bluuebunny/crossref_metadata_2025 huggingface dataset:
Design parameters for lateral carrier confinement in quantum-dot lasers
Authors: Kim J. K. , Strand T. A. , Naone R. L. , Coldren L. A.
Published: July 2002

Quantum-dot (QD) lasers have fallen short of their promise of ultralow threshold and high characteristic temperature. Here, we report that QDs show great promise for controlling lateral carrier leakage. While oxide apertures continue to enable improved performance in vertical cavity surface emitting lasers (VCSELs) by reducing optical losses and current spreading, lateral carrier losses remain uncontrolled. We investigate QD active material in which lateral diffusion is intentionally reduced. Cathodoluminescence results demonstrate reduced lateral diffusion in the material with which we expect >50% reduction in the threshold current for 1-μm-wide edge emitters or 5-μm-diam VCSELs. However, initial edge-emitter results demonstrated 10% reduction due to unintended current spreading and lasing from higher states.
Linear optical properties of a heavily Mg-doped Al0.09Ga0.91N epitaxial layer
Authors: Bergmann M. J. , Özgür Ü. , Casey H. C. , Muth J. F. , Chang Y. C. , Kolbas R. M. , Rao R. A. , Eom C. B. , Schurman M.
Published: July 2002

The room-temperature absorption coefficient and ordinary refractive index for a ∼0.4-μm-thick p-type wurtzite Al0.09Ga0.91N epitaxial layer were determined via optical transmission measurements. The layer was grown by metal organic chemical vapor deposition and heavily doped (∼5×1019 cm−3) with Mg. Additional measurements of the refractive index by prism coupling to the layer confirmed the transmission results. The low-temperature AlN buffer layer altered the expected interference fringes of the transmission spectrum below the band-gap energy and had to be accounted for in the analysis. The absorption coefficient exhibited band-tail effects and had a reduced slope near band-gap energy as compared to undoped GaN. Using a detailed balance argument, the reduced slope was consistent with the lack of a peak in the continuous-wave photoluminescent emission.
Electron mobility exceeding 104 cm2/V s in an AlGaN–GaN heterostructure grown on a sapphire substrate
Authors: Wang T. , Ohno Y. , Lachab M. , Nakagawa D. , Shirahama T. , Sakai S. , Ohno H.
Published: July 2002

High-quality AlGaN/GaN undoped single heterostructures (SH) with different Al contents have been grown on sapphire substrates. The magnetotransport investigation was performed on these samples at a low temperature. The observation of Shubnikov–de Hass oscillations in the magnetic fields below 3 T and the integer quantum Hall effect confirmed the existence of the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. The Al0.18Ga0.82N/GaN SH shows a Hall mobility of 10 300 cm2/V s at a carrier sheet density of 6.19×1012/cm2 measured at 1.5 K. To the best of our knowledge, this is the highest carrier mobility ever measured in GaN-based semiconductors grown on sapphire substrates. The Al composition dependence of the mobility and carrier sheet density were also investigated. Based on the piezoelectric field effect, the Al composition dependence of the 2DEG sheet density was calculated, which agreed well with the experimental result. The negative magnetoresistance with parabolic magnetic-field dependence in the low magnetic field was also observed in the sample with the highest 2DEG sheet density.
Ion-beam patterning of magnetic films using stencil masks
Authors: Terris B. D. , Folks L. , Weller D. , Baglin J. E. E. , Kellock A. J. , Rothuizen H. , Vettiger P.
Published: July 2002

Previously, ion-beam irradiation has been shown to locally alter the magnetic properties of thin Co/Pt multilayer films. In this work, we have used ion-beam irradiation through a silicon stencil mask having 1-μm-diam holes to pattern a magnetic film. Regularly spaced micrometer-sized regions of magnetically altered material have been produced over areas of a square millimeter in Co/Pt multilayers. These magnetic structures have been observed by magnetic force microscopy. The patterning technique is demonstrated with mask–sample spacing as large as 0.5 mm. In addition, smaller regions of magnetic contrast, down to 100 nm, were created by using two masks with partially overlapping micrometer holes. Such patterned magnetic films are of interest for application in high-density magnetic recording.
Dynamics of the excimer laser annealing of hydrogenated amorphous silicon thin films
Authors: Ivlev G. , Gatskevich E. , Cháb V. , Stuchlı́k J. , Vorlı́ček V. , Kočka J.
Published: July 2002

Time-resolved reflectivity and time-resolved conductivity spectroscopies have been used to monitor phase changes as a function of pulse-energy density during the recrystallization of amorphous hydrogenated Si by an ArF excimer laser. The simultaneous application of both spectroscopies allowed clear identification of the melting threshold and time of melting. The dc conductivity of irradiated Si was measured as a function of pulse energy and number of pulses. These results, together with Raman spectroscopy, revealed that single-pulse annealing gives a conductive, but still amorphous and rather defective layer. At least two consecutive pulses are necessary for obtaining of the substantial crystalline fraction.
Pixel-less infrared imaging based on the integration of an n-type quantum-well infrared photodetector with a light-emitting diode
Authors: Dupont E. , Liu H. C. , Buchanan M. , Wasilewski Z. R. , St-Germain D. , Chevrette P.
Published: July 2002

