wavelength. A linear regression relationship between the absorption coefficient and the carrier concentration, in agreement with theory, has been obtained and employed to calculate the photon absorption probability in HIWIP detectors.
Effects of elastic and inelastic interactions on phase contrast images in tapping-mode scanning force microscopy
Authors: Tamayo Javier , Garcı́a Ricardo
Published: July 2002

The dependence of phase contrast in tapping-mode scanning force microscopy on elastic and inelastic interactions is studied. The cantilever–tip ensemble is simulated as a driven, damped harmonic oscillator. It is found that for tip–sample elastic interactions, phase contrast is independent of the sample’s elastic properties. However, phase contrast associated with elastic modulus variations are observed if viscous damping or adhesion energy hysteresis is considered during tip–sample contact. The phase shift versus tip–sample equilibrium separation was measured for a compliant material (polypropylene) and for a stiff sample (mica). The agreement obtained between theory and experiment supports the conclusions derived from the model. These results emphasize the relevance of energy dissipating processes at the nanometer scale to explain phase contrast imaging in tapping-mode force microscopy.
Aging properties of high power laser diode arrays analyzed by Fourier-transform photocurrent measurements
Authors: Tomm J. W. , Jaeger A. , Bärwolff A. , Elsaesser T. , Gerhardt A. , Donecker J.
Published: July 2002

We present the results on aging mechanisms acting in high-power laser diode arrays (LDAs) employing Fourier-transform (FT) spectroscopy. The FT spectrometer was used as an excitation source for performing photocurrent (PC) measurements in two sets of aged LDA samples. The PC spectra reveal both the evolution of a defect band located in the optically active layer as well as modifications of the interband part of the spectrum upon aging. Such changes represent sensitive quantitative measures of the aging status and provide insight into the microscopic changes of the device structure upon aging.
Work-hardening effects in the lattice relaxation of single layer heterostructures
Authors: González D. , Araújo D. , Aragón G. , Garcı́a R.
Published: July 2002

A modelization of the strain relaxation in single heteroepitaxial layers is presented in this letter. The calculations consider the energetic variations of the heteroepitaxial structure when introducing one new dislocation into the existing interfacial array of fixed misfit dislocations without continuous readjustment of the spacing array. The interaction energy of the new dislocation with both lattice mismatch and dislocation array is shown to be the limiting factor that controls the mechanism of strain relaxation at the saturation stage of relaxation. The model is shown to be in good agreement with the lattice relaxation behavior of previously published strain/thickness data.
1/f
dielectric polarization noise in silicon p-n junctions
Authors: Akiba Makoto
Published: July 2002

Dielectric polarization noise in silicon p-n junctions was measured at low leakage current, less than 1×10−17 A, and at temperatures between 110 and 200 K. The power spectra of the noise voltages exhibit 1/f characteristics, where f is the frequency. The dielectric loss of the p-n junctions, which was derived from the 1/f noise by using the fluctuation-dissipation theorem, is proportional to the temperature, and is almost independent of the characteristics of the p-n junctions.
X-ray generation in cryogenic targets irradiated by 1 μm pulse laser
Authors: Shimoura Atsushi , Amano Sho , Miyamoto Shuji , Mochizuki Takayasu
Published: July 2002

Soft x-ray spectral radiations from Xe, H2O, and CO2 cryogenic targets irradiated by a 1 μm neodymium doped YAG-slab laser at pulse widths of 12–20 ns and at laser intensities of 5×1010–1012 W/cm2 have been observed. These targets radiate soft x-rays in a wavelength range of 10–13 nm which is useful for projection microlithography. We have found a strong x-ray spectral peak at λ=10.8 nm with a Xe cryogenic target. The measured x-ray conversion efficiency with the Xe target was 0.8%/sr(λ=10.8±0.27 nm) at a laser intensity of 1×1012 W/cm2. This was ten times or more efficient than that with H2O and CO2 targets.
Amplified spontaneous emission of an Nd3+-doped poly(methyl methacrylate) optical fiber at ambient temperature
Authors: Zhang Q. J. , Wang P. , Sun X. F. , Zhai Y. , Dai P. , Yang B. , Hai M. , Xie J. P.
Published: July 2002

