bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum well intermixing for the fabrication of InGaAsN/GaAs lasers with pulsed anodic oxidation
Authors: Qu Y. , Liu C. Y. , Yuan Shu , Wang S. Z. , Yoon S. F. , Chan Michael C. Y. , Hong M. H.
Published: March 2004

Quantum well (QW) intermixing was carried out by post-growth rapid thermal annealing in InGaAsN/GaAs QW laser structures grown by solid-source molecular-beam epitaxy. The intensity and width of the photoluminescence peak showed a dependence on annealing temperature and time, and the maximum intensity and minimum linewidth were obtained after the wafer was annealed at 670 °C for 60 s. The peak luminescence energy blueshifted with increasing annealing time, although it plateaued at an annealing time that corresponded to that yielding the maximum luminescence intensity. The diffusion coefficient for indium was determined from a comparison between experimental data and modeling, but showed that QW intermixing alone was not sufficient to account for the relatively large blueshift after annealing. Defects related to the incorporation of nitrogen in the QW layer were responsible for the low photoluminescence efficiency in the as-grown samples and were annealed out during rapid thermal annealing. During annealing, nitrogen interstitials moved to vacancy sites within the QW and thus suppressed QW intermixing. After annealing wafers under conditions giving the maximum luminescence intensity, lasers were fabricated with pulsed anodic oxidation.
Investigation of excitons in AlGaN/GaN multiple quantum wells by lateral photocurrent and photoluminescence spectroscopies
Authors: Friel I. , Thomidis C. , Fedyunin Y. , Moustakas T. D.
Published: March 2004

We report on the investigation of excitons in Al0.2Ga0.8N/GaN multiple quantum wells (MQWs) by lateral photocurrent, and photoluminescence (PL) spectroscopies over the temperature range from 9 to 300 K. The MQWs were deposited homoepitaxially by plasma-assisted molecular-beam epitaxy on a (0001) GaN template grown by hydride vapor phase epitaxy. Excitonic peaks in the photocurrent spectra due to the bulk GaN template and the MQW structure were observed up to room temperature. The PL excitonic peak in the MQWs was strongly Stokes shifted over the whole temperature range, a result attributed to recombination via disorder-induced excitonic band-tail states in the MQWs, due primarily to well/barrier interface roughness fluctuations. A theoretical calculation estimates these fluctuations to be 0.7±0.2 monolayers. The temperature dependence of the Stokes shift indicates that, at higher temperatures, the excitons in the PL experiments are in thermal equilibrium with the lattice before recombining. At lower temperatures, the data suggest a nonthermal exciton distribution, which we attribute to exciton trapping in local potential minima. The PL intensity was found to be strongly quenched by two thermally activated mechanisms with activation energies of 33±6 meV and 165±33 meV. The splitting of the exciton, which gives rise to the photocurrent, was correlated with these two quenching mechanisms, and a model was developed to account for the temperature dependence of the excitonic photocurrent in the MQWs.
Fiber-launched ultratight photorefractive solitons integrating fast soliton-based beam manipulation circuitry
Authors: DelRe Eugenio , Palange Elia , Agranat Aharon J.
Published: March 2004

Self-integration of single-mode fiber to spatial solitons is achieved through the trapping of fiber-launched beams into ultratight high-aperture nonlinear waves in photorefractive paraelectric potassium-lithium-tantalate-niobate. This allows the integration of soliton-based circuitry and electroholography at longer nonabsorbed wavelengths, demonstrating electro-optic beam manipulation and two-mode routing capabilities.
Modeling the plasma plume of a hollow cathode
Authors: Boyd Iain D. , Crofton Mark W.
Published: March 2004

In this study, a numerical model is developed to simulate the xenon plasma plume from a thermionic hollow cathode employing an orifice plate used for propellant ionization and beam neutralization in an electrostatic space propulsion system. The model uses a detailed fluid model to describe the electrons and a particle-based kinetic approach is used to model the heavy xenon ions and atoms. A number of key assumptions in terms of physical modeling and boundary conditions of the simulations are assessed through direct comparisons with experimental measurements. For two of the three cathode operating conditions considered, good agreement with the measured data is obtained. The third condition appears to lie in a different physical regime where elevated electron and ion temperatures and decreased transport coefficients are required in the simulation to provide agreement between the model and the measured data.
Effect of turbostratic boron nitride buffer layers on stress evolution of cubic boron nitride films
Authors: Kim Hong-Suk , Baik Young-Joon
Published: March 2004

Cubic boron nitride (cBN) films are deposited on the BN buffer layers with aligned or random orientation of turbostratic BN (tBN) laminates. It is clearly shown that the cBN phase nucleates and grows on the buffer layer regardless of its alignment, but the lattice relationship between the tBN and the cBN phases and the behavior of the residual stress variation are different according to the tBN buffer type. Based on these results, the validity of the residual stress and tBN alignment hypothesis for cBN nucleation is hereby discussed.
Modification of stretched-exponentially dependent depopulation dynamics of small polaron in Fe:LiNbO3 crystals
Authors: Zhang De-Long , Pun E. Y. B.
Published: March 2004

