bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of exciton states in GaAs coupled quantum wires on a V-grooved substrate
Authors: Komori Kazuhiro , Wang Xue-Lun , Ogura Mutsuo , Matsuhata Hirofumi
Published: July 2002

The exciton states of GaAs coupled quantum wires are investigated by the measurement of photoluminescence excitation (PLE) in comparison with those of single quantum wires. In the PLE spectra of single quantum wires (wire thickness=4.5 nm), sharp exciton peaks of the first two heavy hole-like transitions are observed with large energy difference of 47 meV, while two adjacent exciton peaks with the small energy splitting of 24 meV are observed in the coupled quantum wires (wire thickness=5 nm, barrier thickness=3 nm). From the measurements of the barrier thickness dependence, these exciton states agree well with the coupled states of the quantum wires calculated by the finite element method.
Electron multiplication in AlxGa1−xAs/GaAs heterostructures
Authors: Chia C. K. , David J. P. R. , Rees G. J. , Robson P. N. , Plimmer S. A. , Grey R.
Published: July 2002

We have measured the electron multiplication characteristics of thin (∼0.1 μm) AlxGa1−xAs(500 Å)/GaAs(500 Å) single heterostructures with 0.3⩽x⩽0.6, grown in a p-i-n configuration. In these devices, at low electric fields, the electron multiplication occurs mainly in the latter half of the i region because of dead space effects. As the electric field increases the multiplication behavior converges to that of the equivalent alloy. Our measurements show that the electron multiplication in these structures never exceeds that of GaAs. A simple Monte Carlo simulation suggests that the advantage obtained from the conduction band edge step down from AlxGa1−xAs to GaAs is offset by the energy loss via higher phonon emission in the AlxGa1−xAs layer.
Emission of excimer radiation from direct current, high-pressure hollow cathode discharges
Authors: El-Habachi Ahmed , Schoenbach Karl H.
Published: July 2002

A novel, nonequilibrium, high-pressure, direct current discharge, the microhollow cathode discharge, has been found to be an intense source of xenon and argon excimer radiation peaking at wavelengths of 170 and 130 nm, respectively. In argon discharges with a 100 μm diam hollow cathode, the intensity of the excimer radiation increased by a factor of 5 over the pressure range from 100 to 800 mbar. In xenon discharges, the intensity at 170 nm increased by two orders of magnitude when the pressure was raised from 250 mbar to 1 bar. Sustaining voltages were 200 V for argon and 400 V for xenon discharges, at current levels on the order of mA. The resistive current–voltage characteristics of the microdischarges indicate the possibility to form arrays for direct current, flat panel excimer lamps.
In-plane grain boundary effects on the magnetotransport properties of La0.7Sr0.3MnO3−δ
Authors: Gu J. Y. , Ogale S. B. , Rajeswari M. , Venkatesan T. , Ramesh R. , Radmilovic V. , Dahmen U. , Thomas G. , Noh T. W.
Published: July 2002

C -axis oriented La0.7Sr0.3MnO3−δ (LSMO) films were fabricated on the top of SrTiO3/YBa2Cu3O7 grown on MgO (001) substrates. From x-ray φ-scan and planar transmission electron microscopy measurements, the LSMO layer in the LSMO/SrTiO3/YBa2Cu3O7/MgO heterostructure is found to have coherent in-plane grain boundaries with a predominance of 45° rotations (between [100] and [110] grains) in addition to the cube-on-cube epitaxial relationship. Also, epitaxial LSMO/Bi4Ti3O12/LaAlO3 (001) and c-axis textured LSMO/Bi4Ti3O12/SiO2/Si (001) with random in-plane grain boundaries are introduced as the counterparts for comparison. The resistivity and magnetoresistance (MR) of LSMO layer were measured and compared. The low field MR at low temperature shows a dramatic dependence on the nature of the grain boundary. An attempt is made to interpret these results on the basis of correlation between the magnetic properties and grain structures.
Time-resolved Raman studies of the decay of the longitudinal optical phonons in wurtzite GaN
Authors: Tsen K. T. , Ferry D. K. , Botchkarev A. , Sverdlov B. , Salvador A. , Morkoc H.
Published: July 2002

Decay of the longitudinal-optical (LO) phonons in wurtzite GaN has been studied by subpicosecond time-resolved Raman spectroscopy. Our experimental results show that among the various possible decay channels, the LO phonons in wurtzite GaN decay primarily into a large wave-vector TO and a large wave-vector LA or TA phonon. These experimental results are consistent with the recent theoretical calculations of the phonon dispersion curves for wurtzite GaN.
Simulations of quantized inversion layer electron transport in 6H–Silicon carbide metal oxide semiconductor structures
Authors: Joshi R. P.
Published: July 2002

Monte Carlo analysis of channel mobility in 6H–SiC metal oxide semiconductors is presented with inclusion of electron quantization. Our simulation results fit available experimental data very well. Interface roughness scattering is shown to play a strong role in determining channel mobility. A roughness parameter of 7.5 Å is extracted from reported data.
Generation of tunable coherent extreme ultraviolet radiation beyond 19 eV by resonant four-wave mixing in argon
Authors: Palm H. , Merkt F.
Published: July 2002