This letter presents the recent developments of large-area focal plane “pseudo” arrays for infrared (IR) imaging. The devices are based on the epitaxial integration of an n-type mid-IR (8–10 μm) GaAs/AlGaAs quantum-well detector with a light-emitting diode. The increase of spontaneous emission by the midinfrared-induced photocurrent is detected with a charge-coupled device camera in the reflection configuration. The mid-IR image of a blackbody object is up-converted to a near-IR transformed image with very small distortion.
High-temperature light emission from InAs quantum dots
Authors: Patanè A. , Polimeni A. , Main P. C. , Henini M. , Eaves L.
Published: July 2002

We study the photoluminescence (PL) properties of InAs/GaAs self-assembled quantum dots in a temperature range (T=300–500 K) above that reported to date. Various power excitation densities were used, allowing us to identify the important contribution of nonradiative channels in quenching the dot PL as the temperature is increased. The role played by the wetting layer on the dot PL intensity has been investigated in samples in which the separation of the dot and wetting layer levels is tuned by post-growth annealing. This experiment reveals that, at a high temperature (>300 K), the relative population of the dot and wetting layer levels is given by a Boltzmann distribution.
Spectroscopic determination of the electron distribution in a quantum cascade structure
Authors: Wilson L. R. , Keightley P. T. , Cockburn J. W. , Duck J. P. , Skolnick M. S. , Clark J. C. , Hill G. , Moran M. , Grey R.
Published: July 2002

Complementary intersubband and interband optical measurements have been employed in order to study the bias dependence of the carrier distribution and energy level alignment within a GaAs–AlGaAs quantum cascade structure. Using these techniques, we have measured the redistribution of electrons throughout the bridging regions and upper states in the active regions with increasing bias. The high quality of the sample gives very narrow linewidths in the optical spectra, permitting the resolution of transitions involving closely spaced energy levels. This has allowed the direct observation of level alignment at the onset of current flow through the device.
Polarization dependent photocurrent spectroscopy of InAs/GaAs quantum dots
Authors: Chu L. , Arzberger M. , Zrenner A. , Böhm G. , Abstreiter G.
Published: July 2002

We have performed interband photocurrent spectroscopy on self-assembled InAs/GaAs quantum dots using a graded index separate confinement heterostructure in waveguide geometry. The photocurrent spectrum of the quantum dots is found to be shifted to higher energies in comparison to the photoluminescence spectrum. The polarization dependent measurements show that the valence band ground states of strained InAs islands have heavy hole character. In a structure with seven vertically stacked quantum dot layers separated by 10 nm GaAs spacers, we find a change in the photocurrent spectra which is evidence for vertical coupling.
Electron energy dependence of metal-oxide-semiconductor degradation
Authors: DiMaria D. J.
Published: July 2002

Using a variety of electron injection techniques, defect generation and degradation in thin gate oxide devices are unequivocally shown to be related to the electron energy of the carriers impinging on the silicon contacts to the oxide layer and not due to the oxide electric field. Also, nonuniform channel-hot-electron degradation is shown to be due to the same fundamental mechanism as that observed under uniform degradation conditions using either direct tunneling, Fowler–Nordheim tunneling, or substrate-hot-electron injection. Additive defect generation from all three degradation modes is discussed in terms of realistic device operation.
Growth and magnetoresistive properties of (LaMnO3)m(SrMnO3)n superlattices
Authors: Salvador P. A. , Haghiri-Gosnet A.-M. , Mercey B. , Hervieu M. , Raveau B.
Published: July 2002

Artificial (LaMnO3)m(SrMnO3)n superlattices, n/(m+n)=0.26, were grown with pulsed-laser deposition, and the superlattice period Λ was varied. Their structural characteristics were investigated using x-ray diffraction and electron microscopy. When Λ<ap (perovskite lattice parameter), bulk-like properties of La0.74Sr0.26MnO3 are obtained: TMI (metal-to-insulator transition temperature) ≈Tc (Curie temperature) ≈345 K and −MR (magnetoresistance) ≈100%. Increasing Λ (to 21 ap) leads to a decrease in the Tc, the low-temperature magnetization, the magnetoresistance, and the TMI—eventually becoming insulating at low temperatures. These effects can be explained by Mn3+/Mn4+ separation arising from the artificially induced La/Sr order.
Thermal strain in GaAs layers grown by epitaxial lateral overgrowth on Si substrates
Authors: Zytkiewicz Z. R. , Domagala J.
Published: July 2002

X-ray diffraction was used to study deformation of GaAs layers grown on Si substrates by liquid phase epitaxial lateral overgrowth (ELO). We show that, in the direction perpendicular to seeding lines, the GaAs ELO stripes bend outwards from the mask due to the tensile strain in the GaAs buffer layer. As narrow as 94 arcsec (004) rocking curves have been measured for the laterally grown parts of ELO stripes what indicates the high quality of ELO GaAs layers grown on GaAs-coated Si substrates. We use our model of strain relaxation via bending of laterally grown parts of ELO layers to explain some recently published results on bending of ELO GaN layers on SiC and sapphire substrates.
Noise suppression in Ti:sapphire laser-based electro-optic sampling
Authors: Zhang J. M. , Ruo R. L. , Jackson M. K.
Published: July 2002

We show that intensity and phase noise are highly correlated in a mode-locked Ti:sapphire laser. This allows electro-optic sampling noise, degraded by the combination of −184 dBc/Hz laser phase noise and birefringent system components, to be reduced 12 dB to the shot noise floor. Two multiorder waveplates can be combined to achieve shot-limited noise while preserving maximum detection efficiency.
Vector imaging of supercurrent flow in YBa2Cu3O7−δ thin films using terahertz radiation
Authors: Morikawa O. , Yamashita M. , Saijo H. , Morimoto M. , Tonouchi M. , Hangyo M.
Published: July 2002