To overcome the difficulty of the stretched-exponential model in characterizing the initial stage (less than 100 μs) of the relaxation of small polarons in Fe:LiNbO3 at high pump light intensities, an improved stretched-exponential model with combined processes of distance-dependent electron transition and distance-dependent short-range energy interaction has been proposed. Both forward and backward transfer of energy from small polarons to energy acceptors are considered. Accordingly, two modified stretched-exponential expressions for the characterization of the relaxation of the small polarons were given. The improved model was then examined by fitting some selected experimental curves reported previously. As a result, an excellent agreement of the simulated plots with the experimental curves was obtained in both the initial relaxation stage and the subsequent relaxation stage, showing that the modified model is valid. The validity of the improved model was also demonstrated by analyzing the values of the fitting parameters.
Relaxational diffusion in guest–host polyphosphazene thin films: Temperature dependence
Authors: Martin G. , Rojo G. , Agulló-López F.
Published: March 2004

The relaxation of poled guest–host polyphosphazene (PPZ) films containing the 4-[(4′-nitrophenyl)azo]phenoxy (NPAP) azo-dye chromophore (Tg=144 °C) has been investigated by measuring the time decay of the nonlinear optical susceptibility tensor in the 80–120 °C temperature range. The kinetic data have been analyzed on the basis of the Legendre polynomial decomposition of the orientational distribution function. In the measuring time range, two components (fast and slow) of the decay have been found together with a constant component that disappears at temperatures close to Tg. This component accounts for the nonreorientable fraction of the molecules at each temperature. The temperature dependence of the two lifetimes has been determined and fitted to an Adam–Gibbs law. At a high temperature, (120 °C) the ratio of lifetimes closely follows the predictions of the orientational diffusion equation. This allows one to determine the two nonlinear components βzzz=−88.10−30 esu and βzxx=71.10−30 esu of the quadratic hyperpolarizability tensor in our rodlike (C∞v symmetry) molecules, showing the non-negligible contribution of the nondiagonal components.
Degradation mechanism limiting the lifetime of ZnSe-based white light-emitting diodes
Authors: Katayama Koji , Nakamura Takao
Published: March 2004

The degradation mechanism responsible for limiting the lifetime of ZnSe-based white light-emitting diodes was studied. A systematic investigation on the effects of pre-existing stacking faults and threading dislocations, along with point defects in the epitaxial layers was conducted by monitoring their behavior during an aging experiment. Our experimental results, in conjunction with numerical calculations performed on degraded device structures, suggest that the deterioration of the internal quantum efficiency owing to the reduction of the net acceptor concentration in the p-ZnMgSSe cladding layer is responsible for the decline of the optical output power. The reduction in the free hole concentration has an effect of increasing the electron leakage current, leading to a decrease in the carrier injection efficiency. It is also proposed that the carrier depletion effect in the p-cladding layer is due to self-compensation resulting from the increase in the concentration of N-related deep donors.
Grid-controlled electron emission from a hollow-anode electron source
Authors: Krokhmal A. , Gleizer J. Z. , Krasik Ya. E. , Gurovich V. Ts. , Felsteiner J.
Published: March 2004

We describe the operation of a hollow-anode electron source with a biased output grid in a diode powered by a 200 kV 400 ns pulse. The hollow anode had a ferroelectric plasma source incorporated in it. Three electrical schemes for the hollow-anode output grid bias were tested and compared. It is shown that the use of an autobias grid allows electron-beam generation with a current amplitude up to 1.2 kA in a plasma emission-limited mode and with insignificant plasma prefilling of the accelerating gap. The use of an externally biased output grid (either with a positive or negative potential) showed the possibility to control the emission properties of the hollow-anode plasma without changing the amplitude of the discharge current. Electron beams with an amplitude up to 2 kA and insignificant plasma prefilling of the accelerating gap were obtained. It was found that the application of the accelerating pulse leads to a drastic increase in the potential of the plasma up to several kV. It is shown that, in spite of the large positive plasma potential, electron emission occurs due to the dynamics of ions inside the sheath near the hollow-anode grid.
Analysis of nanoindentation creep for polymeric materials
Authors: Yang Shuang , Zhang Yong-Wei , Zeng Kaiyang
Published: March 2004

This article presents studies on nanoindentation creep for polymeric materials at room temperature. A semi-empirical method has been established from a generalized Kelvin model. This model includes the elastic-viscoelastic-viscous effects on indentation creep. It is shown that this method can fit the experimental creep data well, to give the exact changes of creep behavior during nanoindentations for numbers of amorphous polymeric materials. A method has also been proposed to determine the elastic modulus based on this model; the elastic modulus determined from this method is independent of the holding and unloading processes of nanoindentations. Creep deformation is further used to derive creep compliance and retardation spectrums for the polymeric materials studied, which are very useful to predict other mechanical properties of polymeric materials.
Time dynamics of the gas discharge around drops of liquids
Authors: Korotkov K. , Krizhanovsky E. , Borisova M. , Korotkin D. , Hayes M. , Matravers P. , Momoh K. S. , Peterson P. , Shiozawa K. , Vainshelboim A.
Published: March 2004