The upper energy limit of broadly tunable coherent radiation sources based on resonant four-wave mixing is investigated. An argon resonance [the two-photon (3p)54p[5/2](J=2)←(3p)6 resonance at 10 5617 cm−1] has been used in resonant four-wave mixing to generate tunable coherent narrow-bandwidth extreme ultraviolet radiation. With this resonance, spectroscopically useful laser intensities (up to 5×106 photons per laser pulse) can be produced beyond 19 eV with a bandwidth as low as 0.1 cm−1. The light source is used to record a pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of the b 4Σg−(v+=1)←X 3Σg−(v=0) photoelectronic transition of O2.
Ultrahigh Si+ implant activation efficiency in GaN using a high-temperature rapid thermal process system
Authors: Cao X. A. , Abernathy C. R. , Singh R. K. , Pearton S. J. , Fu M. , Sarvepalli V. , Sekhar J. A. , Zolper J. C. , Rieger D. J. , Han J. , Drummond T. J. , Shul R. J. , Wilson R. G.
Published: July 2002

Si + implant activation efficiencies above 90%, even at doses of 5×1015 cm−2, have been achieved in GaN by rapid thermal processing at 1400–1500 °C for 10 s. The annealing system utilizes molybdenum intermetallic heating elements capable of operation up to 1900 °C, producing high heating and cooling rates (up to 100 °C s−1). Unencapsulated GaN shows severe surface pitting at 1300 °C and complete loss of the film by evaporation at 1400 °C. Dissociation of nitrogen from the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for AlN and 3.4 eV for InN). Encapsulation with either rf magnetron reactively sputtered or metal organic molecular beam epitaxy-grown AlN thin films provides protection against GaN surface degradation up to 1400 °C, where peak electron concentrations of ∼5×1020 cm−3 can be achieved in Si-implanted GaN. Secondary ion mass spectrometry profiling showed little measurable redistribution of Si, suggesting DSi⩽10−13 cm2 s−1 at 1400 °C. The implant activation efficiency decreases at higher temperatures, which may result from SiGa to SiN site switching and resultant self-compensation.
Absolute quantum photoyield of diamond thin films: Dependence on surface preparation and stability under ambient conditions
Authors: Laikhtman A. , Hoffman A. , Kalish R. , Avigal Y. , Breskin A. , Chechik R. , Shefer E. , Lifshitz Y.
Published: July 2002

Absolute quantum photoyield (QPY) measurements (140–210 nm) of chemical vapor deposited (CVD) diamond films are reported. The dependence of the QPY on hydrogenation by exposure to a hydrogen microwave (MW) plasma and oxidation by a mixture of acids or on exposure to air under ambient conditions have been studied. Films deposited by MWCVD display a higher QPY than those grown by hot filament (HF) CVD. The QPY values are found to depend on the state of the surface. Hydrogen-terminated films exhibit values above 12% at 140 nm, whereas even small amounts of oxygen strongly degrade the QPY. B-doping, at the level of 1500 ppm, has no apparent effect on the photoemission properties. Exposure of the hydrogenated films to ambient conditions results in oxygen adsorption, leading to degradation of the photoemission properties. Analysis of the data within the three-step model of photoemission clearly shows that the state of the surface is a dominant factor determining the QPY.
Sb mediated C60 thin film growth on mica
Authors: Xu Wentao , Hou J. G. , Wu Z. Q.
Published: July 2002

We studied the growth of C60 thin films on a layer of surfactant Sb which was predeposited on mica substrates with vacuum vapor deposition method. It was found that the growth behaviors of C60 films are significantly affected by the surfactant Sb and the substrate temperature. (112)- and (111)-oriented C60 single-crystal films with large C60 single crystals were obtained when the substrate temperatures were 200 and 215 °C, respectively. The growth mechanisms of the Sb mediated C60 films were attributed to the reduced surface diffusion rate of adsorbed C60 molecules and the lowered boundary potential energy of C60 islands by surfactant Sb.
Microstructure refinement and improvements of magnetic properties of two-phase exchange-coupled Sm2Fe15Ga2Cx/α-Fe nanocomposites by additional Zr
Authors: Cheng Zhao-hua , Kronmüller H. , Shen Bao-gen
Published: July 2002

The effect of additive Zr on the microstructure and magnetic properties of nanocomposite Sm2Fe15Ga2Cx/α-Fe permanent magnet has been investigated. A few percent of Zr addition was found to improve significantly its magnetic properties. After an appropriate heat treatment, the remanence, intrinsic coercivity, and maximum energy product increase from 6.4 kG, 5.5 kOe, and 5.6 MGOe for the Zr-free sample to 7.2 kG, 7.7 kOe, and 9.0 MGOe for the Zr-containing one, respectively. The significant improvements of magnetic properties originate from the microstructure refinement by introducing Zr. The “exchange-spring” behavior and the exchange-coupling between the magnetically hard phase Sm2Fe15Ga2Cx and the soft one α-Fe are discussed as well.
Ferromagnetic resonance force microscopy on microscopic cobalt single layer films
Authors: Zhang Z. , Hammel P. C. , Midzor M. , Roukes M. L. , Childress J. R.
Published: July 2002

We report mechanical detection of ferromagnetic resonance (FMR) signals from microscopic Co single layer thin films using a magnetic resonance force microscope (MRFM). Variations in the magnetic anisotropy field and the inhomogeneity of were clearly observed in the FMR spectra of microscopic Co thin films 500 and 1000 Å thick and ∼40×200 μm2 in lateral extent. This demonstrates the important potential that MRFM detection of FMR holds for microscopic characterization of spatial distribution of magnetic properties in magnetic layered materials and devices.
High-efficiency, low-drive-voltage, semitransparent stacked organic light-emitting device
Authors: Gu G. , Khalfin V. , Forrest S. R.
Published: July 2002

