bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral engineering of carrier dynamics in In(Ga)As self-assembled quantum dots
Authors: Boggess Thomas F. , Zhang L. , Deppe D. G. , Huffaker D. L. , Cao C.
Published: July 2002

Time-resolved photoluminescence upconversion with 200 fs resolution is used to investigate the carrier capture, energy relaxation, and radiative recombination in two self-assembled quantum-dot ensembles with distinctly different sizes and energy spectra. When carriers are excited into the wetting layer at low density and low lattice temperature, the relaxation time to the ground state of the larger dots is ∼1 ps, but the corresponding time for the smaller dots with larger energy spacings is ∼7 ps. This, along with the observed temperature dependence, suggests phonon participation in the relaxation process. At low temperatures, the radiative recombination time in the smaller dots is approximately twice that of the larger dots. The reduced oscillator strength in the smaller dots may be due to a reduced electron–hole wave-function overlap in the smaller dots, in addition to a size-dependent super-radiance effect.
Silicon microdischarge devices having inverted pyramidal cathodes: Fabrication and performance of arrays
Authors: Park S.-J. , Chen J. , Liu C. , Eden J. G.
Published: July 2002

Microdischarge devices having inverted, square pyramidal cathodes as small as 50 μm×50 μm at the base and 35 μm in depth, have been fabricated in silicon and operated at gas pressures up to 1200 Torr. For the polyimide dielectric incorporated into these devices (εr=2.9), the discharges produced exhibit high differential resistance (∼2×108 Ω in Ne), ignition voltages for a single device of ∼260–290 V, and currents typically in the μA range. Arrays as large as 10×10 have been fabricated. For an 8 μm thick polyimide dielectric layer, operating voltages as low as 200 V for a 5×5 array have been measured for 700 Torr of Ne. Array lifetimes are presently limited to several hours by the thin (1200–2000 Å) Ni anode.
Tuning the spectral and temporal response in PtAu core–shell nanoparticles
Authors: Hodak José H. , Henglein Arnim , Hartland Gregory V.
Published: July 2002

Ultrafast laser spectroscopy has been used to measure the electron–phonon coupling constant in PtAu core–shell bimetallic nanoparticles. A chemical reduction method was used to prepare Pt cores of 12.5 nm diameter and a γ-radiolytic deposition technique was then used to grow Au shells of variable thickness. The resulting nanoparticles have a spectrum that is characteristic of Au. It is found that the electron–phonon coupling time for these hybrid nanoparticles (τe–ph∼200 fs) is over a factor of 3 shorter than that for plain Au nanoparticles (τe–ph∼650 fs). The faster dynamical response is due to the large electron–phonon coupling constant for Pt, which provides efficient scattering of the excited electrons. Platinum dominates the temporal response, even for a 1:6 molar ratio of Pt to Au, because it has a much larger density of states near the Fermi level compared to Au.
Quantitative metrology study of Cu/SiO2 interconnect structures using fluorescence x-ray microscopy
Authors: Xu Guangyong , Su X. , Stagarescu C. B. , Eastman D. E. , Lai B. , Cai Z. , Noyan I. C. , Hu C.-K.
Published: July 2002

We demonstrate the capability of fluorescence x-ray microscopy with a 0.25 μm beam for in situ measurements of Cu-wiring interconnects of submicron dimensions. We are able to measure submicron line widths, lengths, and thicknesses of both Cu and W structures, and a Ta liner in the test vehicle, to the absolute accuracy of 0.03 μm, and a relative accuracy of ∼4% in lateral dimensions, and ∼10% in heights. The shape of a buried electromigration void was also determined. This nanoscale nondestructive characterization technique promises to be powerful for a variety of materials systems.
Excitonic properties and band alignment in lattice-matched ZnCdSe/ZnMgSe multiple-quantum-well structures
Authors: Bonanni B. , Pelucchi E. , Rubini S. , Orani D. , Franciosi A. , Garulli A. , Parisini A.
Published: July 2002

Lattice-matched Zn0.85Cd0.15Se/Zn0.74Mg0.26Se multiple-quantum-well structures were obtained on GaAs(001) using graded-composition InyGa1−yAs layers to match the II–VI lattice parameter to the III–V substrate. Cross-sectional transmission electron microscopy studies show that the effect of the crosshatch pattern of the InyGa1−yAs surface is limited to long-period coherent undulations of quantum well and barrier layers. Optical measurements of the excitonic properties as a function of well thickness, complemented by self-consistent calculations of the transition energies, indicate good quantum confinement in the well, with a 68% conduction band contribution to the 0.482 eV band gap difference.
Nanofractography of shocked RDX explosive crystals with atomic force microscopy
Authors: Sharma J. , Armstrong R. W. , Elban W. L. , Coffey C. S. , Sandusky H. W.
Published: July 2002

Examination with atomic force microscopy has revealed apparent shear-type cleavage steps with heights as small as 0.05 nm, smaller than the size of cyclotrimethylenetrintramine (RDX) molecules, on the fracture surfaces of crystals that were subjected to aquarium shocks of 61.6 or 129 kbar, both greater than the pressure (38 kbar) required for the alpha-to-gamma phase transformation. The shocked centimeter size, originally transparent crystals became opaque and white from prolific fractures and internal cracks that are associated with their breakup into nanocrystallites of sizes extending from 500 down to 20 nm. The submolecular steps are related geometrically to the macroscale (K∥) fracture mechanics mode of shear fracturing that has obvious consequences at the nanoscale level for nonregistry between molecules across the crack surfaces. The results are of interest in relation to lattice trapping of crack fronts and as a contribution to the possibility of deformation-induced chemical decomposition/detonations.
Plasma sources for high-current electron beam generation
Authors: Krasik Ya. E. , Dunaevsky A. , Felsteiner J.
Published: July 2002