We have investigated the dynamics of gas discharge around drops of conducting (strong electrolytes) and nonconductive (organic oils) liquids. The trend of dynamics of the gas discharge image turns out to distinquish even between chemically close liquids, which have nondistinquishable static gas discharge images. This article is a natural continuation of the work by K. Korotkov and D. Korotkin [J. Appl. Phys. 89, 4732 (2001)], where static images of gas discharge were studied.
Shock gaseous cylinder interactions: Dynamically validated initial conditions provide excellent agreement between experiments and numerical simulations to late–intermediate time
Authors: Zhang Shuang , Zabusky Norman J. , Peng Gaozhu , Gupta Sandeep
Published: March 2004

We present numerical simulations of a planar shock interacting with a two-dimensional sulfur hexafluoride (SF6) cylinder. We have excellent agreement with experiments at two Mach numbers M=1.095 [Jacobs, Phys. Fluids A 5, 2239 (1993)] and M=1.2 [Zoldi, Ph.D. thesis, SUNY Stony Brook, 2002]. This includes intermediate scale features and quantities such as bounding box dimensions of coherent structures and velocity magnitude distribution function. Our simulations use a validated viscous FLASH [ASCI FLASH Center, “FLASH User’s Guide,” University of Chicago, 2002] environment initialized with a cylinder bounded by a finite-thickness interfacial transition layer of specific shape. The shape parameters are determined through iteration, beginning with the uncertain experimental images and optimizing to obtain maximal agreement with early to intermediate time evolving structures. The visiometric approach and the vortex paradigm [Hawley and Zabusky, Phys. Rev. Lett. 63, 1241 (1989)] are essential to obtain insight into this Richtmyer–Meshkov environment. We verify our recent discovery [Zabusky and Zhang, Phys. Fluids 14, 419 (2002)] that after the primary shock-deposition of vorticity by the incident shock, a vortex bilayer of large circulation magnitude grows significantly through intermediate times. The inclusion of physical viscosity allows us to examine some aspects of pre-turbulence at late–intermediate times.
High-temperature phase stability of hafnium aluminate films for alternative gate dielectrics
Authors: Yang Yan , Zhu Wenjuan , Ma T. P. , Stemmer Susanne
Published: March 2004

Hafnium aluminate films with different compositions were deposited at room temperature by jet vapor deposition. The as-deposited films were amorphous. After annealing at 1100 °C, the microstructure of the films was analyzed by high-resolution transmission electron microscopy, electron diffraction and electron energy loss spectroscopy (EELS). The crystalline phase in pure HfO2 films was monoclinic. With an increase in the Al content in the films, the amount of metastable HfO2 with a tetragonal distorted fluorite structure increased. In addition, the grain sizes decreased, making the detection of crystallization by x-ray diffraction difficult. No crystalline Al2O3 phase could be unambiguously detected in electron diffraction patterns in films with up to 30 mol % Al2O3. However, an Al-rich intergranular phase was identified by EELS. Films with ∼64 mol % Al2O3 crystallized as tetragonal HfO2 and metastable cubic Al2O3 with the spinel structure. The complex microstructures of the films should be considered in the interpretation of their dielectric and electrical properties after high temperature anneals.
Identification of vacancy–oxygen complexes in oxygen-implanted silicon probed with slow positrons
Authors: Fujinami M. , Miyagoe T. , Sawada T. , Suzuki R. , Ohdaira T. , Akahane T.
Published: March 2004

Defects and their annealing behavior for low (2×1015/cm2) and high (1.7×1018/cm2) doses of 180 keV oxygen-implanted silicon have been investigated by the coincidence Doppler broadening (CDB) and lifetime measurements in variable-energy positron annihilation spectroscopy. In the low-dose sample, divacancies are induced throughout the entire implantation region. In the vacancy-oxygen coexisting region (300–500 nm depths), by raising the annealing temperature to 600 °C, vacancy–oxygen VxOy complexes with one vacant site are formed and, simultaneously, the migration of oxygen begins to takes place. In the vacancy-rich region (−200 nm depths), the evolution of simple vacancy clusters to V4 is mainly observed below 600 °C. From CDB and lifetime measurements, it has been proven that after annealing at 800 °C, the VxOy complexes are formed throughout the implanted region and they contain four vacant sites and a high ratio of y to x. On the other hand, high-dose implantation at 550 °C produces the VxOy complexes with a lifetime of a 430 ps in the near-surface region (less than 200 nm deep) and annealing at 1100 °C leads to the highest ratio of y to x. These complexes cannot be annealed out even by annealing at 1350 °C, and their structure is found to be very similar to that for the electron-irradiated amorphous SiO2.
Frictional properties of alkanethiol self-assembled monolayers with different thermal annealing
Authors: Zhang Chun , Liang Qi , Wang Bing , Xiao Xudong
Published: March 2004

We have studied the thermal annealing effect on friction of alkanethiol self-assembled monolayers (SAMs) using atomic force microscopy and frictional force microscopy. The friction is found to increase with thermal annealing time once the annealing temperature is high enough. The change in friction is well correlated with the change in the SAM structure. From a densely packed (∛×∛)R30° phase to a (p×∛) stripe phase with lower density, the magnitude of friction is found to increase by ∼10 times. Such an increase in friction is proposed as being due to the less compact surface structure of the stripe phase, which opens up additional energy dissipation channels to the film.
Acoustic chaos for enhanced detectability of cracks by sonic infrared imaging
Authors: Han Xiaoyan , Zeng Zhi , Li Wei , Islam Md. Sawar , Lu Jianping , Loggins Vera , Yitamben E. , Favro L. D. , Newaz G. , Thomas R. L.
Published: March 2004