We report a semitransparent, two-color, stacked organic light-emitting device (SOLED) with high efficiency, low drive voltage, and minimal color distortion. The SOLED emits light from both device surfaces. The external quantum efficiencies of the green and red stacked elements are 1% and 0.4%, respectively, where only the photons emitted from the substrate surface are collected. The drive voltage for the top stack element is decreased to ∼12 V by using a modified indium tin oxide thin film deposition process. Color distortion and angular dependence of the emission spectra are minimal.
Growth and polarization features of highly (100) oriented Pb(Zr0.53Ti0.47)O3 films on Si with ultrathin SiO2 buffer layer
Authors: Lin Y. , Zhao B. R. , Peng H. B. , Xu B. , Chen H. , Wu F. , Tao H. J. , Zhao Z. X. , Chen J. S.
Published: July 2002

Highly (100) oriented Pb(Zr0.53Ti0.47)O3 (PZT) films were prepared on Si substrates with ultrathin SiO2 buffer layer by pulsed laser deposition. Hysteresis measurements show that saturation polarization, remnant polarization and coercive field of the films reach 26 μC/cm2, 10 μC/cm2 and 70 kV/cm, respectively. The thickness of SiO2 buffer layer is found to play a significant role on phase purity and orientation of PZT as well as the prevention of interdiffusion. It is also found that the grain size and the interdiffusion between PZT and Si are the key factors for the ferroelectric properties of the films, which are discussed together with the synthesis condition in detail.
Ultrafast nonlinear subwavelength solid immersion spectroscopy at T=8 K
Authors: Vollmer Martin , Giessen Harald , Stolz Wolfgang , Rühle Wolfgang W. , Ghislain Luke , Elings Virgil
Published: July 2002

Pump–probe measurements with a subwavelength spatial resolution of 355 nm and a temporal resolution of 130 fs have been performed in a multiple quantum well sample at T=8 K. A solid immersion lens was used to increase the spatial resolution to 0.41⋅λ and the effective numerical aperture to 1.23, demonstrating that the limit of conventional microscopy has been surpassed. The lateral carrier transport was investigated on subpicosecond timescales.
Asymmetric electro-optical response in a liquid crystal cell containing a layer of amorphous tungsten trioxide
Authors: Strangi G. , Lucchetta D. E. , Cazzanelli E. , Scaramuzza N. , Versacé C. , Bartolino R.
Published: July 2002

In this work, we present the experimental evidence of a polarity sensitive electro-optic response in a nematic liquid crystal. The liquid crystal cell was made by using a standard sandwich configuration, with one of the indium tin oxide electrodes covered by a thin layer of tungsten trioxide (WO3), deposited by sputtering. The optical response was inhibited when the electrode covered by WO3 film was anodically charged, while the usual optical response occurred under a reverse field. An ionic diffusion process was associated with the establishment of an internal electric field, which inhibited unipolarly the optical switching.
Optical properties of β-Si3N4 single crystals grown from a Si melt in N2
Authors: Munakata F. , Matsuo K. , Furuya K. , Akimune Y. , Ye J. , Ishikawa I.
Published: July 2002

Large colored β-Si3N4 single crystals were successfully grown from a Si melt in N2. The transmission optical absorption of coloring β-Si3N4 single crystal shows that impurities introduce a midgap level of ∼2.4 eV into the wide band gap of ∼5.3 eV in nondoped Si3N4. The infrared transmission spectrum and electron probe x-ray microanalysis of β-Si3N4 samples show that the solution of the Al element affects the silicon–nitrogen molecular vibration and the states within the band gap of β-Si3N4. The obtained results mean that the Al impurity acts as the radiative center and is the origin of the color in the β-Si3N4 single crystal.
Molecular beam epitaxial growth and optical properties of lattice-matched MgxZn1−xSeyTe1−y alloys on InP substrates
Authors: Naniwae Koichi , Iwata Hiroshi , Yashiki Kenichiro
Published: July 2002

Molecular beam epitaxial growth and the optical properties of lattice-matched MgxZn1−xSeyTe1−y alloys on InP were investigated. Two-dimensional growth and high crystalline quality were achieved by forming an InP buffer layer and a Zn0.48Cd0.52Se buffer layer before the MgxZn1−xSeyTe1−y layer growth. The band-gap energy (Eg) of the MgxZn1−xSeyTe1−y films, measured by optical-reflection measurements, increased with the Mg content and is expressed by the quadratic equation Eg=0.90x2+0.83x+2.37. We found that the Stokes shifts of MgxZn1−xSeyTe1−y quaternary alloys, which seemed to originate from deep trap levels related to Te clusters, were larger than those of ZnSeTe ternary alloys. The refractive index of MgxZn1−xSeyTe1−y at a wavelength of 600 nm decreased as the Mg content rose, as expressed by the linear equation n600(x)=−1.09x+2.99.
Acoustic waveguide properties of a thin film of nanoporous silica on silicon
Authors: Rogers John A. , Case Carlye
Published: July 2002

This letter reports picosecond laser-based measurements of acoustic waveguiding in a thin film of nanoporous silica (similar to silica aerogel or xerogel) on silicon for wavelengths between 2.5 and 25 μm. The large mismatch between the acoustic properties of the film and substrate in this system creates pronounced dispersion in the velocities and leads to unusual acoustic behavior: over a relatively large range of wavelengths, the group velocities of the lowest order Rayleigh mode and certain other modes are less than 50 m/s—nearly ten times slower than the intrinsic velocities of the nanoporous silica and more than one hundred times slower than those of silicon. An isotropic model of the waveguide reproduces these and other features. Nonlinear least-squares fitting of the data to this model determines the intrinsic acoustic velocities of the nanoporous glass.
Dose dependence of microstructural evolution in oxygen-ion-implanted silicon carbide
Authors: Ishimaru Manabu , Sickafus Kurt E.
Published: July 2002

Transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy and ion channeling (RBS/C) have been used to characterize the microstructures and damage distributions in oxygen-ion-implanted silicon carbide (SiC). 6H–SiC single crystals with [0001] orientation were irradiated with 180 keV O+ ions at 650 °C to fluences ranging from 0.2×1018 to 1.4×1018/cm2. No continuous buried amorphous layer was formed in the specimen with 0.2×1018 O+/cm2, although there were striated regions consisting of amorphous and crystalline structures at depth between 200 and 300 nm. A continuous buried amorphous layer appeared above 0.4×1018 O+/cm2, and the amorphous regions grew in size with increasing fluence. TEM and RBS/C results indicated that microstructures and elemental distributions change drastically between 0.7×1018 and 1.4×1018 O+/cm2.
Effect of magnetocrystalline anisotropy on the magnetic properties of Fe-rich R–Fe–B nanocomposite magnets
Authors: Zhao T. , Xiao Q. F. , Zhang Z. D. , Dahlgren M. , Grössinger R. , Buschow K. H. J. , de Boer F. R.
Published: July 2002

Nanocomposite ribbons consisting of hard-magnetic R2Fe14B (R=rare-earth) and soft-magnetic Fe3B phases were produced by melt spinning and subsequent heat treatment. Alloys of composition Nd4−xSmxFe77.5B18.5, with x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5 all showing strong remanence enhancement, were selected to investigate the effect of the magnetocrystalline anisotropy of the R2Fe14B-type phase on the exchange coupling with the soft-magnetic phase in the nanocomposite magnets. An enhancement of exchange coupling due to anisotropy reduction was found, as expected on the basis of the model of Kneller and Hawig. A maximum of the energy product (BH)max was achieved for x=0.2 after annealing at 700 °C. The thermomagnetic behavior also changes upon substitution of Sm for Nd.
Effect of growth temperature on photoluminescence of GaNAs/GaAs quantum well structures
Authors: Buyanova I. A. , Chen W. M. , Monemar B. , Xin H. P. , Tu C. W.
Published: July 2002

The effect of growth temperature on the optical properties of GaAs/GaNxAs1−x quantum wells is studied in detail using photoluminescence (PL) spectroscopies. An increase in growth temperature up to 580 °C is shown to improve the optical quality of the structures, while still allowing one to achieve high (>3%) N incorporation. This conclusion is based on: (i) an observed increase in intensity of the GaNAs-related near-band-edge emission; (ii) a reduction in band-edge potential fluctuations, deduced from the analysis of the PL line shape; and (iii) a decrease in concentration of some extended defects detected under resonant excitation of the GaNAs. The thermal quenching of the GaNAs-related PL emission, however, is almost independent of the growth temperature and is attributed to a thermal activation of an efficient nonradiative recombination channel located in the GaNAs layers.
Room-temperature single-electron memory made by pulse-mode atomic force microscopy nano oxidation process on atomically flat α-alumina substrate
Authors: Matsumoto Kazuhiko , Gotoh Yoshitaka , Maeda Tatsuro , Dagata John A. , Harris James S.
Published: July 2002

A single-electron memory was fabricated using the improved pulse-mode atomic force microscopy nano oxidation process which oxidized the surface of the thin titanium (Ti) metal on the atomically flat α-alumina (α-Al2O3) substrate and formed the narrow oxidized titanium (TiOx) line that works as a tunnel junction for the device. This single-electron memory consists of the multitunnel junction and a memory capacitance. The single-electron transistor, which works as an electrometer, was connected to the memory node of the single-electron memory to detect the potential change of the memory node by the injection of the individual electrons. The fabricated single-electron memory showed the hysteresis loop even at room temperature by the return trip of the memory bias when starting from 0 to 10 V and again coming back to 0 V. About 25 electrons were stored at the memory node.
Structure and stability of ultrathin zirconium oxide layers on Si(001)
Authors: Copel M. , Gribelyuk M. , Gusev E.
Published: July 2002

We have examined the structure of ultrathin ZrO2 layers on Si(001) using medium energy ion scattering and cross-sectional transmission electron microscopy. Films can be deposited on SiO2 layers with highly abrupt interfaces by atomic layer deposition. On HF stripped Si(001), nucleation was inhibited, resulting in poorer film morphology. ZrO2 showed remarkable stability against silicate formation, with no intermixing even after high temperature oxidation. The oxide is vulnerable to high temperature vacuum annealing, with silicidation occurring at temperatures above 900 °C.
Anisotropic conduction behavior in metal-induced laterally crystallized polycrystalline silicon thin films
Authors: Wang Mingxiang , Meng Zhiguo , Wong Man
Published: July 2002

The conduction behavior of metal-induced laterally crystallized (MILC) polycrystalline silicon (poly-Si) was studied and compared to that of the conventional solid-phase crystallized and low-pressure chemical vapor deposited poly-Si. MILC poly-Si was found to exhibit superior electrical properties, with significantly lower grain-boundary (GB) trap density, as well as much higher carrier mobility and conductivity. Furthermore, a unique anisotropic conduction behavior was discovered in MILC poly-Si, with the resistivity and its activation energy showing remarkable difference for conduction transverse or parallel to the MILC direction. These phenomena have been related to the fluctuation of the potential barrier associated with the longitudinal GBs separating the relatively ordered elongated grains in MILC poly-Si.
Ultrafast saturable absorption at 1.55 μm in heavy-ion-irradiated quantum-well vertical cavity
Authors: Mangeney J. , Oudar J. L. , Harmand J. C. , Mériadec C. , Patriarche G. , Aubin G. , Stelmakh N. , Lourtioz J. M.
Published: July 2002