A review of experimental studies of the operation of cathodes made of metal-ceramic, velvet, corduroy, carbon fibers, carbon fabric, and different types of ferroelectrics is presented. These cathodes operated at electric fields in the range of 5–60 kV/cm that allowed the generation of electron beams with duration of several hundreds of nanoseconds while keeping a quasi-constant diode impedance. All cathodes had the same diameter and were tested in a diode powered by a high-voltage generator (300 kV, 85 Ω, 250 ns, ⩽5 Hz). It was shown that the source of electrons for all the studied cathodes is a plasma which is formed as a result of surface discharges. Different types of electrical and optical diagnostics were used to study the formation and parameters of the plasma, the potential distribution inside the anode–cathode gap, and the uniformity and divergence of the extracted electron beam as a function of the amplitude and rise time of the accelerating pulse. Results of the lifetime of the tested cathodes and their compatibility with vacuum requirements are presented as well.
Atomic arrangements and formation mechanisms of the CuPt-type ordered structure in CdxZn1−xTe epilayers grown on GaAs substrates
Authors: Kim T. W. , Lee D. U. , Choo D. C. , Lee H. S. , Lee J. Y. , Park H. L.
Published: July 2002

Selected area electron diffraction pattern (SADP) and transmission electron microscopy (TEM) measurements were carried out to investigate the ordered structure in CdxZn1−xTe epitaxial layers grown on (001) GaAs substrates. The SADP showed to sets of superstructure reflections with symmetrical intensities, and the corresponding high-resolution TEM image showed a doublet periodicity in the contrast of the {111} lattice planes. The results of the SADP and the TEM measurements showed the existence of a CuPt-type ordered structured in the CdxZn1−xTe epitaxial layers. This CuPt-type ordered structure had two different variants with an antiphase boundary existing between the two variants. The formation of a CuPt-type ordered structure in a CdxZn1−xTe epitaxial layer might originate from the minimization of the strain relaxation energy in the reconstructed GaAs (001) surface. A possible atomic arrangement of and a formation mechanism for the CuPt-type ordered structure in the CdxZn1−xTe epitaxial layer are presented based on the TEM results. These results provide important information on the microstructural properties for improving the efficiencies of optoelectronic devices operating in blue-green spectral regions.
Wet etching of GaAs using synchrotron radiation x rays
Authors: Ma Qing , Moldovan Nicolaie , Mancini Derrick C. , Rosenberg Richard A.
Published: July 2002

The results of room-temperature wet etching of GaAs using synchrotron-radiation x rays are described. Under x-ray illumination, etching occurs on the n-GaAs surface in contact with an acid or base solution or even deionized water. The etching process is studied as functions of the electrolytes, their concentration, semiconductor doping level, and x-ray intensity and energy. The etching mechanism is determined to be primarily electrochemical in nature, but the x-ray radiation chemistry plays a role in the etching. Smoothly etched surfaces are achievable with a root-mean-square surface roughness of 0.7–2.0 nm. We also found that the etching rate increases substantially with the ratio of the sample size to the x-ray exposure size. This is accounted for by the rate-limiting effect on the charge transfer across the semiconductor-electrolyte junction. The chemistry of etched surfaces is studied using x-ray photoelectron spectroscopy and compared to that of as-received surfaces.
Holographic scattering as a technique to determine the activation energy for thermal fixing in photorefractive materials
Authors: Ellabban M. A. , Mandula G. , Fally M. , Rupp R. A. , Kovács L.
Published: July 2002

We introduce holographic scattering as a technique to determine the activation energy for thermal fixing of refractive index patterns in photorefractive crystals. After recording a parasitic hologram at ambient temperature, we measured the time dependence of the transmitted intensity at the fixing temperature, to determine the time constant. The temperature dependence of the latter allowed us to evaluate the thermal activation energy. For comparison, we performed an equivalent experiment employing the standard two-wave mixing method. The values obtained using the two techniques agree very well.
Contrast mechanisms for neutron radiography
Authors: McMahon P. J. , Allman B. E. , Nugent K. A. , Jacobson D. L. , Arif M. , Werner S. A.
Published: July 2002

The use of thermal neutrons in contact radiography and tomography [Schlenker and Baruchel, Physica B 137, 309 (1986)] provides a powerful nondestructive analysis technique for materials that are difficult to study with x rays [Kirz et al., Quart. Rev. Biophys. 28, 33 (1995)]. Since x rays are strongly absorbed by the heavier elements and neutrons are preferentially scattered by light elements, neutron radiography is well suited to the visualization of light elements in the interior of metallic objects. In this letter, we demonstrate neutron radiography using a number of different contrast mechanisms. We find we are able to obtain radiographs showing features hitherto unobservable.
Ab initio study of substitutional nitrogen in GaAs
Authors: Orellana W. , Ferraz A. C.
Published: July 2002

We investigate the atomic geometry, formation energies, and electronic structure of nitrogen occupying both arsenic and gallium sites in GaAs (NAs and NGa) using first-principles total-energy calculations. We find that both neutral defects induce impurity-like empty levels in the band gap acting as acceptors. While NAs shows a s-like a1 level in the middle of the band gap, NGa shows a p-like t2 level close to the bottom of the conduction band. The gap level of NAs gives theoretical support for the experimentally observed band-edge redshift on the GaAsN alloy for a N concentration ∼3%. Strong inward relaxations preserving the Td symmetry characterize the NAs equilibrium geometry in all the charge states investigated. In contrast, NGa exhibits a structural metastability in neutral charge state and Jahn–Teller off-center distortions in negative charge states forming a negative-U center. Formation energies of competing NAs and NGa defects are also discussed.
Interdiffusion of CdS and Zn2SnO4 layers and its application in CdS/CdTe polycrystalline thin-film solar cells
Authors: Wu X. , Asher S. , Levi D. H. , King D. E. , Yan Y. , Gessert T. A. , Sheldon P.
Published: July 2002