The technique of sonic infrared imaging (SIR) consists of the excitation of an object with a short pulse of 15 to 40 kHz sound, in combination with IR imaging of the object’s surface temperature as a function of time. Sonic infrared imaging is effective for detecting surface and subsurface cracks. The recent discovery of acoustic chaos has provided a means of greatly enhancing the effectiveness of SIR. We describe the properties of chaotic sound in the context of SIR crack detection, and show examples of the enhancement of the detectability of cracks through the use of chaotic sound.
Microstructure evolution upon devitrification and crystallization kinetics of Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 melt-spun glassy ribbon
Authors: Scudino S. , Kühn U. , Schultz L. , Nagahama D. , Hono K. , Eckert J.
Published: March 2004

The crystallization behavior of glassy Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 produced by melt spinning was investigated by differential scanning calorimetry, x-ray diffraction, electron microscopy, and atom probe investigations. The devitrification of the as-spun ribbon occurs by primary crystallization of a metastable nanoscale quasicrystalline phase during the first stage of the crystallization process, followed by successive transformation into intermetallic compounds at higher temperatures. The kinetics investigation reveals that quasicrystal formation is characterized by two overlapping processes: the first step probably linked with the redistribution of one or more elements, most likely Al and Zr, between the quasicrystalline phase and the remaining amorphous matrix, and the second step corresponding to the crystallization itself. Furthermore, a higher value of the activation energy for quasicrystal formation compared to other quasicrystal-forming alloys suggests an increased complexity of the crystallization mechanism.
Photoluminescence decay dynamics of noninteracting silicon nanocrystals
Authors: Guillois O. , Herlin-Boime N. , Reynaud C. , Ledoux G. , Huisken F.
Published: March 2004

Time-resolved photoluminescence measurements on size-selected silicon nanocrystals have been carried out in order to elucidate the nonexponential behavior of the photoluminescence decay kinetics. The nanoparticles are gas-phase synthesized, extracted as a supersonic beam, size selected, and deposited downstream as films of variable densities. The nanoparticle number densities were determined by atomic force microscopy. The photoluminescence properties appear totally independent of the film density. Even in the very low density film where nanoparticles are completely isolated from each other, the decay kinetics corresponds to a stretched exponential law. This means that the stretched exponential kinetics does not originate from the interaction between nanoparticles, but is actually a characteristic of the silicon nanocrystals.
The hairpin resonator: A plasma density measuring technique revisited
Authors: Piejak R. B. , Godyak V. A. , Garner R. , Alexandrovich B. M. , Sternberg N.
Published: March 2004

A microwave resonator probe is a resonant structure from which the relative permittivity of the surrounding medium can be determined. Two types of microwave resonator probes (referred to here as hairpin probes) have been designed and built to determine the electron density in a low-pressure gas discharge. One type, a transmission probe, is a functional equivalent of the original microwave resonator probe introduced by R. L. Stenzel [Rev. Sci. Instrum. 47, 603 (1976)], modified to increase coupling to the hairpin structure and to minimize plasma perturbation. The second type, a reflection probe, differs from the transmission probe in that it requires only one coaxial feeder cable. A sheath correction, based on the fluid equations for collisionless ions in a cylindrical electron-free sheath, is presented here to account for the sheath that naturally forms about the hairpin structure immersed in plasma. The sheath correction extends the range of electron density that can be accurately measured with a particular wire separation of the hairpin structure. Experimental measurements using the hairpin probe appear to be highly reproducible. Comparisons with Langmuir probes show that the Langmuir probe determines an electron density that is 20–30% lower than the hairpin. Further comparisons, with both an interferometer and a Langmuir probe, show hairpin measurements to be in good agreement with the interferometer while Langmuir probe measurements again result in a lower electron density.
X-ray absorption spectroscopy of transition-metal doped diluted magnetic semiconductors Zn1−xMxO
Authors: Okabayashi J. , Ono K. , Mizuguchi M. , Oshima M. , Gupta Subhra Sen , Sarma D. D. , Mizokawa T. , Fujimori A. , Yuri M. , Chen C. T. , Fukumura T. , Kawasaki M. , Koinuma H.
Published: March 2004

We have investigated the electronic structure of Zn1−xMxO (M: 3d transition metal) by x-ray absorption spectroscopy. Using configuration–interaction cluster-model analyses, electronic structure parameters have been deduced and their chemical trend is discussed. Results show that the p–d exchange constant Nβ is negative and large in cases of Mn, Fe, and Co, which is consistent with the enhancement of magnetic circular dichroism.
Simulation of a two-dimensional sheath over a flat insulator–conductor interface on a radio-frequency biased electrode in a high-density plasma
Authors: Kim Doosik , Economou Demetre J.
Published: March 2004