Measurements of absorption saturation in heavy-ion-irradiated InGaAs/InAlAs multiplequantum-well reflection-mode vertical-cavity devices have been performed with short pulses at 1.55 μm and repetition rates up to 10 GHz. The relaxation time was essentially independent of the pulse repetition rate and optical excitation fluence, with a lower value of 2.4 ps for an ion dose of 1012 cm−2. Efficient optical switching was obtained, with a saturation energy smaller than 12 pJ, a contrast ratio up to 3.5:1, and a switching amplitude up to 20% of the incident signal. A relaxation model accounting for capture and recombination on defect levels indicates an upper limit of 2 ps of the defect level recombination time.
Imaging and local current transport measurements of AlInP quantum dots grown on GaP
Authors: Reddy C. V. , Narayanamurti V. , Ryou J. H. , Chowdhury U. , Dupuis R. D.
Published: July 2002

Individual AlInP self-assembled quantum dots grown on a (100) GaP substrate are imaged and probed using ballistic electron emission microscopy (BEEM). The excellent nanometer scale lateral resolution of BEEM is utilized to inject carriers directly into a single quantum dot, and thus, current transport through the dot investigated without any direct electrical contact. The BEEM spectra taken on and off the dot revealed a local conduction-band offset between GaP and AlInP with a barrier height of ΔEc∼0.13±0.01 eV.
Bonding topologies in diamondlike amorphous-carbon films
Authors: Siegal M. P. , Provencio P. N. , Tallant D. R. , Simpson R. L. , Kleinsorge B. , Milne W. I.
Published: July 2002

The carbon ion energy used during filtered cathodic vacuum arc deposition determines the bonding topologies of amorphous-carbon (a-C) films. Regions of relatively low density occur near the substrate/film and film/surface interfaces; their thicknesses increase with deposition energy. The ion subplantation growth results in mass density gradients in the bulk portion of a-C in the growth direction; density decreases with distance from the substrate for films grown using ion energies <60 eV and increases for films grown using ion energies >160 eV. Films grown between these energies are the most diamondlike with relatively uniform bulk density and the highest optical transparencies. Bonding topologies evolve with increasing growth energy consistent with the propagation of subplanted carbon ions inducing a partial transformation of σ- to π-bonded carbon atoms.
Room-temperature triangular-lattice two-dimensional photonic band gap lasers operating at 1.54 μm
Authors: Hwang Jeong-Ki , Ryu Han-Youl , Song Dae-Sung , Han Il-Young , Song Hyun-Woo , Park Hong-Kyu , Lee Yong-Hee , Jang Dong-Hoon
Published: July 2002

Thermally and mechanically dependable two-dimensional (2D) photonic band gap lasers are proposed and realized at room temperature. The thin slab photonic band gap laser structure is sandwiched between air and a drilled aluminum oxide layer provided by wafer fusion techniques. In this thin slab structure, the optical confinement of photons is achieved by 2D triangular photonic lattice in horizontal plane and total internal reflection in vertical direction. Pulsed lasing action is observed at 1.54 μm by optical pumping with duty cycle up to 10%. Polarization characteristics of two-dimensional photonic band gap defect modes are also studied below and above the lasing threshold.
Electron tunneling experiments on skutterudite Co1−xFexSb3 semiconductors
Authors: Nagao Jiro , Ferhat Marhoun , Anno Hiroaki , Matsubara Kakuei , Hatta Eiji , Mukasa Kōichi
Published: July 2002

Electron tunneling experiments were performed on p-Co1−xFexSb3–Al-oxide–Al junctions for x=0 and x=0.1 at 4.2 K. A U-shaped tunneling conductance curve obtained for polycrystalline p-CoSb3 clearly shows an energy-band gap of ∼50 meV. For p-Co0.9Fe0.1Sb3 skutterudites, a strong zero-bias conductance anomaly is observed. This anomaly may be due to a structural disorder arising from defects such as vacancies and interstitial Fe atoms.
Influence of ion energy and arrival rate on x-ray crystallographic properties of thin ZrOx films prepared on Si(111) substrate by ion-beam assisted deposition
Authors: Matsuoka M. , Isotani S. , Chubaci J. F. D. , Miyake S. , Setsuhara Y. , Ogata K. , Kuratani N.
Published: July 2002

Thin zirconium oxide films, formed on Si(111) substrate by ion-beam assisted deposition, have been investigated by x-ray diffractometry with respect to the microstructure of the films, such as preferred orientation, interplanar spacing, crystallite size. The results of the interplanar spacing and diffraction intensity analysis could be interpreted in terms of relative amount of Zr4+ ions estimated by analyses of Zr 3d x-ray photoelectron spectroscopy spectra for the films.
Avalanche multiplication in submicron AlxGa1−xAs/GaAs multilayer structures
Authors: Chia C. K. , David J. P. R. , Plimmer S. A. , Rees G. J. , Grey R. , Robson P. N.
Published: July 2002

A systematic study of the role of band edge discontinuities on ionization rates in periodic AlxGa1−xAs/GaAs structures has been performed by measuring the electron and hole multiplication characteristics in a series of submicron AlxGa1−xAs/GaAs multilayer p–i–n and n–i–p structures. These structures comprise alternating 500Å AlxGa1−xAs and GaAs layers in the intrinsic multiplication regions, with a total thickness of up to 0.5 μm. The results show little dependence on initiating carrier type for multiplication region widths above 0.3 μm, nor on whether they originate in GaAs or AlxGa1−xAs. Only alloy-like behavior is observed at all values of multiplication up to the breakdown voltage in contrast to earlier work on single heterojunction structures where a large difference was seen at low values of multiplication between carriers starting in GaAs and AlxGa1−xAs. The microscopic aspects of hot carrier transport in these devices were studied numerically using a simple Monte Carlo model. Simulations suggest that the energy gained from the conduction band edge discontinuity from AlxGa1−xAs to GaAs is offset by the increased energy loss via the higher phonon scattering rate in the preceding AlxGa1−xAs layer. We conclude that AlxGa1−xAs/GaAs multilayer structures offer no electron ionization enhancement.
Atomic force microscope based patterning of carbonaceous masks for selective area growth on semiconductor surfaces
Authors: Avramescu Adrian , Ueta Akio , Uesugi Katsuhiro , Suemune Ikuo
Published: July 2002