Polymer optical fibers have received much attention in recent years as they can replace silica glass fibers in local area nets in the future. A neodymium ion (Nd3+)-doped poly(methyl methacrylate) (PMMA) fiber has been made from Nd3+ containing PMMA as a core and silica resin as claddings. Fluorescence of the fiber results from the transition of 4G5/2 to 4I9/2 in a three energy level system. Amplified spontaneous emission (ASE) at 575 nm has also been observed and the critical power at a pump wavelength of 514.5 nm for the onset of ASE has been found to be 85 mW.
Critical thickness for the saturation state of strain relaxation in the InGaAs/GaAs systems
Authors: González D. , Araújo D. , Aragón G. , Garcı́a R.
Published: July 2002

Using previously published relaxation models [D. J. Dunstan, P. Kidd, L. K. Howard and R. H. Dixon, Appl. Phys. Lett. 59, 3390 (1991) and D. González, D. Araújo, G. Aragón, and R. Garcı́a, Appl. Phys. Lett. 71, 2475 (1997)] that predict the strain relaxation in the InGaAs/GaAs system, before and during the stage of relaxation saturation, the critical thickness where dislocation interactions begin to limit the plastic relaxation is estimated. The approximations used to deduce an analytical expression are shown to be appropriate for describing the regime of relaxation considered. A good agreement with experimental data previously published by other authors permits a physical explanation for the different observed regimes of relaxation to be given.
Far-infrared photoconductivity in self-organized InAs quantum dots
Authors: Phillips J. , Kamath K. , Bhattacharya P.
Published: July 2002

We report far-infrared photoconductivity in self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy. Through use of a Fourier transform infrared spectrometer, a photoconductivity signal peaked at 17 μm is observed from a n–i–n detector structure with doped InAs quantum dots in the intrinsic region. Comparison of photoluminescence and band-to-band photocurrent absorption spectra suggests the far-infrared response is due to intersubband transitions in the quantum dots.
Mössbauer and magnetic study of substituted magnetites
Authors: Sorescu M. , Mihaila-Tarabasanu D. , Diamandescu L.
Published: July 2002

Co, Ni, Mn, Cr, and Cu substituted magnetites were prepared by the hydrothermal method at 300 °C, with concentrations ranging from 8.2% to 12.5%. Transmission electron microscopy determined the average particle diameter 〈Φ〉 to be in the hundred of nm range and the morphological modifications induced by the various substitutions employed. Hysteresis loop measurements were performed to determine the coercive field Hc, and saturation magnetic moment ms. While Hc decreased with increasing 〈Φ〉, the particle shape was found to play an important role in explaining the dependence of ms on 〈Φ〉. Transmission Mössbauer spectroscopy was used to determine the site preference of the substitutions and their effect on the hyperfine magnetic fields. The room-temperature Mössbauer spectra were analyzed assuming a random distribution of substitutents using the binomial distribution from the ionic crystal point of view. Superparamagnetic particles were observed at room temperature in the case of Cu and Cr substituted magnetites.
Granular Fe–Pb–O films with large tunneling magnetoresistance
Authors: Huang Yi-Hong , Hsu Jen-Hwa , Chen J. W. , Chang Ching-Ray
Published: July 2002

A method of manufacturing granular Fe–Pb–O films is described. The temperature dependence of resistivity in these samples exhibits a semiconductorlike behavior indicating that the electronic transport takes place via a tunneling process. The magnetoresistance ratio has been found to be about 10% at room temperature. The enhancement of the tunneling magnetoresistance effect is believed to be due to spin-dependent tunneling through the tunneling barrier containing magnetic ferrite formed by PbO and α-Fe2O3.
Raman analysis of short-range clustering in laser-deposited CdSxTe1−x films
Authors: Fischer A. , Anthony L. , Compaan A. D.
Published: July 2002

Raman line shapes of the longitudinal optical phonon have been analyzed for the pseudobinary alloy system CdSxTe1−x over the full alloy range. The polycrystalline thin films were grown by pulsed laser deposition at, typically, 370 °C including films with x values throughout the miscibility gap (0.06<x<0.97). Peak shift, broadening, and asymmetry arising from spatial correlation effects yield details of the microstructural clustering. The dependence of phonon coherence length on the x value cannot be explained simply from a random occupancy of the anion sublattice. We employed a linear chain model with site probabilities modified by the Warren–Cowley short-range order parameter to infer coherence lengths versus x. The data are best fit with a short-range order parameter of 0.73 at x=1/2.
Si adatom surface migration biasing by elastic strain gradients during capping of Ge or Si1−xGex hut islands
Authors: Kubler L. , Dentel D. , Bischoff J. L. , Ghica C. , Ulhaq-Bouillet C. , Werckmann J.
Published: July 2002