A combined fluid/Monte Carlo (MC) simulation was developed to study the two-dimensional (2D) sheath over a flat insulator/conductor interface on a radio-frequency (rf) biased electrode in a high-density plasma. The insulator capacitance increased the local impedance between the plasma and the bias voltage source. Thus, for uniform ion density and electron temperature far away from the wall, the sheath potential over the insulator was only a fraction of that over the conductor, resulting in a thinner sheath over the insulator. The fluid model provided the spatiotemporal profiles of the 2D sheath electric field. These were used as input to the MC simulation to compute the ion energy distribution (IED) and ion angular distribution (IAD) at different locations on the surface. The ion flux, IED, and IAD changed drastically across the insulator/conductor interface due to the diverging rf electric field in the distorted sheath. The ion flux was larger on the conductor at the expense of that on the insulator. Both the ion impact angle and angular spread increased progressively as the material interface was approached. The ion impact energy and energy spread were smaller on the insulator as compared to the conductor. For given plasma parameters, as the insulator thickness was increased, the sheath potential and thickness over the insulator decreased, and sheath distortion became more pronounced.
Propagation of shock-induced chemistry in nanoenergetic materials: The first micrometer
Authors: Yang Yanqiang , Wang Shufeng , Sun Zhaoyong , Dlott Dana D.
Published: March 2004

The propagation of shock-induced chemical reactions over nanometer distances is studied in energetic materials consisting of Al nanoparticles (30, 62, and 110 nm) in the polymer oxidizers nitrocellulose (NC) and Teflon. Picosecond laser flash heating vaporizes the Al particles, which react with surrounding oxidizer and generate a spherical shock wave with a rapidly dropping pressure, that decomposes the NC or Teflon out to a diameter drxn. A methodology is developed to measure drxn as a function of laser energy, that uses the average distance between nanoparticles davg as a length scale and identifies the ablation threshold as occurring when the reaction spheres from multiple particles coalesce. At minimal laser fluences, drxn is slightly larger than the diameter of the polymer sphere needed to just oxidize the nanoparticle. The excess diameter is attributed to the chemical energy of oxidation. At larger laser fluences where chemical energy is unimportant, drxn∝E over the length scale of 50–1500 nm, where E is the energy in the spherical shock. Shock-induced chemical reactions propagate farther with larger nanoparticles and farther in Teflon than in NC. The linear dependence of drxn on E is explained using a hydrodynamic model that assumes chemistry occurs when a pressure P is applied for a given time t, so that Pt=constant.
Properties of a metamaterial element: Analytical solutions and numerical simulations for a singly split double ring
Authors: Shamonin M. , Shamonina E. , Kalinin V. , Solymar L.
Published: March 2004

An equivalent circuit, consisting of bulk and distributed elements, is derived for describing the properties of a potential metamaterial element capable of providing negative effective permeability. It is the singly split double ring (SSDR), a special case of the split ring resonator (J. B. Pendry et al., IEEE Trans. Microwave Theory Tech. 47, 2075 (1999)), obtained when the gap capacitance in the inner ring is infinitely large. The variables are the inter-ring voltage and the currents flowing in the inner and outer rings. The excitation is assumed in the form of a spatially constant temporally varying magnetic field. The functions, showing the angular variation of the variables, are found by solving a set of differential equations with boundary conditions imposed at the position of the split. It is shown from the analytical solution that the SSDR can have resonant frequencies in the full spectrum from very low to very high frequencies. It is pointed out in particular that whenever the mean diameter of the ring is equal to an odd multiple of the half wavelength it is always possible to find a set of parameters which will give rise to resonance. As examples the resonant frequencies are determined for eight sets of parameters. Results are also derived by replacing the distributed circuit with a number of discrete circuits. It is finally shown that the results obtained from the equivalent circuit model are in excellent agreement with those derived from the MICRO-STRIPES numerical package which solves Maxwell’s equations in the time domain.
Far-infrared optical properties of SrLaAlO4 single crystal
Authors: Chen G. X. , Ge Y. J. , Bi C. Z. , Qiu X. G. , Zhao B. R.
Published: March 2004

Near-normal incidence infrared reflectivity spectra of (001) strontium lanthanum aluminate (SrLaAlO4) single crystal have been measured under different temperatures in the frequency region between 100 and 6500 cm−1. Five phonon modes are distinguished at low frequency while no spectral features were observed above 1000 cm−1. All the reflectivity spectra are fitted with the factorized form of the dielectric function. The dielectric properties of the SrLaAlO4 crystal in far-infrared region are presented and analyzed.
Investigations on alloy potential fluctuations in AlxGa1−xN epilayers using optical characterizations
Authors: Chung S. J. , Kumar M. Senthil , Lee H. J. , Suh E.-K.
Published: March 2004

The optical properties of AlxGa1−xN epilayers were extensively investigated by using photoluminescence, optical absorption, photocurrent, and persistent photoconductivity measurements. For the AlxGa1−xN epilayers containing high Al composition, we observed the Stokes shift, and an anomalous temperature-dependent photoluminescence and persistent photoconductivity effect. These results show a strong dependence on Al contents in AlxGa1−xN epilayers. The magnitude of the Stokes shift is close to the localization depth determined from the persistent photoconductivity decay kinetics. The increase in the degree of localized states with large Al compositions is due to alloy compositional fluctuations. Also, the S-shaped behavior and Stokes shift can be described in terms of localized states formed by alloy potential fluctuations in AlxGa1−xN epilayers.
On the differential gain in coupled quantum wells
Authors: Wartak M. S. , Weetman P.
Published: March 2004