Carbonaceous masks for selective growth on GaAs substrates were fabricated with high resolution by anodization with an atomic force microscope (AFM). Mask deposition is made by a 15-kV accelerated electron-beam irradiation in a scanning electron microscope. The local anodization of the carbonaceous film under intense electric field is investigated and the main factors for improving resolution and reproducibility are discussed. The “edge effect” of the anodized region, revealed in the electric-field distribution at the tip–water–film interfaces is identified as the main factor responsible for the resolution degradation during patterning. Short forward bias pulse for anodizing the carbonaceous film and the subsequent reverse bias pulse for neutralizing the space charge, locally accumulated during the forward bias, are shown to be effective for the higher pattern resolution and also for deepening the patterning depth. Based on the analysis, a modulated-amplitude pulsed bias mode is proposed and is demonstrated to bring a significant improvement in the resolution and the aspect ratio of patterns made by the anodization. Carbonaceous masks ready for selective area growth of semiconductors alloys were fabricated with the pattern resolution of ∼26 nm, limited by the curvature of AFM cantilever tips.
Structure, phase stability, and thermodynamics in charged colloidal solutions
Authors: Linse Per
Published: July 2002

Model systems of charged spherical macroions and point counterions interacting solely through hard-sphere and Coulomb interaction were investigated by means of extensive Monte Carlo simulations. The macroion-charge to counterion-charge ratio was varied from 10 to 80, the macroion volume fraction from 0.001 25 to 0.08, and the reduced parameter that remained, an electrostatic coupling parameter, over a range of two orders in magnitude. The part of the parameter space investigated includes nearly all experimental colloidal solutions (in the salt-free limit) in which the colloids are carrying up 80 (monovalent counterions) or 160 (divalent counterions) elementary charges. The effects on the structure of varying the counterion charge, the macroion charge, the macroion size, the temperature, the dielectric permittivity, and the macroion volume fraction are presented. At a low charge ratio, a low volume fraction, and/or a low electrostatic coupling, the counterion distribution is only weakly perturbed by the macroions, the system being nearly ideal. At a higher electrostatic coupling, the counterions accumulate near the macroions and screen the electrical field of the macroions. At a very high electrostatic coupling, the system displays a thermodynamic instability and separates into two phases of widely differing electrolyte concentration. Parts of the binodal surface were determined and critical points were estimated. The concept of effective macroion charge is discussed.
Series operation of direct current xenon chloride excimer sources
Authors: El-Habachi Ahmed , Shi Wenhui , Moselhy Mohamed , Stark Robert H. , Schoenbach Karl H.
Published: July 2002

Stable, direct current microhollow cathode discharges in mixtures of hydrochloric acid, hydrogen, xenon, and neon have been generated in a pressure range of 200–1150 Torr. The cathode hole diameter was 250 μm. Sustaining voltages range from 180 to 250 V at current levels of up to 5 mA. The discharges are strong sources of xenon chloride excimer emission at a wavelength of 308 nm. Internal efficiencies of approximately 3% have been reached at a pressure of 1050 Torr. The spectral radiant power at this pressure was measured as 5 mW/nm at 308 nm for a 3 mA discharge. By using a sandwich electrode configuration, consisting of five perforated, alternate layers of metal and dielectric, a tandem discharge—two discharges in series—could be generated. For an anode–cathode–anode configuration the excimer irradiance, recorded on the axis of the discharge, was twice as large as that of a single discharge. The extension of this basic tandem electrode structure to a multiple electrode configuration allows the generation of high irradiance excimer sources. Placing such a structure with a string of microhollow cathode discharge into an optical resonator promises to lead to a direct current microexcimer laser.
Photofragmentation dynamics of the thiomethoxy radical
Authors: Pushkarsky Michael B. , Applegate Brian E. , Miller Terry A.
Published: July 2002

The radiative and nonradiative decay of the Ã 2E electronic state of the CH3S radical has been investigated using a variety of experimental techniques. Lifetimes have been measured for a number of vibrational levels; these data have been analyzed along with similar results previously obtained for other methoxy radical family members, CH3O, CD3O, and CF3S. It is concluded that the totality of the data is best described by a model which postulates mode-selective fragmentation into a methyl (or fluoromethyl) radical and an O or S atom. For CH3S there appears a second nonradiative decay channel, possibly producing H+CH2S.
Effect of electron irradiation on poly(vinylidene fluoride-trifluoroethylene) copolymers
Authors: Tang Yiwen , Zhao Xing-Zhong , Chan Helen L. W. , Choy C. L.
Published: July 2002

Electrical field-induced strain response of electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer has been studied by a Mach–Zehnder type heterodyne interferometer in the frequency range of 3–9 kHz. The electrostrictive constant is calculated from the strain results, which is of the same order of magnitude as those obtained at 1 Hz by a bimorph-based strain sensor but at much lower electrical field. Changes in the dielectric property, phase transition behavior and crystal structure of the same copolymer have also been studied. The reversible solid–state transition between the polar and nonpolar phase in the crystalline regions of the copolymer driven by the external electric field is suggested to be responsible for the significant high electrostrictive strain of the electron-irradiated copolymer.
Dynamic plasma screening effects on electron–ion collisional ionizations in a generalized Lorentzian distribution plasma
Authors: Kim Chang-Geun , Jung Young-Dae
Published: July 2002