Hut cluster formation during Ge or Si1−xGex solid source molecular beam epitaxial growth on Si(001) is a well-known kinetic pathway for partial strain relief. It results in undulated morphologies with {105} facets allowing a∥ lattice parameter relaxation on the island apexes. Here, we show how subsequent Si coverages, grown at 500 °C, avoid being tensile strained and impede further increase of stored elastic strain energy. Dominant inhomogeneous Si surface diffusions take place as proven by a Ge marker technique able to provide transmission electron microscopy or high-resolution transmission electron microscopy images of the initial Si morphology stages and by reflection high-energy electron diffraction examinations. This mechanism prevails for high enough Si growth rates, able to quench lateral Ge diffusion and limit chemical strain relief. Mediated by stress variations on the noncapped island curvatures, Si is depleted from the top of the islands and accumulates in the troughs of the ripples where it accommodates mostly unstrained. By this selective Si coverage, the surface undergoes a rapid smoothing and a∥ recovery toward the Si bulk value. When the Ge containing islands are completely buried, their strain, dictated by the Si buffer and cap layers, ends by being mainly along the growth direction or tetragonal (Δa∥=0).
Dynamic properties and nonequilibrium processes in electron-beam scribed YBa2Cu3O7 Josephson junctions
Authors: Davidson B. A. , Nordman J. E. , Hinaus B. M. , Rzchowski M. S. , Siangchaew K. , Libera M.
Published: July 2002

We present a detailed characterization of the dynamic properties of proximity-coupled Josephson junctions in YBa2Cu3O7 fabricated by electron-beam scribing. A full description of the low-temperature behavior includes nonequilibrium processes in the normal barrier as well as wide-junction effects resulting from the planar geometry. Above ∼40 K these junctions obey the standard (equilibrium) resistively-shunted junction (RSJ) model in applied magnetic field. At lower temperatures, the volt–ampere V(I) curves develop a temperature-dependent “excess critical current,” saturating at 0.5–0.75 of the total critical current. Below ∼10 K hysteresis is observed. The observed temperature dependence and magnitude of the excess current and hysteresis are qualitatively consistent with published calculations based on the time-dependent Ginzburg–Landau equations. At low temperatures, the V(I) curves in applied field deviate significantly from the RSJ model, which we attribute to wide-junction behavior with a nonuniform bias-current distribution.
Nanoscale reconstruction of surface crystallography from three-dimensional polarization distribution in ferroelectric barium–titanate ceramics
Authors: Eng L. M. , Güntherodt H.-J. , Schneider G. A. , Köpke U. , Muñoz Saldaña J.
Published: July 2002

The three-dimensional orientation of polarization vectors at the surface of ferroelectric barium–titanate (BaTiO3) ceramics is evaluated using voltage-modulated scanning force microscopy (SFM). By applying an ac voltage to the conductive SFM tip, we measure the relative amount of the three orthogonal components Px, Py, and Pz, of the polarization vector at any surface point. The measured polarization orientation together with the actual domain-wall orientation allows a precise reconstruction of the crystallographic orientation of the investigated grains down to a 40 nm resolution. Excellent agreement is obtained when comparing this orientation with the crystallographic reconstruction revealed by etch patterns from the sample surface topography. We show that the surface topography manifests a domain structure, which was present in the past, while the actual ferroelectric domain configuration is revealed by the modulation technique.
Emission of prompt electrons during excimer laser ablation of aluminum targets
Authors: Amoruso S. , Armenante M. , Bruzzese R. , Spinelli N. , Velotta R. , Wang X.
Published: July 2002

We report on prompt emission of fast electrons occurring during xenon–fluoride (351 nm) laser ablation of aluminum targets in the nanosecond regime. We have measured both the kinetic energy distribution and the energy-integrated time-of-flight distribution of these electrons. Experimental data evidence that the energetic electrons are produced during the laser pulse as a consequence of two-photon processes, and that space-charge effects influence the photoemitted electron kinetic energy, leading to prompt electrons kinetic energy distributions extending up to ≈15 eV.
Heating of photogenerated electrons and holes in highly excited GaN epilayers
Authors: Tamulaitis G. , Žukauskas A. , Yang J. W. , Khan M. A. , Shur M. S. , Gaska R.
Published: July 2002