We have analyzed the combined self-consistent and well coupling effects on differential gain in quantum wells within the self-consistent solution of the Poisson, Schrödinger, and 4×4 Luttinger–Kohn (LK) equations. The many-body effects of band-gap renormalization, Coulombic scattering interactions, and a non-Markovian distribution are also included. The analysis has been performed for a 1.55 μm InGaAsP/InP lattice-matched system grown in the [001] direction. The analysis shows that self-consistent effects significantly affect the differential gain.
High-power limitation of passive ring-resonator-coupled lasers in the presence of material nonlinearity
Authors: Bian Zhixi , Shakouri Ali , Liu Bin
Published: March 2004

We investigate the high-power limitation and stability of a passive ring-resonator-coupled laser (RCL). A detailed numerical analysis of the bidirectional transmission of the microring resonator in the presence of material nonlinearity is presented. The two-photon absorption is the limiting factor for high-power operation of the RCL.
Enhancement of electron injection in organic light-emitting devices using an Ag/LiF cathode
Authors: Wang X. J. , Zhao J. M. , Zhou Y. C. , Wang X. Z. , Zhang S. T. , Zhan Y. Q. , Xu Z. , Ding H. J. , Zhong G. Y. , Shi H. Z. , Xiong Z. H. , Liu Y. , Wang Z. J. , Obbard E. G. , Ding X. M. , Huang W. , Hou X. Y.
Published: March 2004

A LiF-buffered silver cathode has been used in organic light-emitting devices (OLEDs) with structure indium–tin–oxide/N,N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine (50 nm)/Alq3 (100 nm)/cathode. The efficiency of electron injection from the cathode is strongly dependent on the thickness of the LiF buffer layer. While a LiF layer thinner than 1.0 nm leads to higher turn-on voltage and decreased electroluminescent (EL) efficiency, a LiF layer of 3.0 nm significantly enhances the electron injection and results in lower turn-on voltage and increased EL efficiency. A brightness of 16 000 cd/m2 and EL efficiency of 4.8 cd/A can be achieved with an Ag/LiF cathode. This dependence of electron injection on the LiF thickness is quite different from that reported for OLEDs with a Al/LiF cathode, but can be well understood using the tunneling model.
Spin disorder scattering mechanism of ferromagnetic Ga1−xMnxAs layers on (100) GaAs substrates
Authors: Yoon I. T. , Kang T. W. , Kim K. H. , Kim D. J.
Published: March 2004

The temperature-dependent Hall resistivity and carrier concentrations of Ga1−xMnxAs epilayers grown on (100) semi-insulating GaAs substrates by molecular beam epitaxy have been investigated in the temperature range of 10–300 K. A Ga1−xMnxAs sample with x≈4.4% shows typical insulator behavior and Ga1−xMnxAs samples with x≈2.2 and 3.7 % show typical metallic behavior. A model taking into account ionized impurity and spin disorder scattering mechanisms was used to portray properly the observed features of the temperature-dependent Hall resistivity data. The value of the p-d exchange energy was J=59.4±0.5 and 71.9±0.5 eV Å3 for the samples with x≈2.2 and 3.7 %, respectively. Ionized impurity scattering dominates the entire temperature range, with a temperature-independent spin disorder scattering in the paramagnetic region. It was found that the spin disorder scattering mechanism had a strong temperature dependence on 1−T2 in the ferromagnetic region.
Optimized conditions for an enhanced coupling rate between Er ions and Si nanoclusters for an improved 1.54-μm emission
Authors: Gourbilleau F. , Levalois M. , Dufour C. , Vicens J. , Rizk R.
Published: March 2004

The present article deals with the optimized processing conditions leading to the highest density of Si nanoclusters which play the role of sensitizing centers for the nearby Er ions within a silica matrix. The layers were obtained by reactive magnetron sputtering under a plasma of Ar mixed to different rates of hydrogen, and were subsequently annealed at various temperatures. The increase of the dilution degree of the Ar plasma with hydrogen was found to multiply the nucleation sites whose density foreshadows that of the Si nanoclusters formed upon annealing. Both hydrogen content and annealing temperature govern the growth of the clusters. The maximum density of efficient sensitizing centers was obtained for hydrogen rate in the plasma of 50% and annealing at 900 °C. This has directly led to the enhancement of the coupling rate between the Si nanoclusters and the Er ions, as reflected by the ten times increase of the proportion of optically active ions, compared to that for standard conditions. In parallel, the lifetime emission of the active Er ions was found to continuously improve with the annealing temperature and has reached values exceeding 7 ms.
Spectroscopic analysis of electrical properties in polar semiconductors with over-damped plasmons
Authors: Nakashima S. , Harima H.
Published: March 2004

Raman and infrared reflection spectroscopies provide us information on electronic properties in polar semiconductors. In the present work, we have employed a dielectric function including the LO phonon damping in addition to the TO phonon damping and free carrier damping, and analyzed observed spectra of n-type 4H–SiC crystals. The carrier density and mobility obtained from the line shape analysis as the best fit parameters are compared with those obtained using the conventional classical dielectric function. It is recognized that the LO phonon damping has a significant effect on the determination of the carrier mobility and density.
Raman characterization of local electrical properties and growth process in modulation-doped 6H-SiC crystals
Authors: Nakashima S. , Harima H. , Ohtani N. , Katsuno M.
Published: March 2004