We demonstrated the vector mapping of the supercurrent flow in YBa2Cu3O7−δ thin films using a terahertz (THz) radiation imaging system. A wire-grid polarizer was employed to detect the vector components of the THz radiation. The two THz radiation images obtained through the polarizer tilted by +45° and −45° with respect to the detector’s orientation were successfully transferred into the vector image of the supercurrent distribution. The observed distributions in the current-biased and the vortex-penetrated films were qualitatively explained using Bean’s critical-state model [E. H. Brandt and M. Indenbom, Phys. Rev. B 48, 12893 (1993)].
Very low densities of localized states at the Fermi level in hydrogenated polymorphous silicon from capacitance and space-charge-limited current measurements
Authors: Kleider J. P. , Longeaud C. , Gauthier M. , Meaudre M. , Meaudre R. , Butté R. , Vignoli S. , Roca i Cabarrocas P.
Published: July 2002

The density of states at the Fermi level N(EF) has been measured on hydrogenated polymorphous (pm-Si:H) silicon samples using both capacitance measurements on Schottky barriers and space-charge-limited current measurements on n+/i/n+ structures. From both techniques, N(EF) values of 7–8×1014 cm−3 eV−1 have been obtained, which is significantly lower than reported in the literature for hydrogenated amorphous silicon (a-Si:H). Such values demonstrate that pm-Si:H is a very low defect density material which should be able to replace a-Si:H in the field of applications like photovoltaics.
Investigations on the Stranski–Krastanow growth of CdSe quantum dots
Authors: Schikora D. , Schwedhelm S. , As D. J. , Lischka K. , Litvinov D. , Rosenauer A. , Gerthsen D. , Strassburg M. , Hoffmann A. , Bimberg D.
Published: July 2002

We have investigated the growth kinetics of the self-assembled formation of coherently strained CdSe islands. We have found that two distinctly different types of islands are formed in succession. Analyzing the density distribution function of the two dominating size classes of islands, we show that islands of an average diameter of about 16 nm (type B islands) are correlated with a phase transition via a Stranski–Krastanow growth process. The other islands with a diameter of less than 10 nm (type A islands) is formed during the growth of the first 2 ML. At a coverage of about 3.1 ML CdSe stacking faults appear, indicating the beginning of the plastic relaxation of the quantum dot structure.
Large optical nonlinearity and fast response time in low-temperature grown GaAs/AlAs multiple quantum wells
Authors: Okuno Tsuyoshi , Masumoto Yasuaki , Ito Masashi , Okamoto Hiroshi
Published: July 2002

We have investigated optical nonlinearity in low-temperature (LT) molecular-beam-epitaxy-grown GaAs/AlAs multiple quantum wells (MQWs). Minimum saturation intensity, that is, maximum optical nonlinearity, is observed at around the excitonic absorption peak. The saturation intensity of the LT MQW is smaller by an order of magnitude than that of LT bulk GaAs. The response time of the LT MQW is 1/4–1/2 of the LT GaAs, and becomes less than 1 ps, corresponding to ∼1/400 of the standard-temperature-grown MQW. These results demonstrate a clear advantage of the room-temperature excitons in the LT MQW having large optical nonlinearity as well as fast response time.
Flexible microdischarge arrays: Metal/polymer devices
Authors: Park S.-J. , Wagner C. J. , Herring C. M. , Eden J. G.
Published: July 2002

Flexible microdischarge arrays have been fabricated in metal–polymer–metal structures having a total thickness of ∼30 μm (∼1.2 mils). Composed of individual cylindrical devices having a diameter of 150 μm, positive differential resistance (30–120 kΩ), and operating voltages as low as 114 V for a 5 μm thick dielectric layer, the arrays operate at pressures beyond 700 Torr of Ne and in 1 atm of air. For Ne pressures ⩽ 200 Torr, emission is produced from Ne ion excited states lying more than 55 eV above the neutral ground state (2p6). The structures reported here are inexpensive to fabricate and have lifetimes beyond 50 h. Arrays that have been sealed by conventional lamination have also been operated successfully.
High-frequency properties of 1.55 μm laterally complex coupled distributed feedback lasers fabricated by focused-ion-beam lithography
Authors: Rennon S. , Bach L. , Reithmaier J. P. , Forchel A. , Gentner J. L. , Goldstein L.
Published: July 2002

Laterally complex coupled distributed feedback lasers have been fabricated by focused-ion-beam lithography on completely grown InGaAsP/InP laser structures emitting at 1.55 μm. The grating definition is based on implantation-enhanced quantum-well intermixing and wet chemical etching and allows the fabrication of complex coupled antiphase gratings without any further overgrowth step. Side-mode suppression ratios of 45 dB and bandwidths for direct modulation beyond 13 GHz were obtained for 300-μm-long devices. The lasers exhibit frequency-modulation response values lower than 200 MHz/mA and feature a low sensitivity to back-reflected light. Preliminary lifetime measurements over 7000 h continuous-wave operation at room temperature show no significant indication for long-time degradation.
The CO dimer millimeter wave spectrum: Detection of tunneling transitions
Authors: Roth Daniel A. , Surin Leonid A. , Dumesh Boris S. , Winnewisser Gisbert , Pak Igor
Published: July 2002