Heating of nonequilibrium carriers under intense off-resonant photogeneration was investigated in GaN epilayers at T=80 K. We determined that under these extreme conditions hot electrons and hot holes exhibit different patterns of energy dissipation. A good agreement between the experimental data and modeling results was achieved by assuming negligible efficiency of optical-phonon emission by holes. This implies that hot holes, contrary to hot electrons, are unable to establish a smooth distribution function above the threshold of optical phonon emission. We attribute this effect to a large hole mass, strong carrier–phonon coupling, and large optical-phonon energies in GaN.
Transmission electron microscopy study on ferroelectric domain structure in SrBi2Ta2O9 ceramics
Authors: Ding Y. , Liu J. S. , Wang Y. N.
Published: July 2002

A transmission electron microscopy investigation has been conducted on the domain structure in SrBi2Ta2O9 ceramics. From the 90° rotation relationship of the electron diffraction pattern of the [001] zone axis, a 90° domain wall can be confirmed. It is due to the failure of Friedel’s law that the contrast of a 180° domain wall can be identified. The antiphase boundary can be seen clearly in the dark-field image, which is taken by the (300) superlattice reflection. The 90° domain wall, as well as antiphase the boundary (APB), has an irregular configuration. The APB combined with the 90° domain wall is also identified.
Continuous-wave operation of λ=3.25 μm broadened-waveguide W quantum-well diode lasers up to T=195 K
Authors: Bewley W. W. , Lee H. , Vurgaftman I. , Menna R. J. , Felix C. L. , Martinelli R. U. , Stokes D. W. , Garbuzov D. Z. , Meyer J. R. , Maiorov M. , Connolly J. C. , Sugg A. R. , Olsen G. H.
Published: July 2002

Mid-infrared (λ=3.25 μm) broadened-waveguide diode lasers with active regions consisting of 5 type-II “W” quantum wells operated in continuous-wave (cw) mode up to 195 K. At 78 K, the threshold current density was 63 A/cm2, and up to 140 mW of cw output power was generated. A second structure with ten quantum wells operated up to 310 K in pulsed mode.
Importance of intermolecular interactions in the nonlinear optical properties of poled polymers
Authors: Liakatas I. , Cai C. , Bösch M. , Jäger M. , Bosshard Ch. , Günter P. , Zhang C. , Dalton L. R.
Published: July 2002

A series of phenylbithiophene stilbenes and phenyltetraenes were synthesized and their first-order molecular hyperpolarizabilities determined. Optical nonlinearities up to μβo=9300×10−69 C m5/V were measured at 1907 nm. We show that intermolecular interactions have a large influence on the optical nonlinearity of the molecules in solution and in guest-host polymers with polymethylmethacrylate and polyquinoline as the host. We propose the use of a bulky donor group and a side chain perpendicular to the molecule’s conjugate bridge to reduce aggregation. Electro-optic coefficients as high as r33=30 pm/V at 1.55 μm in polymethylmethacrylate with 25 wt % chromophore loading were measured.
Single axial-mode selection in a far-infrared p-Ge laser
Authors: Muravjov A. V. , Withers S. H. , Weidner H. , Strijbos R. C. , Pavlov S. G. , Shastin V. N. , Peale R. E.
Published: July 2002

Single axial-mode operation of the pulsed far-infrared p-Ge laser with an intracavity Fabry–Perot type frequency selector has been observed by means of Fourier-transform spectroscopy. A spectral resolution better than 1 GHz has been achieved on an ordinary continuous-scan spectrometer using the event-locked technique for pulsed emission sources. A laser active-cavity finesse of at least unity has been directly confirmed from the measured emission spectral width. Analysis of the envelope of the corresponding interferogram suggests that the finesse exceeds 10.
Conductance oscillations induced by longitudinal resonant states in heteroepitaxially defined Ga0.25In0.75As/InP electron waveguides
Authors: Wang Qin , Carlsson N. , Maximov I. , Omling P. , Samuelson L. , Seifert W. , Sheng Weidong , Shorubalko I. , Xu H. Q.
Published: July 2002