In this work, we found that the interdiffusion of the CdS and Zn2SnO4 (ZTO) layers can occur either at high temperature (550–650 °C) in Ar or at lower temperature (400–420 °C) in a CdCl2 atmosphere. By integrating a Zn2SnO4 film into a CdS/CdTe solar cell as a buffer layer, this interdiffusion feature can solve several critical issues and improve device performance and reproducibility of both SnO2-based and Cd2SnO4-based CdTe cells. Interdiffusion consumes the CdS film from both the ZTO and CdTe sides during the device fabrication process and improves quantum efficiency at short wavelengths. The ZTO film acts as a Zn source to alloy with the CdS film, which results in increases in the band gap of the window layer and in short-circuit current density Jsc. Interdiffusion can also significantly improve device adhesion after CdCl2 treatment, thus providing much greater process latitude when optimizing the CdCl2 process step. The optimum CdCl2-treated CdTe device has high quantum efficiency at long wavelength, because of its good junction properties and well-passivated CdTe film. We have fabricated a Cd2SnO4/Zn2SnO4/CdS/CdTe cell demonstrating an NREL-confirmed total-area efficiency of 15.8% (Voc=844.3 mV, Jsc=25.00 mA/cm2, and fill factor=74.82%). This high-performance cell is one of the best thin-film CdTe solar cells in the world.
Tunable pulsed vacuum ultraviolet light source for surface science and materials spectroscopy based on high order harmonic generation
Authors: Riedel D. , Hernandez-Pozos J. L. , Palmer R. E. , Baggott S. , Kolasinski K. W. , Foord J. S.
Published: July 2002

We report here the development of a tunable vacuum ultraviolet light source, providing subpicosecond pulses over the wavelength range from 114 (∼11 eV) to 32 nm (∼39 eV), designed for surface science experiments. The source is based on high order harmonic generation. The experimental setup is described in detail and the harmonic yield as a function of the gas type (xenon or argon) is compared with theoretical descriptions. We address in particular the tunability of the source, desirable for surface science applications. Absolute characterization of the harmonic yield has also been performed and validated with time-resolved fluorescence measurements. The results are extremely promising, with intensities of ∼1010 photons/s, with regard to the use of this coherent source for surface science and spectroscopy.
Controlling hole injection in organic electroluminescent device by sputter-grown Cu-phthalocyanine thin films
Authors: Kim Shin Cheul , Lee Gun Bae , Choi Myung-Woon , Roh Youngsuk , Whang Chung Nam , Jeong Kwangho , Lee Jae-Gyoung , Kim Sunwook
Published: July 2002

A sputter-grown Cu-phthalocyanine (SG-CuPc) thin films have been employed to control the anode interface of organic electroluminescent device (OELD). Insertion of a thin SG-CuPc between the indium tin oxide and hole-transport layers enhances the hole injection in a controllable manner, which can increase the device efficiency and decrease the operation voltage without increasing the interface roughness. Time–voltage–luminescence measurements show the improved operational durability of the thin SG-CuPc inserted OELD.
Stability of doubly occupied N2 clathrate hydrates investigated by molecular dynamics simulations
Authors: van Klaveren E. P. , Michels J. P. J. , Schouten J. A. , Klug D. D. , Tse J. S.
Published: July 2002

Classical molecular dynamics calculations were performed for a structure II clathrate hydrate with N2 guest molecules in order to investigate the possibility of double occupancy, i.e., two N2 molecules inside one large cage. For all of the pressures, temperatures, and compositions at which the simulations have been performed, the doubly occupied clathrate remained stable. The structure of the host lattice is indistinguishable from that of a singly occupied clathrate hydrate. The volumes and energies are linearly dependent on the filling fraction. The range of values are the same for both the singly as well as doubly occupied clathrates. In the doubly occupied cages, the O–N2 radial distribution function, and therefore the structure in the vicinity of the N2 molecule, is similar to that of the mixed fluid. An extensive investigation of the distances in the cages shows a large similarity between singly and doubly occupied clathrates. All these results indicate that, upon filling the large cages with pairs of molecules, the stresses on the host lattice are comparable to those in single occupation. Small changes in the intermolecular interactions do not affect the results. The clathrate hydrate destabilizes upon increasing the size of the guest molecules by more than about 10%. We also find that the clathrate hydrate remains stable when experimental conditions are closely followed, i.e., for partially doubly occupied clathrates with a pressure dependent filling fraction. The calculated compressibility is the same as in experiments.
A simple continuum model for boron clustering based on atomistic calculations
Authors: Chakravarthi Srinivasan , Dunham Scott T.
Published: July 2002

Boron exhibits anomalous diffusion during the initial phases of ion implant annealing. Boron transient enhanced diffusion is characterized by enhanced tail diffusion coupled with an electrically inactive immobile peak. The immobile peak is due to clustering of boron in the presence of excess interstitials which also enhance boron diffusion in the tail region. We present a simple model for the formation of immobile boron interstitial clusters and associated point defect interactions derived based on atomistic calculations.
Properties of high κ gate dielectrics Gd2O3 and Y2O3 for Si
Authors: Kwo J. , Hong M. , Kortan A. R. , Queeney K. L. , Chabal Y. J. , Opila R. L. , Muller D. A. , Chu S. N. G. , Sapjeta B. J. , Lay T. S. , Mannaerts J. P. , Boone T. , Krautter H. W. , Krajewski J. J. , Sergnt A. M. , Rosamilia J. M.
Published: July 2002