A portion of the CO dimer millimeter wave absorption spectrum has been studied by using our highly sensitive intracavity-jet OROTRON spectrometer in the frequency range from 131 to 174 GHz. By varying the CO concentration in the Ne/CO gas mixture feeding the supersonic jet expansion, the effective temperature of the beam could be changed, revealing a correlation between the observed line intensity and the relative energy of the respective lower state energy levels. Using this temperature dependence and the technique of combination differences together with the data from the infrared study of Brookes and McKellar [J. Chem. Phys. 111, 7321 (1999)], out of over 200 observed transitions, a total of 19 lines could be assigned. All assigned millimeter-wave transitions are tunneling transitions. They belong to four subbands, which connect seven lower energy levels with A+ symmetry to ten previously unknown upper energy levels with A− symmetry. The A+ and A− separation signifies the tunneling splitting of the CO stretching ground state vCO=0 energy levels. The energies of all levels were determined to microwave accuracy. The discovered energy levels fall into two substates, corresponding to the projection K=0 and to K=1 of the total angular momentum J onto the intermolecular axis. The effective intermolecular CO–CO separation for these new K=0 and K=1 states is 4.26 and 4.17 Å, respectively.
Structure and dynamics of nonaqueous mixtures of dipolar liquids. I. Infrared and far-infrared spectroscopy
Authors: Venables Dean S. , Chiu Allan , Schmuttenmaer Charles A.
Published: July 2002

Mixtures of acetone/methanol, acetonitrile/methanol, and acetone/acetonitrile over their whole composition range were studied with infrared and far-infrared (THz) spectroscopy. Experimental spectra of all mixtures were obtained below 120 cm−1, and spectra of methanolic mixtures were also measured from 400 to 1000 cm−1. The mixtures display ideal absorption spectra below 120 cm−1, contrasting with significant nonidealities in the absorption of aqueous mixtures in the same frequency range. Dramatic changes as a function of composition were found in methanolic mixtures at higher frequencies. The high frequency librational band of methanol, centered at 670 cm−1, shifts to substantially lower frequencies upon dilution, indicating marked changes in the librational motion of the hydroxyl hydrogen of methanol. This band is a sensitive probe of the hydrogen bonding environment experienced by methanol molecules.
Visualization of a Taylor–Couette flow avoiding parasitic reflections
Authors: Prigent A. , Dauchot O.
Published: July 2002

Fluorescent lighting is used to avoid reflections in the visualization of various flows in a large Taylor–Couette device. The inner cylinder is covered by a fluorescent film and the entire apparatus is illuminated with UV lights. The visible light emitted by the film allows visualization by transmission through the flow containing reflective flakes. This technique is ideal to identify different flows coexisting in the whole apparatus, whose detailed structure has been determined locally by other traditional means. The major advantage of the technique is the lighting uniformity and the absence of parasitic reflections on the whole setup. The technique will soon be applied to the discrimination between laminar and turbulent areas in the coexisting regimes.
Application of split-gate structures as tunable spin filters
Authors: Gilbert M. J. , Bird J. P.
Published: July 2002

We describe an electron filter that exploits the known magnetotransport properties of quantum point contacts to provide local and tunable control of the spin polarization in a semiconductor. When properly configured, we show that the conductance of this device gives a direct measure of the spin polarization of carriers transmitted through it. By modeling the transport through a potential barrier with experimentally realistic parameters, we discuss the factors which must be satisfied in order to successfully implement such a device.
The effect of microstructure on the temperature dependence of the interlayer coupling in Co/Cu multilayers
Authors: Christides C.
Published: July 2002

Three classes of giant magnetoresistance Co(1 nm)/Cu(2.1 nm) multilayers were sputter grown with different microstructures in respect to grain size and interface roughness, depending on deposition conditions. Magnetization and current in-plane giant-magnetoresistance (GMR) isothermal loops reveal an unusually high increase of coercivity from 280 down to 5 K. In addition, a systematic variation was observed in the temperature dependence of the indirect exchange coupling as the Co–Cu layering is modified in the three classes of Co/Cu multilayers. Specifically, the temperature dependence of the saturation (switching) field in the GMR-loops, and the indirect coupling strength, vary as (T/T0)/sinh(T/T0) whereas the spin-blocking temperature T0 is found equal to 84(4), 96(11), and 105(10) K for class A, B, and C multilayers, respectively. These results indicate that the desirable low hysteresis appears in the GMR loops at room temperature because the spin structure becomes unstable above the obtained T0 due to domain wall fluctuations. Such magnetic fluctuations define a short–range order state above T0 that depends on Co–Cu intermixing and geometric factors of the grains.
Effects of self-generated magnetic fields and nonlocal electron transport in laser produced plasmas
Authors: Nicolaï P. , Vandenboomgaerde M. , Canaud B. , Chaigneau F.
Published: July 2002

Experiments using planar gold targets were performed at the Phebus facility [G. Thiell, Laser Part. Beams 16, 253 (1998)] in order to characterize the deposition of laser energy. These experiments have been simulated with a two-dimensional radiation-hydrodynamic code. Using flux limited Spitzer–Härm transport [L. Spitzer and R. Härm, Phys. Rev. 89, 977 (1953)], no agreement was obtained between simulations and experiments. Soft x-ray streak camera and two-dimensional time-resolved radiography data cannot be reproduced. For the first time, self-generated magnetic fields and nonlocal heat fluxes have been taken into account in a two-dimensional code. Both effects are simultaneously computed. The artificially limited Spitzer–Härm fluxes are not needed anymore with this treatment of the heat conduction. The simulations obtained with the latter are in good agreement with all the experimental results. Based on the numerical results, effects of the magnetic fields and nonlocal fluxes on electron temperature and x-ray conversion of the plasma are also presented.
Sub-femtosecond dynamics and dissociation of C–H bonds in the condensed phase: Effects of entangled protonic states
Authors: Chatzidimitriou-Dreismann C. A. , Abdul-Redah T. , Sperling J.
Published: July 2002