We have measured at low temperatures the conductance of electron waveguides fabricated from modulation-doped quantum wells by wet etching and regrowth. We have found that, for a waveguide with abruptly changed geometry at the entrance and exit, the conductance shows oscillations, which are superimposed on a conventional conductance plateau structure. The periods and amplitudes of conductance oscillations depend on the length to width aspect ratio of the waveguide. In addition, the amplitudes of conductance oscillations decrease with increasing temperature. We propose that the observed oscillations are caused by the formation of longitudinal resonant electron states in the waveguide, in analogy with optical Fabry–Perot effects.
Measurement of negative-ion density in high-density C4F8 plasma using a laser photodetachment technique combined with a millimeter-wave open resonator
Authors: Kono A. , Kato K.
Published: July 2002

A technique for measuring negative-ion density in high-density plasmas used for materials processing has been developed; negative ions were detected by measuring the electron-density perturbation caused by laser photodetachment using a millimeter-wave open resonator. The measured negative-ion density for an inductively-coupled C4F8(5%–20%)/Ar plasma with electron densities around 1011 cm−3 at a pressure of 25 mTorr was comparable to the electron density. The results suggest that species produced via dissociation of C4F8 attaches electrons as effectively as C4F8.
Saturation of THz-frequency intraband absorption in InAs/GaAs quantum dot molecules
Authors: Boucaud P. , Gill K. S. , Williams J. B. , Sherwin M. S. , Schoenfeld W. V. , Petroff P. M.
Published: July 2002

We have investigated the far-infrared absorption in InAs/GaAs quantum dot molecules. The quantum dot molecules consist of two vertically coupled InAs self-assembled quantum dots separated by a GaAs barrier. The electronic coupling between the dot states results in an intraband absorption at THz frequencies. We show that this absorption can be bleached under high excitation intensity delivered by a free-electron laser. The saturation intensity is found to be on the order of 1 W cm−2. The electron relaxation time T1 is estimated from the saturation intensity. A lower limit for T1 of the order of 30 ps is deduced.
Structure information in rapid distortion analysis and one-point modeling of axisymmetric magnetohydrodynamic turbulence
Authors: Widlund Ola , Zahrai Said , Bark Fritz H.
Published: July 2002

It has recently been suggested that dimensionality information, as carried by the Reynolds dimensionality tensor, should be included in an extended Reynolds stress closure for modeling of magnetohydrodynamic (MHD) turbulence at low magnetic Reynolds numbers. This would enable more accurate modeling of the Joule dissipation, and capture the length-scale anisotropies and tendencies towards two-dimensionality characteristic of MHD turbulence. In the present work, an evolution equation for the Reynolds dimensionality tensor is derived, based on the spectral formulation of the Navier–Stokes equations. Most of the terms in the equation require modeling. Rapid distortion theory (RDT) is applied to study the behavior of the different magnetic terms of the dimensionality and Reynolds stress tensor equations; a variety of different anisotropy states could be examined by letting magnetic forcing act on a number of initial spectral energy distributions obtained from axisymmetric strain. The properties and limitations of linear or bilinear invariant tensor models for the magnetic terms are evaluated. In the limit of large interaction numbers (where Joule dissipation dominates), the resulting model equations for the energy decay have analytic solutions. By choosing one model constant appropriately, these are made consistent with the asymptotic energy decay K∼t−1/2 predicted earlier by Moffatt. The long-term objective of these efforts is the development of an effective second-moment closure for engineering applications.
Rate constants for energy transfer in carbon monoxide
Authors: Coletti Cecilia , Billing Gert D.
Published: July 2002

A large number of rate constants for vibrational energy transfer in CO–CO collisions, including both single quantum and multiquantum processes, where at least one of the colliding molecules is in a highly excited vibrational state, have been calculated. A semiclassical method has been used together with an updated interaction potential.
Frequency doubling and tripling of terahertz radiation in a GaAs/AlAs superlattice due to frequency modulation of Bloch oscillations
Authors: Winnerl S. , Schomburg E. , Brandl S. , Kus O. , Renk K. F. , Wanke M. C. , Allen S. J. , Ignatov A. A. , Ustinov V. , Zhukov A. , Kop’ev P. S.
Published: July 2002

We report on frequency doubling and tripling of THz radiation in a voltage-biased GaAs/AlAs superlattice. By use of a corner cube antenna system, radiation from the Santa Barbara free-electron laser (frequency 0.7 THz) was guided into a superlattice mesa element and the second and third harmonic were coupled out of the mesa. Without bias only radiation of the third harmonic was generated, while the biased superlattice emitted radiation of both the second and third harmonic. We attribute the harmonic generation to frequency modulation of damped Bloch oscillations of the miniband electrons in the superlattice.
Coarsening and phase separation in ultraviolet cured polymer dispersed liquid crystals
Authors: Lucchetti L. , Simoni F.
Published: July 2002