Raman microspectroscopy has been applied to the determination of free carrier distributions in modulation-doped 6H-SiC crystals in order to examine the process of the crystal growth. The spatial distribution of the carrier concentration and mobility is extracted from the line shape analysis of LO phonon plasmon coupled modes using a dielectric function which takes into account the LO-phonon damping. The residual time of dopants in a growth furnace after the shut-off of the doping gas is inferred from the carrier density profiles. The growth rate of the crystals and incorporation of the dopants are also studied. The relationship between the carrier mobility and carrier concentration is determined from these quantities at different positions in the sample. It is found that the LO-phonon damping has much influence on the carrier damping and changes linearly with the carrier concentration in a range below ∼2×1018 cm−3.
Field emission and photofluorescent characteristics of zinc oxide nanowires synthesized by a metal catalyzed vapor-liquid-solid process
Authors: Li Seu Yi , Lin Pang , Lee Chia Ying , Tseng Tseung Yuen
Published: March 2004

The ZnO nanowires synthesized by vapor-liquid-solid growth mechanism with Cu and Au as the catalyst were investigated. The principal differences in morphology between Cu and Au catalyzed ZnO nanowires are observed and lead to significant differences in their field emission and photofluorescent characteristics. The Cu catalyzed ZnO nanowires with a high-quality wurtzite structure were grown vertically on p-type Si(100) substrate along [0002] direction. A strong ultraviolet emission at 381 nm is observed. These ZnO nanowires show excellent field emission properties with turn-on field of 0.83 V/μm and corresponding current density of 25 μA/cm2. The emitted current density of the ZnO nanowires is 1.52 mA/cm2 at a bias field of 8.5 V/μm. The large field emission area factor, β arising from the morphology of the nanowire field emitter, is partly responsible for the good emission characteristics. The ZnO nanowires with high emission current density and low turn-on field are expected to be used in field emission flat panel display.
Method using the primary knock-on atom spectrum to characterize electrical degradation of monocrystalline silicon solar cells by space protons
Authors: Alurralde M.
Published: March 2004

In this work, a method to calculate the equivalent dose for orbits in which the main part of the damage is due to protons, is presented. The method compares the primary knock-on atom distribution and the depth dependence of damage for the cases of space-proton irradiation and laboratory irradiation of silicon solar cells, as obtained from simulations using the transport of ions in matter code. It is shown that, in order to simulate the right depth damage distribution in space for a solar cell covered by glass, it is convenient to irradiate the cell from the backside with a proton energy dependent of the glass thickness. The method is applied to predict the electrical degradation of a silicon solar cell at the end of its life in a low altitude orbit and to estimate the damage produced as consequence of the October 19, 1989 solar proton event. The results obtained are compared with predictions of the U.S. Jet Propulsion Laboratory method.
Chemical reaction mediated by excited states of Si nanocrystals—Singlet oxygen formation in solution
Authors: Fujii Minoru , Usui Motofumi , Hayashi Shinji , Gross Egon , Kovalev Dmitri , Künzner Nicolei , Diener Joachim , Timoshenko Victor Yu.
Published: March 2004

Formation of singlet oxygen in solution by using Si nanocrystals as photosensitizers has been demonstrated. It has been shown that the absorption band of 1,3-diphenylisobenzofuran (DPBF) in benzene centered at 416 nm decreases by irradiating green (514.5 nm) or red (632.8 nm) light if fresh porous Si powder is dispersed in the solution. The decomposition of DPBF was observed only when fresh porous Si was irradiated by light, i.e., without light irradiation no effects were observed. Furthermore, the effect was drastically suppressed if porous Si powder was annealed and a monolayer of oxide was formed on the surface of nanocrystals. The rate of the decomposition of DPBF was accelerated when the solution was bubbled by oxygen gas. These results indicate that electronic excitation of Si nanocrystals is transferred to molecular oxygen dissolved in solution, resulting in the formation of singlet oxygen. Generated singlet oxygen reacts with DPBF (1,4-cycloaddition reaction), forming endoperoxides, which in turn decompose to yield irreversible products. In addition to the singlet-oxygen-mediated decomposition of DPBF, the possibility of direct reaction between triplet excited states of Si nanocrystals and DPBF is discussed.
Large second-order optical nonlinearities of fresnoite-type crystals in transparent surface-crystallized glasses
Authors: Takahashi Yoshihiro , Benino Yasuhiko , Fujiwara Takumi , Komatsu Takayuki
Published: March 2004