According to quantum theory, adjacent particles of condensed systems may exhibit quantum interference and/or correlation phenomena being caused by interparticle interactions. Here we present, for the first time, direct experimental evidence for short-lived quantum entanglement of protons of C–H bonds. Neutron Compton scattering results from solid polystyrene reveal that, in the sub-femtosecond time scale, the measured cross-section density of the protons is “anomalously” reduced by ca. 20%. A first-principles theoretical treatment of this effect is given, which is based on the well-known van Hove formalism, and also takes explicitly into account quantum entanglement of protonic states and its decoherence. Besides its fundamental physical character, the effect may have considerable chemical and biological applications, and also be of technical importance, e.g., for the engineering of some neutron moderators.
The electronic structure and chemical bonding of aluminum acetylide: Al2C2 and Al2C2−: An experimental and theoretical investigation
Authors: Cannon Nathan A. , Boldyrev Alexander I. , Li Xi , Wang Lai-Sheng
Published: July 2002

We have investigated the electronic structure and chemical bonding of Al2C2 and Al2C2− both experimentally and theoretically. Photoelectron spectra of Al2C2− were obtained at several photon energies. Two anionic isomers were observed: one with a very sharp ground state feature and a low vertical electron binding energy (0.71 eV) and another with a very broad ground state feature with a much higher vertical electron binding energy (1.58 eV). Theoretical calculations were performed at various levels of theory for both the anion and the neutral. We found two isomers with relatively close energies for the anion: a quasilinear acetylide species and a planar-bridged D2h structure. However, only one stable isomer was found for the neutral, which has the acetylide structure. Adiabatic and vertical detachment energies were also calculated for the two anionic isomers and were used to interpret and assign the experimental spectra. We found that the sharp 0.71 eV feature was from the acetylide isomer, whereas the broad 1.58 eV feature was from the D2h isomer. The excellent agreement between the calculated and experimental electron affinities and excitation energies lends considerable credence for the assignments of the two anionic isomers. The structures and bonding of the acetylide neutral and anion and the D2h anion are discussed. © 2000 American Institute of Physics.
Stimulated radiation from spatiotemporally gyrating relativistic electron beams
Authors: Davies J. A. , Chen C.
Published: July 2002

A stability analysis is made of an electron beam, propagating along and gyrating about a uniform magnetic field, for the case of a spatiotemporal equilibrium distribution in the phase angle of the transverse electron momentum component. The axial momentum component and the magnitude of the transverse momentum component are assumed to have definite values in the equilibrium distribution. The analysis is carried out by applying Lorentz transformations to previous results for nongyrotropic equilibrium distributions. The dispersion matrix, its eigenmodes (which relate field amplitudes), and the dispersion relation are obtained. Numerical results show that varying the spatiotemporal properties of a nongyrotropic equilibrium distribution has only a small effect on maximum growth rates of radiation, but has a strong effect on the frequencies and wavenumbers at which instability occurs. A novel mechanism is found by which electrons emit stimulated radiation at frequencies that, in principle, can be greater than the usual Doppler-shifted electron cyclotron frequency by orders of magnitude.
Photodissociation of CF2=CFCl at 193 nm by translational spectroscopy
Authors: Lee Y. R. , Wang L. D. , Lee Y. T. , Lin S. M.
Published: July 2002

The photodissociation of CF2=CFCl at 193 nm has been studied by product translational spectroscopy. Results show that the molecule undergoes competing dissociation into three pairs of products: (1) CFCl+CF2, (2) C2F2Cl+F, and (3) C2F3+Cl with a relative yield of 1.0:∼0.1:1.4. When photolyzed with polarized laser light, reactions (1) and (3) were detected with fragments recoiling anisotropically, indicating that dissociation occurs primarily on the excited potential energy surface. From the maximum kinetic energy derived for reactions (2) and (3), a upper limit of 110 and 90 kcal/mol was determined for the C–F bond and the C–Cl bond dissociation energy, respectively. In addition to these primary dissociation channels, we have observed the secondary products, CF+CF2 and F+C2F2 from the primary fragment C2F3 by absorbing an additional photon.
Normalized Uq(sl(3)) Gel’fand–(Weyl)–Zetlin basis and new summation formulas for q-hypergeometric functions
Authors: Dobrev V. K. , Mitov A. D. , Truini P.
Published: July 2002

An explicit realization of the normalized Gel’fand–(Weyl)–Zetlin (GWZ) basis for Uq(sl(3)) in terms of polynomial functions in three variables (real or complex) is given. The construction uses two different realizations of the Uq(sl(3)) unnormalized GWZ basis which were given previously, and whose transformation properties were not known. It turns out that finding these properties enables us to find the (GWZ-dependent) proportionality constant between these two realizations. The scalar product is also fixed by this in both unnormalized realizations, and then, by normalization, the normalized GWZ states are obtained. As by-products new summation formulas are obtained which seem new also for q=1. The main new formula is a double sum which is given in terms of the proportionality constant mentioned above. This double sum can be written as single sum over a q−3F2 hypergeometric function, or as a q-hypergeometric function of two variables.
Examination of tunnel junctions in the AlGaN/GaN system: Consequences of polarization charge
Authors: Singh Madhusudan , Zhang Yifei , Singh Jasprit , Mishra Umesh
Published: July 2002