We present the materials growth and properties of both epitaxial and amorphous films of Gd2O3 (κ=14) and Y2O3 (κ=18) as the alternative gate dielectrics for Si. The rare earth oxide films were prepared by ultrahigh vacuum vapor deposition from an oxide source. The use of vicinal Si (100) substrates is key to the growth of (110) oriented, single domain films in the Mn2O3 structure. Compared to SiO2 gate oxide, the crystalline Gd2O3 and Y2O3 oxide films show a reduction of electrical leakage at 1 V by four orders of magnitude over an equivalent oxide thickness range of 10–20 Å. The leakage of amorphous Y2O3 films is about six orders of magnitude better than SiO2 due to a smooth morphology and abrupt interface with Si. The absence of SiO2 segregation at the dielectric/Si interface is established from infrared absorption spectroscopy and scanning transmission electron microscopy. The amorphous Gd2O3 and Y2O3 films withstand the high temperature anneals to 850 °C and remain electrically and chemically intact.
Resonance Raman spectroscopy in the dissociative A band of nitrosyl chloride
Authors: Mackey Jeffrey L. , Johnson Bruce R. , Kittrell Carter , Le Linh D. , Kinsey James L.
Published: July 2002

Resonance Raman spectra measured for ClNO photoexcited at 212.5, 219, and 222 nm produce evidence that the strong transition at the vacuum ultraviolet end of the merged A band system dominates to wavelengths as long as 222 nm. The spectral resolution is sufficient to enable definitive assignments of excited vibrational levels in the ground electronic state, several of which have not been previously observed. A curvilinear coordinate model of the ground state potential surface around the Franck–Condon region has been constructed. This model yields vibrational eigenvalues in excellent agreement with all known transitions.
Ion oxidation mechanisms controlling the formation of barriers in magnetic tunnel junctions
Authors: Roos B. F. P. , Beck P. A. , Demokritov S. O. , Hillebrands B. , Ozkaya D.
Published: July 2002

The oxidation mechanisms of the formation of thin insulating barriers in magnetic tunnel junctions are studied. The barriers are prepared by means of an oxidation technique using a highly dissociated low-energy (30–80 eV) ionized oxygen atom beam. The barrier formation is investigated using two independent in situ techniques, resistance and an optical reflectivity method. The oxidation depth varies from 1.5 to 1.9 nm depending on the initial ion energy in agreement with performed Monte Carlo simulations. Two different oxidation mechanisms (ion embedding and diffusion) are identified. The electrical and magnetotransport properties of the junctions are studied.
Collective dynamics at high wave vector in the glass-forming liquid Ca0.4K0.6(NO3)1.4
Authors: Ribeiro Mauro C. C.
Published: July 2002

The nature of the phonons at high wave vectors (0.2<k<7.0 Å−1) in the glass-forming liquid Ca0.4K0.6(NO3)1.4 has been investigated by using molecular dynamics (MD) simulations. The MD simulations have been performed with a previously proposed polarizable model for the nitrate anion [M. C. C. Ribeiro, Phys. Rev. B 61, 3297 (2000)]. Time correlation functions of mass and charge current fluctuations have been calculated in order to obtain the spectra of the acoustic and the optic modes, respectively, both the longitudinal and the transverse ones (LA, TA, LO, and TO). Phonon dispersion curves, ω(k), and the damping, Γ(k), of the LA modes are shown to oscillate out of phase with the static structure factor, S(k), in a wide range of wave vectors. Corresponding dynamic structure factors, S(k,ω), are calculated as k-dependent, ω-constant cuts of the LA spectra. It is shown that S(k,ω) oscillates in phase with S(k) for low energy modes. A similar analysis is applied to the LO spectra, the corresponding dynamic structure factor following the charge static structure factor, Sq(k).
Strong adhesion in nanocrystalline diamond films on silicon substrates
Authors: Sharda T. , Umeno M. , Soga T. , Jimbo T.
Published: July 2002

Strong adhesion is shown to be achieved in the growth of smooth nanocrystalline diamond (NCD) thin films on silicon substrates at 600 °C using biased enhanced growth in microwave plasma chemical vapor deposition. The strong adhesion is evident from the films sustaining compressive stress, which may be as high as 85 GPa. The substrates are bent spherically after deposition, however, films are not peeled off, in spite of having enormous in-plane stress. The strong adhesion may be a result of implanted carbon below the substrate surface with an optimized ion flux density in the initial stages of growth. The compressive stress in the films is shown to be generating from the graphitic and other nondiamond carbon impurities in the films. It was observed that the NCD grain size decreases with biasing hence increasing grain boundary area in the films accommodating more graphitic impurities, which in turn results in an increase in compressive stress in the films.
Template-based carbon nanotubes and their application to a field emitter
Authors: Jeong Soo-Hwan , Hwang Hee-Young , Lee Kun-Hong , Jeong Yongsoo
Published: July 2002

Anodic aluminum oxide (AAO) templates were fabricated by anodizing Al films. After the Co catalyst had been electrochemically deposited into the bottom of the AAO template, carbon nanotubes (CNTs) were grown by the catalytic pyrolysis of C2H2 at 650 °C. Overgrowth of CNTs on the AAO templates was observed. The diameter of the CNTs strongly depends on the size of the pores in the AAO template. The electron field emission measurements on the samples showed a turn-on field of 1.9–2.1 V/μm and a field enhancement factor of 3360–5200. Our observation concerning the low turn-on field and high field enhancement factors is explained in terms of a low field screening effect.
Collinear light modulation by surface acoustic waves in laterally structured semiconductors
Authors: Santos Paulo V.
Published: July 2002