Dynamic plasma screening effects on semiclassical ionization probabilities for electron-impact ionization of hydrogenic ions are investigated in a generalized Lorenzian (kappa) distribution plasma. The projectile–target interaction Hamiltonian is obtained by introduction of the longitudinal plasma dielectric function. A semiclassical straight-line trajectory method is applied to the path of the projectile electron and the bound electron is treated quantum mechanically. The semiclassical ionization probability is obtained as a function of the impact parameter, projectile energy, spectral index, and Debye length. It is found that the non-Maxwellian effect is quite small for low plasma temperatures. However, the non-Maxwellian effect is significant for high plasma temperatures. It is also found that the dynamic plasma screening effect is increased with decreasing the spectral index.
Kinetic theory of alpha particles production in a dense and strongly magnetized plasma
Authors: Cereceda Carlo , Deutsch Claude , De Peretti Michel , Sabatier Michel , Basko Mikhail M. , Kemp Andreas , Meyer-ter-Vehn Jurgend
Published: July 2002

In connection with fundamental issues relevant to magnetized target fusion, the distribution function of thermonuclear alpha particles produced in situ in a dense, hot, and strongly magnetized hydrogenic plasma considered fully ionized in a cylindrical geometry is investigated. The latter is assumed in local thermodynamic equilibrium with Maxwellian charged particles. The approach is based on the Fokker–Planck equation with isotropic source S and loss s terms, which may be taken arbitrarily under the proviso that they remain compatible with a steady state. A novel and general expression is then proposed for the isotropic and stationary distribution f(v). Its time-dependent extension is worked out numerically. The solutions are valid for any particle velocity v and plasma temperature T. Higher order magnetic and collisional corrections are also obtained for electron gyroradius larger than Debye length. f(v) moments provide particle diffusion coefficient and heat thermal conductivity. Their scaling on collision time departs from Braginski’s.
Experimental studies of the NaK 1 3Δ state
Authors: Huennekens J. , Prodan I. , Marks A. , Sibbach L. , Galle E. , Morgus T. , Li Li
Published: July 2002

The NaK 1 3Δ state has been studied by the perturbation-facilitated optical–optical double resonance technique. Mixed singlet–triplet levels, A(2)1Σ+(vA,J)∼b(1)3Π(vb,J), were pumped from thermally populated rovibrational levels of the ground state, X(1)1Σ+(vX,J±1), using a single-mode cw dye laser. A single-mode cw Ti:Sapphire laser was then used to further excite the NaK molecules to various 1 3Δ(vΔ,NΔ,JΔ) rovibrational levels which were detected by observing collision-induced Λ3→a(1)3Σ+ fluorescence in the green part of the spectrum. The measured energies of the 1 3Δ(vΔ,NΔ) levels were fit to a Dunham expansion, and the Dunham coefficients were used to construct the RKR potential curve. Absolute numbering of the 1 3Δ state vibrational levels was established by a comparison of experimental and calculated 1 3Δ(vΔ,NΔ,JΔ)←b(1)3Π(vb,Jb) absorption line strengths. A deperturbation program was used to determine the vibration-dependent 1 3Δ state spin–orbit interaction parameter. Hyperfine structure of the 1 3Δ state was studied, and the Fermi-contact interaction term for this state was determined to be ∼0.0111 cm−1.
Nonresonant radiative Pierce instability and its saturation—Chaos
Authors: Klochkov D. N. , Pekar M. Yu. , Rukhadze A. A.
Published: July 2002

The review of theoretical investigations of a nonresonance stimulated Pierce radiation by the relativistic electron beam (REB) in a waveguide is presented. REB is supposed to be stabilized by an infinitely strong longitudinal magnetic field. Using the analytical methods of linear theory, the conditions leading to the rise of radiative Pierce instabilities, their growth rates and the radiation spectrum are determined. By numerical modeling of the problem, the efficiency of beam energy transformation into electromagnetic field energy as a function of the beam current, its relativism, and geometry are calculated. The physical nature of the instability is clarified and its saturation mechanisms are discussed. In particular, in the saturated state the chaotic motion of beam particles and chaos in the radiation spectrum are observed.
Third-order nonlinear optical properties of an expanded porphyrin cadmium complex
Authors: Sun Wenfang , Byeon Clare C. , Lawson Chris M. , Gray Gary M. , Wang Duoyuan
Published: July 2002

The third-order nonlinear optical properties of a pentaazadentate porphyrin-like cadmium complex, [(DBP–APPC)Cd]Cl, have been investigated using 40-ps and 5-ns laser pulses at 532 nm. The molecular second-order hyperpolarizability of this complex is 9.7×10−31 esu, and the nonlinearity is predominantly electronic in origin with picosecond laser pulses. Temporal response of the nonlinear process is composed of at least two components with different time responses and shows a strong fluence dependence that indicates the participation of excited states in the nonlinear process. Nonlinear absorption measurements demonstrate that this complex shows strong reverse saturable absorption for nanosecond laser pulses, but exhibits a transformation from reverse saturable absorption to saturable absorption at high fluence for picosecond pulses. These results suggest that [(DBP–APPC)Cd]Cl is a very promising candidate for optical limiting of nanosecond laser pulses.
Magnetized implosions driven by intense ion beams
Authors: Basko M. M.
Published: July 2002