A strong piezoelectric effect and a large spontaneous polarization allow one to incorporate a large electric field (>106 V/cm) and high sheet charge (>1013 cm−2) without doping in the AlGaN/GaN heterostructure. Theoretical studies are done to examine how polarization effects can be exploited to design metal–AlGaN/GaN tunnel junctions. We find that with a proper choice of AlGaN thickness undoped junctions can be made with very high metal to two-dimensional electron gas tunneling. Thus, a Schottky junction can be converted to a tunnel junction without doping. The tunneling probabilities approach those produced in a system doped at ∼4×1019 cm−3. This work suggests that very interesting tunnel junctions can be made from undoped AlGaN/GaN heterostructures.
Bending electrostriction in polyurethane using in situ ultraviolet and visible spectroscopies
Authors: Watanabe Masashi , Shirai Hirofusa , Hirai Toshihiro , Suzuki Makoto , Hirako Yoshiyuki
Published: July 2002

We measured the ultraviolet and visible spectra of polyurethane films while a high electric field (2 MV/m) was applied to the films in order to investigate the detailed mechanism of the bending electrostriction. The polyurethane films were composed of segmented polyester polyurethane to which a small amount of sodium acetate (∼10−4 mol/kg) was added. The films had ∼9-nm-thick gold electrodes on both surfaces. The obtained spectra showed reversible absorption that appeared during the application of the electric field and disappeared with its removal. The response of the absorption to the field application was very similar to that of the film’s bending due to the bending electrostriction. This suggests that the reversible absorption is directly related to the mechanism of the bending electrostriction. We also demonstrated that the absorption was attributed to the moiety “Ph–NH–COO” on the polyurethane, suggesting that this moiety is a trap site of injected charges that can be considered the cause of the bending electrostriction.
Excitonic gain and stimulated ultraviolet emission in nanocrystalline zinc-oxide powder
Authors: Sun Y. , Ketterson J. B. , Wong G. K. L.
Published: July 2002

We have studied ultraviolet photoluminescence from high-purity zinc-oxide powder over a wide temperature range (2–293 K). At low temperatures, the spontaneous emission is due to radiative recombination of excitons bound to donors and acceptors. At high temperature (>90 K), it mainly consists of recombination of free excitons, with exciton–exciton collision-induced recombination dominating the spectrum at higher pumping intensities. Emission from the exciton–exciton collision process shows clear stimulated-emission behavior. At sufficiently high pumping intensity, the stimulated emission band shifts abruptly to a longer wavelength due to the formation of an electron–hole plasma. Sharp emission peaks are observed in the stimulated emission bands. The possible origins of these sharp peaks are discussed.
A new basic effect in retarding potential analyzers
Authors: Sanmartı́n Juan R. , López-Rebollal O.
Published: July 2002

The Retarding Potential Analyzer (RPA) is the standard instrument for in situ measurement of ion temperature and other ionospheric parameters. The fraction of incoming ions rejected by a RPA produces perturbations that reach well ahead of a thin Debye sheath, a feature common to all collisionless, hypersonic flows past ion-rejecting bodies. This phenomenon is here found to result in a correction to Whipple’s classical law for the current characteristic of an ideal RPA (sheath thin; inverse ram ion Mach number M−1, and ram angle of RPA aperture θ, small or moderately small). The current correction increases with the temperature ratio Te/Ti, and ranges from a 15%–30% reduction at Mθ=0 to a 15%–30% increase at Mθ=2, for typical values of M, Te/Ti and transparency of aperture grid. Linear analysis of the perturbed plasma beyond the sheath rests on the fact that a Maxwellian undisturbed ion distribution is Vlasov-stable against quasineutral–ionacoustic waves.
Stabilization of Z pinch by velocity shear
Authors: DeSouza-Machado S. , Hassam A. B. , Sina Ramin
Published: July 2002

A numerical experiment is run to assess the stabilization of ideal magnetohydrodynamic (MHD) instabilities by externally applied velocity shear. A Z pinch, unstable to both kink and interchange (sausage) instabilities, is subjected to an external force that drives sheared mass flow along the axis. The turbulence from the MHD instabilities is found to be suppressed significantly with increasing Mach number of the flow. At sonic Mach numbers of 4–5, the “discharge” is seen to have recovered to its laminar state in more than 95% of the volume, there being a small residual wobble at the center of the column. For lower Mach numbers, the wobble becomes more significant. This is consistent with analytic theories that predict supersonic sheared flows are needed to stabilize MHD instability.
Nonlinear dynamics of feedback modulated magnetic islands in toroidal plasmas
Authors: Fitzpatrick Richard , Waelbroeck François L.
Published: July 2002

An analysis is presented of the dynamics of a helical magnetic island chain embedded in a toroidal plasma, in the presence of an externally imposed, rotating, magnetic perturbation of the same helicity. Calculations are carried out in the large aspect-ratio, zero-β, resistive magnetohydrodynamical limit, and incorporate a realistic treatment of plasma viscosity. There are three regimes of operation, depending on the modulation frequency (i.e., the difference in rotation frequency between the island chain and the external perturbation). For slowly modulated islands, the perturbed velocity profile extends across the whole plasma. For strongly modulated islands, the perturbed velocity profile is localized around the island chain, but remains much wider than the chain. Finally, for very strongly modulated islands, the perturbed velocity profile collapses to a boundary layer on the island separatrix, plus a residual profile which extends a few island widths beyond the separatrix. Analytic expressions are obtained for the perturbed velocity profile, the island equation of motion, and the island width evolution equation in each of these three regimes. The ion polarization correction to the island width evolution equation, which has previously been reported to be stabilizing, is found to be destabilizing in all three regimes.
Effect of temperature on the optical properties of (InGa)(AsN)/GaAs single quantum wells
Authors: Polimeni A. , Capizzi M. , Geddo M. , Fischer M. , Reinhardt M. , Forchel A.
Published: July 2002