The modulation of light beams by surface acoustic waves (SAW) in periodically modulated photonic materials is investigated. The periodic modulation allows for new scattering processes which can improve the efficiency of SAW-based modulators, as proposed by P. St. J. Russell [Phys. Rev. Lett. 56, 596 (1986)]. In particular, scattering geometries with collinear propagation paths for the SAW and the light beam become possible using SAW wavelengths much larger than the light wavelength. We analyze the performance of such modulators based on semiconductor materials and examine the role of random fluctuations in structure dimensions on their operation.
Disorder-driven hysteresis-loop criticality in Co/CoO films
Authors: Berger A. , Inomata A. , Jiang J. S. , Pearson J. E. , Bader S. D. , Dahmen Karin
Published: July 2002

The effect of magnetic disorder on the magnetization reversal process in thin Co/CoO films has been investigated. The antiferromagnetic CoO layer allows a reversible tuning of the magnetic disorder by simple temperature variation. For temperatures above a critical temperature Tc, we observe a discontinuous magnetization reversal, whereas smooth magnetization loops occur for T<Tc. Our measurements establish the existence of a disorder-driven critical point in the nonequilibrium phase diagram. In addition, we observe scaling behavior in the vicinity of the critical point and determine the critical exponents to β=0.022±0.006 and βδ=0.30±0.03.
Semimagnetic self-organized Cd1−xMnxTe quantum dots generated by postgrowth thermal annealing
Authors: Prechtl G. , Heiss W. , Mackowski S. , Janik E.
Published: July 2002

We demonstrate the possibility to fabricate self-organized semimagnetic quantum dots by the growth of two-dimensional layers and subsequent thermal annealing. In particular, CdTe/CdMgTe quantum-well samples containing four narrow MnTe barriers are characterized by optical methods. After annealing well above the growth temperature, from time-resolved photoluminescence (PL), PL experiments under selective excitation, as well as micro-PL measurements, we find clear evidence for the presence of quantum dots. For a sample with a average Mn content of 5%, the dot luminescence shows a redshift of 18 meV under an applied magnetic field of 4 T, due to the giant Zeeman effect.
Behavior of hydrogen atoms in ultrahigh-frequency silane plasma
Authors: Takashima Seigou , Hori Masaru , Goto Toshio , Yoneda Katsumi
Published: July 2002

We have investigated the behavior of the absolute density of hydrogen (H) atoms in ultrahigh-frequency (UHF), (500 MHz) silane (SiH4) plasma by using a vacuum ultraviolet absorption spectroscopy technique with a microdischarge hollow cathode lamp. In the UHF plasma using SiH4 highly diluted with hydrogen molecule (H2) at a pressure of 20 Pa, an UHF power of 1000 W, and a total flow rate of 200 sccm, the absolute density of H atoms slightly increased from 7.4×1011 to 7.9×1011 cm−3 with increasing the SiH4 flow rate ratios from 0% to 2.5% and then the H atom density decreased at the ratio of 5%. The decrease of the density is due to the increase of the reaction between the H atom and the SiH4 molecule. The behavior of the absolute density of H atoms was compared with that of the Balmer α(Hα) emission intensity. It was found that the behaviors of the absolute H atom density and the Hα emission intensity were quite different. Moreover, the kinetics of H atom density in SiH4 plasmas have been clarified on the basis of measured results.
Polymer photonic crystal slab waveguides
Authors: Liguda C. , Böttger G. , Kuligk A. , Blum R. , Eich M. , Roth H. , Kunert J. , Morgenroth W. , Elsner H. , Meyer H. G.
Published: July 2002

We present details of the fabrication, calculations, and transmission measurements for finite two-dimensional (2D) polymer photonic crystal (PC) slab waveguides, which were fabricated from a benzocyclobutene polymer on a low refractive index substrate from Teflon. A square air hole lattice (500 nm lattice constant, 300 nm hole diameter) was realized by electron beam lithography and reactive ion etching. Polarization and wavelength dependent transmission results show TE-like and TM-like stop gaps at 1.3 μm excitation wavelengths and are in good agreement with the calculated data obtained by 2D and three-dimensional finite difference time domain methods. Transmission was suppressed by 15 dB in the center of the TE-like stop gap for a PC length of ten lattice constants.
Exciton–erbium coupling and the excitation dynamics of Er3+ in erbium-doped silicon-rich silicon oxide
Authors: Seo Se-Young , Shin Jung H.
Published: July 2002

The exciton–erbium coupling and the excitation dynamics of Er3+ in erbium-doped silicon-rich silicon oxide are investigated using time-resolved measurements of Er3+ luminescence. The dependence of the Er3+ luminescence on the pump power and duration indicates that the exciton–erbium coupling is dominant over carrier–exciton coupling. The results further support the idea that the luminescent Er3+ ions are not in the Si nanoclusters but in the interface region surrounding the nanoclusters.
Thermal plume turbulence
Authors: Celani A. , Mazzino A. , Vergassola M.
Published: July 2002

A two-dimensional Boussinesq model of thermal convection is investigated by numerical simulations. The turbulent heat transport is mainly due to thermal plumes. It is shown that their thin, wrinkled interfaces control the statistics of large temperature excursions. Away from the interfaces, the temperature fluctuations are strongly suppressed and the field in those regions is well mixed. Their statistical signature is in the low-order moments of the temperature increments, whose scaling exponents depend linearly on the order.
Influence of the local As antisite distribution on ferromagnetism in (Ga, Mn)As
Authors: Sanvito Stefano , Hill Nicola A.
Published: July 2002