Intense beams of heavy ions, envisaged for the near future at the Institute for Theoretical and Experimental Physics (Moscow) and Gesellschaft für Schwerionenforschung (Darmstadt), will be well suited for conducting implosion experiments in cylindrical geometry. In such implosions, the initial pressure generated by the direct beam heating can be enhanced by more than a factor of 10. If, in addition, an external magnetic field is introduced, the effect of magnetothermal insulation may allow to reach kilovolt temperatures and significant thermonuclear neutron yields in magnetized implosions driven by the beam heating intensities as low as ε̇≃1 TW/g. It is shown how the combined effect of the electrical resistivity and thermal conductivity sets a lower limit on the product UR (R is the radius, and U is the velocity of an imploding plasma volume) as a necessary condition for the regime of self-sustained magnetized implosion (SSMI). The optimal plasma parameters required for initiation of this regime are evaluated. In cylindrical geometry, the threshold for the SSMI regime is determined by the total driver energy deposition per unit areal density of the cylinder, ρ−1|dEb/dz| (kJ cm2/g). The results of one-dimensional magnetohydrodynamic simulations indicate that the advantages of magnetized implosions begin to manifest themselves at a beam energy level of Eb≈100 kJ.
The triple sum formulas for 9j coefficients of SU(2) and uq(2)
Authors: Ališauskas Sigitas
Published: July 2002

Seven different triple sum formulas for 9j coefficients of the quantum algebra uq(2) are derived, using for these purposes the usual expansion of q-9j coefficients in terms of q-6j coefficients and recently derived summation formula of twisted q-factorial series (resembling the very well-poised basic hypergeometric φ45 series) as a q-generalization of Dougall’s summation formula of the very well-poised hypergeometric F34(−1) series. This way for q=1 Rosengren’s second proof of the SU(1,1) case is adapted for the SU(2) case to derive the known triple sum formula of Ališauskas and Jucys, as well as six new independent triple sum formulas for the Wigner 9j coefficients of the angular momentum theory. The mutual rearrangement possibilities of the derived triple sum formulas by means of the Chu–Vandermonde summation formulas are considered and applied to derive several versions of double sum formulas for the stretched q-9j coefficients, which give new rearrangement and summation formulas of special Kampé de Fériet functions and their q-generalizations.
Plasma rotation due to fluctuations caused by instabilities
Authors: Taguchi M.
Published: July 2002

An expression for the ion flow velocity due to fluctuating electric and magnetic fields is presented for an axisymmetric toroidal plasma. When the source term due to fluctuations can be approximated as a parallel momentum source, this expression reduces to that obtained by the moment approach in the conventional neoclassical transport theory. As an application, the plasma rotation due to the slab ion-temperature-gradient mode is estimated.
Experimental determination of attractor dimension of E×B turbulence
Authors: Chiu J. S. , Sen A. K.
Published: July 2002

The time series analysis method for chaotic attractor dimension by Grassberger and Procaccia [P. Grassberger and I. Procaccia, Physica D 9, 189 (1983)] was applied to fluctuations due to an E×B rotational flute mode in the Columbia Linear Machine (CLM) [G. A. Navratil, J. Slough, and A. K. Sen, Plasma Phys. 24, 184 (1982)]. After frequency domain filtering, the results show that the fluctuations carry a dimension close to three. A dimension of three supports a possible three-wave mode coupling process revealed in bi-coherence studies. The Grassberger Procaccia method was also performed on cases where partial feedback suppression was applied to the instability. It is shown that feedback does not alter the attractor dimension of the instability, supporting previous findings that the linear feedback only affects the linear drive of the instability, and not its non-linear saturation mechanism.
Mechanism for rapid thermal annealing improvements in undoped GaNxAs1−x/GaAs structures grown by molecular beam epitaxy
Authors: Buyanova I. A. , Pozina G. , Hai P. N. , Thinh N. Q. , Bergman J. P. , Chen W. M. , Xin H. P. , Tu C. W.
Published: July 2002

A systematic investigation of the effect of rapid thermal annealing (RTA) on optical properties of undoped GaNAs/GaAs structures is reported. Two effects are suggested to account for the observed dramatic improvement in the quality of the GaNxAs1−x/GaAs quantum structures after RTA: (i) improved composition uniformity of the GaNxAs1−x alloy, deduced from the photoluminescence (PL), PL excitation and time-resolved measurements; and (ii) significant reduction in the concentration of competing nonradiative defects, revealed by the optically detected magnetic resonance studies.
Evidence of a two-dimensional nucleation and growth mechanism for metastable nanocrystals embedded in Pd40.5Ni40.5P19 glass
Authors: Yao K. F. , Kui H. W.
Published: July 2002

Nanocrystals, with a body-centered-cubic crystal structure of lattice parameter 18.2525 Å , were found in amorphous Pd40.5Ni40.5P19 specimens that had been annealed at 628 K for 30 min. The crystal surfaces are smooth and dislocations are absent, suggesting that the growth of these nanocrystals is through a two-dimensional nucleation and growth mechanism (formation of surface nuclei of critical size and their subsequent growth).
Fowler–Nordheim hole tunneling in p-SiC/SiO2 structures
Authors: Chanana R. K. , McDonald K. , Di Ventra M. , Pantelides S. T. , Feldman L. C. , Chung G. Y. , Tin C. C. , Williams J. R. , Weller R. A.
Published: July 2002

We report the confirmed occurrence of Fowler–Nordheim hole tunneling in p-4H–SiC metal-oxide-semiconductor capacitor structures. The effective mass for holes in the oxide is found to be in the range of 0.35m–0.52m, where m is the free electron mass.