Second-order optical nonlinearities, d33 values, of fresnoite-type crystals, Ba2TiSi2O8 (BTS), Sr2TiSi2O8 (STS), and Ba2TiGe2O8 (BTG), have been evaluated by measurements on transparent surface-crystallized glasses consisting of fresnoite-type crystals with the Maker fringe technique. It is found that all these crystals show large d33 values, i.e., d33=13±2 pm/V for BTS, d33=7.2±0.4 pm/V for STS, and d33=22±3 pm/V for BTG. In particular, it is emphasized that BTG has an extremely large d33 value, comparable to LiNbO3 single crystal. It is clarified that the d33 value of fresnoite-type crystals increases with increasing c/a ratio in the lattice parameters of their structures. It is suggested that the increase in the c/a ratio results in the elongation of TiO5 pyramidal units along the c axis, consequently enhancing spontaneous polarizations in the fresnoite-type structure. Transparent surface-crystallized glasses consisting of fresnoite-type crystals are candidates for nonlinear optical materials.
On the vaporization of Teflon and heated compound-materials in ablation-controlled arcs
Authors: Zaghloul Mofreh R.
Published: March 2004

In this article, we highlight and discuss overlooked physics and point out inaccuracies in a recently developed kinetic model of Teflon ablation caused by plasma in ablation controlled discharges. The effects of internal degrees of freedom on the energy flux conservation and the change of the average molecular mass with temperature on the mass, momentum, and energy fluxes conservation are investigated. Modifications to the governing set of equations, to account for these two effects, are derived and presented. Simple assessment of these effects showed the significance of including such modifications in the study of vaporization of Teflon and compound materials in ablation controlled arcs.
Nanoscratch evaluation of adhesion and cohesion in SiC/low-k/Si stacked layers
Authors: Ye J. , Kojima N. , Ueoka K. , Shimanuki J. , Nasuno T. , Ogawa S.
Published: March 2004

A nanoscratch method combined with atomic force microscope and transmission electron microscope observations was used to estimate the adhesive and cohesive strengths of SiC/low-k/Si stacked layers with the aim of correlating the fracture strength with the results of chemical mechanical polishing (CMP) tests. It was found that the friction coefficient was an effective signal for detecting the occurrence of failure in the microstructure during the scratch test. Fracture strength was characterized using the critical normal load at the first abrupt decrease in the friction coefficient. A specimen (A) having a low-k layer with a lower modulus and lower hardness displayed a lower friction coefficient, a lower critical load, and ductile adhesive failure resulting from delamination at the interface between the low-k layer and Si substrate. In contrast, a specimen (B) having a low-k layer with a higher modulus and higher hardness exhibited a higher friction coefficient, a higher critical load and brittle cohesive failure that occurred in the low-k layer. It was also found that specimen A always had a higher critical load level than specimen B regardless of the scratching speed, loading rate, and stylus radius. Dependence of the critical load on specimen thickness was also examined. These nanoscratch measurements corresponded with the results of the CMP process, which suggests that the process characteristics can be predicted with this nanoscratch method.
Formation and evolution of luminescent Si nanoclusters produced by thermal annealing of SiOx films
Authors: Iacona Fabio , Bongiorno Corrado , Spinella Corrado , Boninelli Simona , Priolo Francesco
Published: March 2004

Si nanoclusters embedded in SiO2 have been produced by thermal annealing of SiOx films prepared by plasma enhanced chemical vapor deposition. The structural properties of the system have been investigated by energy filtered transmission electron microscopy (EFTEM). EFTEM has evidenced the presence of a relevant contribution of amorphous nanostructures, not detectable by using the more conventional dark field transmission electron microscopy technique. By also taking into account this contribution, an accurate quantitative description of the evolution of the samples upon thermal annealing has been accomplished. In particular, the temperatures at which the nucleation of amorphous and crystalline Si nanoclusters starts have been determined. Furthermore, the nanocluster mean radius and density have been determined as a function of the annealing temperature. Finally, the optical and the structural properties of the system have been compared, to demonstrate that the photoluminescence properties of the system depend on both the amorphous and crystalline clusters.
Splitting of the recombination zone in organic light emitting diodes by dye doping
Authors: Berner D. , Nuesch F. , Tutiš E. , Ma C. , Wang X. , Zhang B. , Zuppiroli L.
Published: March 2004

In organic light emitting devices, doping of the electroactive organic layer with highly luminescent molecules has been shown to considerably increase device performance and lifetime. In most cases, the doping molecule does not act as a charge donor or an acceptor as in classical semiconductors, but is used to enhance light emission and to tune the emission color. By using the laser dye derivative 4-(dicyanomethylene)-2-methyl-6-{2-[(4-diphenylamino)phenyl]ethyl}-4H-pyran as dopant in a standard organic light emitting device, we have achieved highly efficient red to yellow emission depending on doping concentration. Furthermore the emission color changes with increasing current density. Using device model simulations, we have found that this color change is caused by the splitting of the recombination zone into two zones due to a decrease of the electron mobility inside the doped area.
Size-dependent effective modulus of elastic composite materials: Spherical nanocavities at dilute concentrations
Authors: Yang Fuqian
Published: March 2004

The effect of surface energy on the effective elastic properties was analyzed for elastic composite materials containing spherical nanocavities at dilute concentration. Closed-form solutions of the effective shear modulus and bulk modulus were obtained, which turn out to be a function of the surface energy and size of the nanocavity. The dependence of the elastic response on size of the nanocavity in composite materials is different from the classic results obtained in the linear elasticity theory, suggesting the importance of the surface energy of the nanocavity in analyzing the deformation of nanoscale structures.