The effect of the inclusion of As antisites in the diluted magnetic semiconductor (Ga, Mn)As is studied within the density functional theory in the local spin density approximation. In the case of the homogeneous distribution of Mn ions, we find that the ferromagnetism is weakened by the presence of the antisites. This is due to the compensation of the free holes which mediate the long-range ferromagnetic order. In contrast, when two Mn ions are coupled through only one As ion, the ferromagnetic and antiferromagnetic states are comparable in energy. In this case, the magnetic ground state depends on: (i) the position of the As antisites relative to the Mn, and (ii) the As antisite concentration. We explain our results using a model of competing antiferromagnetic super exchange and ferromagnetic double exchange via localized Zener carriers.
Photon energy dependence of contrast in photoelectron emission microscopy of Si devices
Authors: Ballarotto V. W. , Siegrist K. , Phaneuf R. J. , Williams E. D. , Yang W.-C. , Nemanich R. J.
Published: July 2002

We investigate the variation in doping-induced contrast with photon energy in photoelectron emission microscopy images of Si pn devices using a free-electron laser as a tunable monochromatic light source. Photoyield is observed from p-doped regions of the devices for photon energies as low as 4.5 eV. Band tailing is the dominant effect contributing to the low energy photoyield from the heavily doped p regions. The low intensity tail from the n regions, however, may be from surface states.
Growth and deep ultraviolet picosecond time-resolved photoluminescence studies of AlN/GaN multiple quantum wells
Authors: Nam K. B. , Li J. , Kim K. H. , Lin J. Y. , Jiang H. X.
Published: July 2002

AlN/GaN multiple quantum wells (MQWs) with a well thickness of 26 Å have been grown by metal–organic chemical-vapor deposition. A specially designed photoluminescence (PL) spectroscopy system, which is capable of measuring picosecond time-resolved PL up to 6.2 eV, has been employed to probe the optical properties as well as the carrier transfer and decay dynamics in these MQWs. Optical transitions at 4.039 and 5.371 eV at T=10 K, resulting from the interband recombination between the electrons and holes in the n=1 and n=2 subbands in the wells, have been observed. The band-offset parameter for the AlN/GaN heterostructure has been obtained by comparing the experimental results with the calculations. Carrier dynamics including the relaxation of the electrons and holes from the n=2 and n=1 subband in the conduction and valence bands and the decay lifetimes of the interband transitions have also been measured and analyzed. Detailed subband structures for both the conduction and valence bands in the wells were determined. The implications of our findings on the potential applications of AlN/GaN quantum wells have been discussed.
Clustering equilibrium and deactivation kinetics in arsenic doped silicon
Authors: Nobili D. , Solmi S. , Shao J.
Published: July 2002

The equilibrium between clusters and dopant in solution was studied on silicon on insulator specimens uniformly doped with As at concentrations CAs from 1 to 7.6×1020 cm−3. The values of the carrier density n* after equilibration at 700, 800, and 900 °C are reported. With increasing dopant concentration n* rapidly saturates to the limiting value of the carrier density ne, thus simulating a precipitation process. It is shown that the values of n* at different temperatures and dopant concentrations can be calculated by an equation derived in the Appendix by using a simple cluster model. The deactivation was analyzed by isothermal annealing of the specimens at temperatures in the range 550–800 °C. At high temperature the kinetics accurately complies with the rate equation −dn/dt=A{exp[−(E−αn)/kT]−(n0−n)/(n0−n*)exp[−(E−αn*)/kT]} which is the one reported in [D. Nobili, S. Solmi, M. Merli, and J. Shao, J. Electrochem. Soc. 146, 4246 (1999)] complemented by the second term on the right to account for the declustering process. Deviations leading to rates lower than predicted by the above equation are presented by the most heavily doped compositions after partial deactivation at temperatures ⩽ 700 °C. The analysis of this phenomenon puts into evidence that clustering presents a limiting rate which only depends on temperature and carrier density, and is insensitive to As concentration.
Brillouin scattering investigation of elastic properties of Cu–Mo solid solution thin films
Authors: Djemia P. , Ganot F. , Moch P. , Branger V. , Goudeau P.
Published: July 2002

Brillouin scattering has been used to investigate the elastic properties of CuxMo1−x thin films with thicknesses varying from 200 up to 430 nm, elaborated by ion beam sputtering. Two compositions around Cu30Mo70 and Cu70Mo30 and pure Cu and Mo films have been studied. The samples are essentially randomly oriented polycrystals, leading to an isotropic effective symmetry. By fitting all the Brillouin spectral lines, two independent effective elastic constants of these films have been determined, allowing the calculation of their elastic moduli. Results indicate that pure copper thin films’ elastic properties are similar to the bulk copper ones whereas pure molybdenum thin films' Young's modulus is weaker (about 10%) than the bulk Mo one. Concerning Cu–Mo solid solutions, elastic constants values lie between pure copper and molybdenum ones. Finally, the effect of annealing at 760 K on the mechanical properties of the solid solutions in relation with the microstructural evolution is discussed. The annealed solid solutions show a hardening of the elastic moduli with respect to the as-deposited samples, which is supposed to reflect their full demixing.
Low driving voltages and memory effect in organic thin-film transistors with a ferroelectric gate insulator
Authors: Velu G. , Legrand C. , Tharaud O. , Chapoton A. , Remiens D. , Horowitz G.
Published: July 2002

In this letter, two organic thin-film transistors with SiO2 and ferroelectric PbZrTiO3 (PZT) gate insulator are compared. The fabrication of the devices is described and their electrical properties estimated. The PZT-based devices show better performance: Low driving voltage, high Ion/Ioff ratio, etc. Moreover, a memory effect is reported in correlation with ferroelectric properties of PZT thin films.
InGaAsNSb/GaAs quantum wells for 1.55 μm lasers grown by molecular-beam epitaxy
Authors: Yang X. , Héroux J. B. , Mei L. F. , Wang W. I.
Published: July 2002