An extended investigation of the phase separation process in polymer dispersed liquid crystals is presented. A detailed analysis of the experimental results shows that the nucleation and growth of the liquid crystal droplets from the polymeric matrix can be described as the coarsening process occurring in binary alloys.
Effects of highly conducting interface and particle size distribution on optical nonlinearity in granular composites
Authors: Gao L. , Wan Jones T. K. , Yu K. W. , Li Z. Y.
Published: July 2002

The optical nonlinearity has been investigated in granular metal/dielectric composites taking the effects of highly conducting interfaces between the constituent phases as well as the distribution of particle sizes into account. We compute analytically the spectral function for composites with a binary distribution of particle sizes. For a log-normal distribution of width σ, numerical results show that the spectral density m(s) changes from a delta function for zero width to a prominent peak, accompanied by a broad spectrum for a finite width σ. As a result, the locations of the nonlinearity enhancement peak and the absorption spectrum shift to small frequencies with the increase of the interfacial factor I. The strength of the absorption and the optical nonlinearity are always decreased near resonance with the increase of σ, while the absorption peak and optical nonlinearity peak shift to smaller volume fraction f as I is increased. Moreover, the variation of I will further increase the optical absorption (optical nonlinearity) by more than one (three) order of magnitude, depending on the volume fractions.
From small to large behavior: The transition from the aromatic to the Peierls regime in carbon rings
Authors: Bylaska Eric J. , Kawai Ryoichi , Weare John H.
Published: July 2002

Results of local density approximation (LDA) and Hartree–Fock (HF) calculations for even numbered monocyclic rings are reported. Small Cn rings satisfying n=4N+2 show aromatic stability with equal bond-length structures, whereas rings of size n=4N show antiaromatic destabilization with bond-length alternation. For large rings a transition, the Peierls transition, from aromatic and antiaromatic to nonaromatic behavior, takes place. Above the Peierls transition, both n=4N and n=4N+2 rings show bond-length alternation and no differences in stability. The critical size for the transition to nonaromatic behavior depends on the electron-phonon coupling strength and therefore depends on the choice of ab initio method. HF predicts nonaromatic behavior for ring sizes n=14 and above. Fully optimized LDA results are presented up to n=42, which still has a cumulenic structure. Calculations based on periodic infinite ring systems show that within LDA the onset of nonaromatic behavior does not occur until n=82. Experimental results suggest that aromatic behavior exists in these ring systems to at least n=22. The force constant for in-plane angle bending may also be estimated from these calculations and was found to be 0.022 kcal mol−1 deg−2 per atom, not a strong function of size beyond n=22, and in good agreement with experimental estimates.
Preparation and performance of transmission-mode GaAs photocathodes as sources for cold dc electron beams
Authors: Pastuszka S. , Hoppe M. , Kratzmann D. , Schwalm D. , Wolf A. , Jaroshevich A. S. , Kosolobov S. N. , Orlov D. A. , Terekhov A. S.
Published: July 2002

Photoemission from GaAs cathodes with negative electron affinity (NEA) is applied for producing electron beams with very low longitudinal and transverse velocity spread. GaAs transmission-mode cathodes were activated with Cs and either O2 or NF3 in an extremely high vacuum setup (base pressure below 10−12 mbar). Quantum efficiencies of 20%–25% (at 670 nm) and long dark lifetimes (about 1000 h) could be achieved for both types of activation in a reproducible way. Using a method based on the adiabatic transverse expansion of an electron beam in a spatially decreasing magnetic field, the mean transverse energy (MTE) of the photoemitted electrons was measured systematically, recording longitudinal energy distribution curves. Both the MTE and the longitudinal energy spread strongly depend on the value of NEA and the position of the extracted distribution relative to the bulk conduction band minimum. Electrons with energies above the conduction band minimum are thermalized with the lattice temperature of the cathode, while electrons with energies below this level show a non-Maxwellian distribution with enhanced transverse energies. Thus, when extracting all electrons in a current limited emission mode, the MTE increases with the absolute value of NEA and reaches values up to ≈100 meV. By cutting off the low energy electrons with an external potential barrier, the longitudinal as well as transverse energy spread of the extracted electron ensemble are reduced. The MTE could be reduced down to about 28 meV at room temperature and to about 14 meV at liquid nitrogen temperature. The behavior of the MTE was found equivalent for (Cs, O) and for (Cs, F) activation layers on the same cathode. Conclusions about energy loss and scattering in the emission of photoelectrons from NEA GaAs cathodes are discussed.
Thermal conductivity of molecular fluids from molecular dynamics simulations: Application of a new imposed-flux method
Authors: Bedrov Dmitry , Smith Grant D.
Published: July 2002