In x Ga 1−x As 1−y N y / GaAs single quantum wells emitting at room temperature in the wavelength range λ=(1.3–1.55) μm have been studied by photoluminescence (PL). By increasing temperature, we find that samples containing nitrogen have a luminescence thermal stability and a room temperature emission efficiency higher than that of the corresponding N-free heterostructures. The temperature dependence of the PL line shape shows a progressive carrier detrapping from localized to extended states as T is increased. Finally, the extent of the thermal shift of the free exciton energy for different y indicates that the electron band edge has a localized character which increases with nitrogen content.
Optical transitions and gain in group-III nitride quantum wells
Authors: Litvinov V. I.
Published: July 2002

The convenient basis representation for the valence band Hamiltonian in wurtzite semiconductors is proposed and the polarization selection rules in GaN-based quantum wells are found. The polarization dependent optical matrix elements in quantum wells are calculated for a finite in-plane electron momentum, that allows to calculate peak optical gain and laser threshold in the GaN-based quantum well structures. This article also addresses the role of alloy inhomogeneities in the optical gain in quantum well lasers. The peak gain is found as a function of statistical broadening caused by the In-content fluctuations. It is shown that the composition fluctuations in the active region influence the gain differently depending on a light polarization. The absolute value of the calculated gain is found to be in agreement with the experiment.
Broadband calibration of long lossy microwave transmission lines at cryogenic temperatures using nichrome films
Authors: Stutzman M. L. , Lee Mark , Bradley R. F.
Published: July 2002

A one-port method to calibrate the S parameters of a long, lossy microwave transmission line in a cryogenic system is presented. This method exploits the relative temperature independence and frequency stability of nichrome thin films used as a known termination load. We have used the calibration on coaxial lines 1.2 m long from 0.5 to 12 GHz and at temperatures down to 100 mK.
Delamination behavior of Pt in a SiO2/Pt/Pb(ZrxTi1−x)O3/Pt ferroelectric thin-film capacitor
Authors: Park Youngsoo , Lee June Key , Chung Ilsub , Lee Jai-Young
Published: July 2002

Silicon dioxide was deposited on a Pt/Pb(ZrxTi1−x)O3/Pt capacitor by the plasma-enhanced chemical-vapor deposition (PECVD) method. We have been investigated the mechanism of blister formation on the PECVD SiO2/Pt/PZT/Pt capacitor. The blisters were observed at the temperature of 325 °C in O2 atmosphere, while in N2 and Ar atmosphere blisters were not formed even at 500 °C. Hydrogen evolution analysis from the PECVD SiO2 layer showed a sharp peak near 320 °C. The results indicated that the accumulation of water-vapor pressure developed by chemical reaction between oxygen and hydrogen could be the dominant mechanism of blister formation in PECVD SiO2/Pt/PZT/Pt capacitors.
Terahertz near-field microscopy based on a collection mode detector
Authors: Mitrofanov O. , Brener I. , Harel R. , Wynn J. D. , Pfeiffer L. N. , West K. W. , Federici J.
Published: July 2002

We report on the development of a collection mode near-field probe for the terahertz spectral range. The near-field detector is based on an aperture type probe with dimensions of 30×30 μm2. The collection mode technique provides higher sensitivity and higher resolution than the similar illumination mode approach. Spatial resolution better than 40 μm is demonstrated for a broad spectrum of 300–600 μm, which equals to λ/15 for the longest wavelength. The observed resolution is determined by the size of the probe aperture.
Reduced dynamics with initial correlations: Multiconfigurational approach
Authors: Burghardt I.
Published: July 2002

Dynamical equations for a subsystem interacting with an environment are proposed which are adapted to a multiconfigurational form of the density operator. Initial correlations are accounted for in a non-Markovian master equation. Two variants of the latter are derived by projection operator techniques and cumulant expansion techniques, respectively. The present scheme is developed in view of describing the ultrafast dynamics in solute–solvent complexes where the details of system–environment correlations are of importance. The master equation is readily integrated into the equations of motion derived by the multiconfiguration time-dependent Hartree method, which provides an efficient scheme for the numerical propagation of the density operator.
Experimental characterization and modeling of the reliability of interconnect trees
Authors: Hau-Riege Stefan P. , Thompson Carl V.
Published: July 2002

Interaction of the segments of interconnect trees during electromigration experiments has been studied using several simple tree structures. It is demonstrated that the reliability of a segment can not be predicted without knowledge of the conditions for stress migration and electromigration in connecting segments. Simulations of stress evolution during electromigration in interconnect trees have also been developed, and have been shown to predict behavior consistent with experimental results. Based on experiments and simulations, an analytic model for estimating the reliability of interconnect trees is developed. This analysis is based on calculations of the lifetimes of individual nodes within a tree, by estimating the times for void nucleation, void growth to failure, and formation of extrusions, using current density information for all of the segments connecting at a node. Models, simulations and experimental results on the reliability of interconnect trees are shown to yield mutually consistent results. Accurate circuit-level reliability analyses can not be based on segment-level models, but require instead, tree-based models such as the one developed here.
High-quality p–n junctions with quaternary AlInGaN/InGaN quantum wells
Authors: Chitnis A. , Kumar A. , Shatalov M. , Adivarahan V. , Lunev A. , Yang J. W. , Simin G. , Khan M. Asif , Gaska R. , Shur M.
Published: July 2002