InGaAsNSb/GaAs quantum wells (QWs) were grown by solid-source molecular-beam epitaxy using a N2 radio frequency plasma source. The effect of adding Sb during growth of InGaAsN/GaAs QWs was studied. X-ray diffraction, reflection high-energy electron diffraction and transmission electron microscopy studies indicate that Sb suppresses the three-dimensional growth and improves the interface of the QWs. X-ray diffraction and secondary ion mass spectroscopy analysis show that Sb gets incorporated into the quantum well, which becomes a quinternary compound that was previously unexplored. The introduction of Sb during growth of InGaAsN/GaAs QWs significantly enhances the optical properties of the QWs. 1.53 μm room-temperature photoluminescence was obtained from InGaAsNSb/GaAs QWs, which demonstrates the potential of fabricating 1.55 μm InGaAsNSb/GaAs QW lasers for long-haul applications.
Compact holographic memory system using a one-beam geometry in a photorefractive crystal
Authors: Mitsuhashi Hideta , Obara Minoru
Published: July 2002

We have demonstrated a compact holographic memory system by using a one-beam geometry, which eliminates the process of dividing the incident beam into object and reference beams, by using a 635 nm laser diode as a compact light source. As the recording media, a photorefractive crystal (Fe-doped 45° cut LiNbO3) is used. One crucial advantage of the developed system is improved mechanical stability because hologram recording is sensitive to mechanical vibrations. An image is successfully recorded and reconstructed, and the ability of the system is also discussed.
Enhancement of electromechanical coupling coefficient and acoustic power density in conventional “Hard” Pb(Zr1−xTix)O3 ceramics by application of uniaxial stress
Authors: Viehland D. , Tito F. , McLaughlin E. , Robinson H. , Janus R. , Ewart L. , Powers J.
Published: July 2002

Investigations of the polarization versus electric field (P-E) and strain versus electric field (ε-E) responses for “hard” Pb(Zr1−xTix)O3 piezoelectric ceramics have been performed under various uniaxial stresses (σ) and ac electrical drive fields. Investigations revealed a significant decrease in the remanent polarization of specimens with increasing σ. Subsequent calculations of the longitudinal electromechanical coupling coefficient (k33) and acoustic power density revealed significant enhancements with increasing σ.
Localized charge injection in SiO2 films containing silicon nanocrystals
Authors: Boer Elizabeth A. , Brongersma Mark L. , Atwater Harry A. , Flagan Richard C. , Bell L. D.
Published: July 2002

An atomic-force microscope (AFM) is used to locally inject, detect, and quantify the amount and location of charge in SiO2 films containing Si nanocrystals (size ∼2–6 nm). By comparison with control samples, charge trapping is shown to be due to nanocrystals and not ion-implantation-induced defects in samples containing ion-beam-synthesized Si nanocrystals. Using an electrostatic model and AFM images of charge we have estimated the amount of charge injected in a typical experiment to be a few hundred electrons and the discharge rate to be ∼35±15 e/min.
Electrical properties of gold–polymer composite films
Authors: Augelli V. , Ligonzo T. , Masellis M. C. , Muscarella M. F. , Schiavulli L. , Valentini A.
Published: July 2002

Electrical conductivity measurements in the range 50–300 K have been carried out on gold–fluorocarbon–polymer composite films. Such measurements have been performed on samples with different percentages in volume of gold, before and after thermal annealing up to 450 K. The experimental temperature dependence of conductivity, in the range 50–300 K, follows the exponential law σ=σ0 exp[−(T0/T)γ], where T0 and γ are constants. Best fit analyses carried out on experimental conductivity data, before and after thermal annealing, give values of γ that can be related to the gold content of the film and to the size of grains embedded in the polymeric matrix. Such results suggest that grain redistribution can take place after thermal annealing and that the grains tend to coalesce. Dependence of conductivity on temperature during heating and cooling processes is also discussed.
Influence of hydrostatic pressure on the Jahn–Teller effect in the T2g4 excited state of CrCl63− doped Cs2NaScCl6
Authors: Wenger Oliver S. , Valiente Rafael , Güdel Hans U.
Published: July 2002

The T2g4→4A2g luminescence of a 4.1% Cr3+ doped Cs2NaScCl6 crystal is studied as a function of hydrostatic pressure at room temperature and 15 K. The vibrational fine structure observed in the low-temperature variable pressure emission spectra is analyzed with a two configurational coordinate approach, involving the totally symmetric a1g and the eg Jahn–Teller normal coordinate. Increasing hydrostatic pressure is found to reduce the tetragonal distortion of the CrCl63− unit in the electronic T2g4 state resulting from the Jahn–Teller effect. Additionally, pressure impedes expansion along the a1g coordinate of the CrCl63− complex upon T2g4→4T2g photo-excitation, and thus has a greater influence on the T2g4 excited state than on the A2g4 ground state. The absolute Cr3+–Cl− average distance reduction upon increasing pressure is estimated using a simple point charge model.
Single-crystalline rocksalt CdO layers grown on GaAs (001) substrates by metalorganic molecular-beam epitaxy
Authors: Ashrafi A. B. M. A. , Kumano H. , Suemune I. , Ok Y.-W. , Seong T.-Y.
Published: July 2002

In this letter, we report the growth of single-crystalline rocksalt CdO layers on (001) GaAs substrates using ZnS buffer layers. The growth processes of CdO layers were studied by reflection high-energy electron diffraction (RHEED), and the grown CdO layers were evaluated with atomic force microscopy (AFM), and x-ray diffraction (XRD) measurements. After an initial growth delay, the formation of polycrystalline CdO was observed in RHEED measurements during the initial growth of very thin CdO layers. With the increase of the CdO layer thicknesses, streaky RHEED patterns were observed, which indicate the formation of single-crystalline cubic-phase CdO layers. Surface morphology of the CdO layers observed by AFM was atomically flat with root-mean-square roughness of ∼1 nm. The crystalline structures were elucidated from XRD measurements by the determination of the lattice constant to be 4.686±0.001 Å, indicating the single-phase rocksalt CdO structure.
Characteristics of ferroelectric Pb(Zr,Ti)O3 thin films having Pt/PtOx electrode barriers
Authors: Lee Kwangbae , Rhee Byung Roh , Lee Chanku
Published: July 2002