We have applied a new nonequilibrium molecular dynamics (NEMD) method [F. Müller-Plathe, J. Chem. Phys. 106, 6082 (1997)] previously applied to monatomic Lennard-Jones fluids in the determination of the thermal conductivity of molecular fluids. The method was modified in order to be applicable to systems with holonomic constraints. Because the method involves imposing a known heat flux it is particularly attractive for systems involving long-range and many-body interactions where calculation of the microscopic heat flux is difficult. The predicted thermal conductivities of liquid n-butane and water using the imposed-flux NEMD method were found to be in a good agreement with previous simulations and experiment.
Ultrahigh transparency of Ni/Au ohmic contacts to surface-treated p-type GaN
Authors: Jang Ja-Soon , Park Seong-Ju , Seong Tae-Yeon
Published: July 2002

We report on ultrahigh-transparency and low-resistance Ni/Au ohmic contacts to surface-treated p-GaN:Mg (3.6×1017 cm−3). It is shown that annealing at 500 °C for 1 min in a N2 ambient improves ohmic contact properties. Specific contact resistance is measured to be 5.0(±1.0)×10−3 and 2.5(±1.0)×10−3 Ω cm2 for the as-deposited and annealed samples, respectively. It is also shown that the light transmittance is 90.3(±0.6) and 97.3(±0.8) % (at 470 nm) for the as-deposited and annealed contacts, respectively. Furthermore, the surface of the annealed contact is found to be fairly smooth with a root-mean-square roughness of 0.84 nm. These results are compared with those previously reported for the Ni/Au contacts.
Relation between short-range order and crystallization behavior in Zr-based amorphous alloys
Authors: Xing L. Q. , Hufnagel T. C. , Eckert J. , Löser W. , Schultz L.
Published: July 2002

We have examined the effect of Ti and cooling rate on the crystallization of Zr62−xTixCu20Ni8Al10(0⩽x⩽10) amorphous alloys. Ti stabilizes an icosahedral phase in Zr62−xTixCu20Ni8Al10(0⩽x⩽10) alloys. Without Ti (x=0), crystallization produces cubic and tetragonal intermetallic phases, and the crystallization temperature shows no dependence on the cooling rate at which the amorphous alloy was produced. The alloys containing Ti (3⩽x⩽10) precipitate an icosahedral quasicrystalline phase upon annealing, and show a significant reduction of crystallization temperatures with decreasing cooling rates of casting. We propose that the undercooled melts and amorphous alloys have icosahedral short-range order. The degree of short-range order or medium-range order in the amorphous alloys increases with decreasing cooling rate. Crystallization is easier when the precipitating phase resembles the short-range order of the amorphous solid. Therefore, the crystallization temperature is reduced when the precipitates are icosahedral. The dissimilarity between the structures of the precipitates and the short-range order in amorphous Zr62Cu20Ni8Al10 may be one of the reasons for its very wide supercooled liquid region.
Real-time extraction of plasma equilibrium parameters in KSTAR tokamak using statistical methods
Authors: Na Yong-Su , Jeon Young-Mu , Hong S. H. , Hwang Y. S.
Published: July 2002

To improve inherent shortcomings of statistical methods and apply them to the extraction of plasma equilibrium parameters in a fast timescale for real-time plasma control, new concepts of statistical methods such as principal component analysis-based neural network (NN), functional parametrization (FP)-based NN and double network are introduced by modifying NN and FP. These new methods are benchmarked and compared with the conventional techniques of NN and FP in a simple single-filament system. As a result of their applications to identification of plasma equilibrium parameters in the Korea Superconducting Tokamak Advanced Research tokamak, particularly, the double network concept among them has successfully achieved the improvement of drawbacks in the conventional methods. It is shown that more reliable results from the double network method can be obtained by combining several different statistical treatments as a primary network. Even in the case of nonoptimized methods united as a primary network, quite acceptable results can be achieved in the double network method.