We report on quaternary AlInGaN/InGaN multiple quantum well (MQW) light emitting diode structures grown on sapphire substrates. The structures demonstrate high quality of the p–n junctions with quaternary MQW. At low forward bias (below 2 V), the temperature dependent of current–voltage characteristics are exponential with the ideality factor of 2.28, which is in a good agreement with the model of the injected carrier recombination in the space charge region. This ideality factor value is approximately three times lower than for conventional GaN/InGaN light emitting diodes (LEDs). The obtained data indicate the recombination in p–n junction space charge region to be responsible for a current transport in LED structures with quaternary quantum wells. This is in contrast to InGaN based LEDs, where carrier tunneling dominates either because of high doping of the active layer or due to the high density of localized states.
Epitaxy of Pd thin films on (100) SrTiO3: A three-step growth process
Authors: Wagner T. , Richter G. , Rühle M.
Published: July 2002

Control of the orientation of thin Pd films on (100) SrTiO3 surfaces was obtained by changing the growth temperature. In particular, a three-step growth method was applied to deposit thin single-crystal Pd films on single-crystal (100) SrTiO3 surfaces. This was realized by first growing epitaxial Pd seeds at elevated temperatures. Subsequently, the seeds were overgrown at room temperature by polycrystalline Pd which fully covered the substrate at a low thickness. Annealing of these films promoted growth of the epitaxial seeds, resulting in single-crystal Pd films: (100) SrTiO3∥(100) Pd, [010] SrTiO3∥[010] Pd. The three-step growth method turned out to be a useful method to overcome surface roughening and the creation of crystalline imperfections in thin Pd films. This is essential for the growth of thin metallic epitaxial buffer layers. The microstructure of the films was analyzed by reflection high-energy electron diffraction, scanning electron microscopy, x-ray diffraction, and transmission electron microscopy.
A theory for Landau damping of ion acoustic waves in dope plasma
Authors: Mondal S. , Sarkar Susmita , Basu A. M. , Khan Manoranjan , Bhattacharyya Rajarshi , Chakraborty B.
Published: July 2002

Landau damping, free from the influence of other forms of damping, has been experimentally detected in ion acoustic waves in traces of a dope plasma of a light inert gas, when it is introduced into partially ionized plasma of a heavy inert gas. A theory has been worked out of Landau damping for longitudinal waves in Vlasov plasma for study of this and other familiar Landau dampings.
SiGe/Si THz laser based on transitions between inverted mass light-hole and heavy-hole subbands
Authors: Friedman L. , Sun G. , Soref R. A.
Published: July 2002

We have investigated a SiGe/Si quantum-well laser based on transitions between the light-hole and heavy-hole subbands. The lasing occurs in the region of k space where the dispersion of ground-state light-hole subband is so nonparabolic that its effective mass is inverted. This kind of lasing mechanism makes total population inversion between the two subbands unnecessary. The laser structure can be electrically pumped through tunneling in a quantum cascade scheme. Optical gain as high as 172/cm at the wavelength of 50 μm can be achieved at the temperature of liquid nitrogen, even when the population of the upper laser subband is 15% less than that of the lower subband.
Vibrational spectra of germanium–carbon clusters. I. Identification of the ν3(σu) mode of linear GeC3Ge
Authors: Robbins D. L. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

The first germanium–carbon cluster GeC3Ge, has been detected in the products from the laser evaporation of carbon and germanium rods trapped in solid Ar at ∼10 K. On the basis of very good agreement between Fourier transform infrared (FTIR) measurements and the predictions of density functional theory (DFT) calculations for the frequencies and C13 isotopic shifts, the ν3(σu) mode of linear GeC3Ge has been identified at 1920.7 cm−1.
Structure and magnetic properties of SmxCo5/α-Fe (x=0.65–1.3) prepared by mechanical milling and subsequent annealing
Authors: Zhang Jian , Zhang Shao-ying , Zhang Hong-wei , Shen Bao-gen , Li Bao-he
Published: July 2002

Powder mixtures of SmxCo5 (x=0.65–1.3)+20 wt % α-Fe were mechanically milled. Annealing these as-milled powders results in the formation of a mixture of the hard phase Sm–(Co, Fe) and the soft phase Fe–Co. For the as-milled Sm1Co5+20 wt % α-Fe powder, the hard phase changes with the increase of annealing temperatures. The optimal maximum energy product (BH)max is obtained in the powder annealed at 550 °C for 30 min. Depending on the Sm content in the as-milled SmxCo5 (x=0.65–1.3)+20 wt % α-Fe powders, the hard phases can be 1:7, 1:5, or 2:7 phase after a heat treatment at 550 °C for 30 min. The coercivity of 6.5 kOe and maximum energy product of 17.8 MGOe is achieved for the powder with x=1.0. The highest coercivity of 9.67 kOe is achieved for the powder with x=1.2. From the measurements of the coercivity obtained from minor hysteresis loops, it is concluded that the coercivities of this type of magnets are controlled mainly by the domain wall pinning.