We have investigated the feasibility of the Pt/PtOx multilayer as an electrode barrier for Pb(Zr,Ti)O3 (PZT)-based ferroelectric random access memories. PtOx and Pt layers were prepared on polycrystalline-Si/SiO2/Si substrates by means of the sputtering method in Ar and O2 ambience, and the Pb(Zr0.53Ti0.47)O3 layer was prepared by the sol-gel method. A capacitor consisting of Pt/PtOx/PZT/PtOx/Pt/PtOx/poly-Si had a remanent polarization of 18 μC/cm2 and a low coercive field of 32 kV/cm. The polarization fatigue behavior of test capacitors was improved as compared with that of Pt/PZT/Pt, which showed negligible fatigue loss of 15% after 1011 switching repetitions with a frequency of 1 MHz.
High-resolution energy-selected study of the reaction CH3X+→CH3++X: Accurate thermochemistry for the CH3X/CH3X+ (X=Br, I) system
Authors: Song Y. , Qian X.-M. , Lau K.-C. , Ng C. Y. , Liu Jianbo , Chen Wenwu
Published: July 2002

Using the high-resolution pulsed field ionization-photoelectron (PFI-PE) and PFI-PE-photoion coincidence (PFI-PEPICO) techniques, we have examined the formation of methyl cation (CH3+) from the dissociation of energy-selected CH3X+ (X=Br and I) near their dissociation thresholds. The breakdown diagrams for CH3X thus obtained yield values of 12.834±0.002 eV and 12.269±0.003 eV for the 0 K dissociative threshold or appearance energy (AE) for CH3+ from CH3Br and CH3I, respectively. Similar to the observation in PFI-PE studies of CH4, C2H2, and NH3, the PFI-PE spectrum for CH3Br exhibits a step at the 0 K AE for CH3+, indicating that the dissociation of excited CH3Br in high-n (⩾100) Rydberg states at energies slightly above the dissociation threshold occurs in a time scale of ⩽10−7 s. The observed step is a confirmation of the 0 K AE(CH3+) from CH3Br determined in the PFI-PEPICO study. The adiabatic ionization energies (IEs) for the CH3Br+(X̃ 2E3/2,1/2) spin–orbit states were determined by PFI-PE measurements to be 10.5427±0.0010 and 10.8615±0.0010 eV, respectively, yielding the spin–orbit coupling constant to be 2571±4 cm−1. The AE(CH3+) values from CH3Br and CH3I and the IE[CH3Br+(X̃ 2E3/2)] value obtained here, when combined with the known IE of CH3 (9.8380±0.0004 eV) and IE[CH3I+(X̃ 2E3/2)] (9.5381±0.0001 eV), have allowed accurate determination of the 0 K bond dissociation energies for CH3–Br (2.996±0.002 eV), CH3+–Br (2.291±0.002 eV), CH3–I (2.431±0.003 eV), and CH3+–I (2.731±0.003 eV). Using the AE(CH3+) from CH3Br and CH3I, together with the known 0 K heats of formation (ΔfH00) for Br (117.93±0.13 kJ/mol), I (107.16±0.04 kJ/mol), and CH3+ (1099.05±0.33 kJ/mol), we have obtained more precise ΔfH00 values for CH3Br (−21.30±0.42 kJ/mol) and CH3I (22.43±0.50 kJ/mol). This experiment demonstrated that highly reliable ΔfH00 values for a range of molecules with error limits comparable to those for some of the most precisely measured values, such as ΔfH00(CH4), can be obtained by PFI-PE and PFI-PEPICO measurements.
Intermolecular vibrations of the jet-cooled 2-pyridone⋅2-hydroxypyridine mixed dimer, a model for tautomeric nucleic acid base pairs
Authors: Müller Andreas , Talbot Francis , Leutwyler Samuel
Published: July 2002

The 2-pyridone⋅2-hydroxypyridine mixed dimer, 2PY⋅2HP, is linked by antiparallel N–H⋯N and O–H⋯O hydrogen bonds, closely analogous to tautomeric hydrogen-bonded DNA base pairs such as isoguanine⋅uracil. Mass-selected S1←S0 vibronic spectra of supersonic jet-cooled 2PY⋅2HP and three N–D or O–D deuterated isotopomers were measured by two-color resonant two-photon ionization and UV/UV-holeburning techniques. S1→S0 fluorescence spectra show intense hydrogen bond stretching vibrations ν6″=158 cm−1, observed for all four isotopomers and the weak ν5″ “shear” vibration at 116 cm−1. The “slanting” intermolecular vibration ν4″=89 cm−1 was tentatively identified via the 2ν4″ overtone. The observation of 2-pyridone intramolecular overtone and combination transitions for two out-of-plane modes indicate slight nonplanarity of the 2-pyridone moiety in the S1 state. Ab initio calculations using Hartree–Fock, configuration interaction among single excitations (CIS), and B3LYP density functional methods were employed to calculate ground- and excited-state structures, rotational constants, harmonic intermolecular and intramolecular vibrational frequencies, binding energies De, and dissociation energies D0. The B3LYP/6-311++(d,p) ν4″ and ν6″ frequencies are in very good agreement with the experimental ground-state intermolecular frequencies. Electronic excitation of the 2-pyridone moiety in 2PY⋅2HP leads to a decrease of dissociation energy by 2.36 kcal/mol and a concurrent increase of H bond lengths by ≈0.08 Å, in qualitative agreement with the self-consistent field (SCF) and CIS ab initio calculations.