bluuebunny/crossref_metadata_2025 huggingface dataset:
Superconductivity, critical current density, and flux pinning in MgB2−x(SiC)x/2 superconductor after SiC nanoparticle doping
Authors: Dou S. X. , Pan A. V. , Zhou S. , Ionescu M. , Wang X. L. , Horvat J. , Liu H. K. , Munroe P. R.
Published: July 2003

We investigated the effect of SiC nanoparticle doping on the crystal lattice structure, critical temperature Tc, critical current density Jc, and flux pinning in MgB2 superconductor. A series of MgB2−x(SiC)x/2 samples with x=0–1.0 were fabricated using an in situ reaction process. The contraction of the lattice and depression of Tc with increasing SiC doping level remained rather small most likely due to the counterbalancing effect of Si and C co-doping. The high level Si and C co-doping allowed the creation of intragrain defects and highly dispersed nanoinclusions within the grains which can act as effective pinning centers for vortices, improving Jc behavior as a function of the applied magnetic field. The enhanced pinning is mainly attributable to the substitution-induced defects and local structure fluctuations within grains. A pinning mechanism is proposed to account for different contributions of different defects in MgB2−x(SiC)x/2 superconductors.
Temperature dependence of nonradiative recombination in low-band gap InxGa1−xAs/InAsyP1−y double heterostructures grown on InP substrates
Authors: Gfroerer T. H. , Priestley L. P. , Fairley M. F. , Wanlass M. W.
Published: July 2003

We have used photoexcitation-dependent radiative efficiency measurements to investigate the rates of defect-related, radiative, and Auger recombination in lattice-matched InxGa1−xAs/InAsyP1−y double heterostructures on InP substrates. Temperature dependence is used to discern the underlying mechanisms responsible for the nonradiative recombination processes. We find that defect-related recombination decreases with an increase in the temperature when the epistructure is lattice matched to the substrate (x=0.53). In contrast, when the epistructure is lattice mismatched to the substrate, defect-related recombination increases slowly with the temperature. The difference between the lattice-matched and mismatched cases is related to fundamental changes in the defect-related density of states function. The temperature dependence in the lattice-mismatched structures is attributed to two competing effects: wider carrier diffusion, which augments the capture rate, and thermally activated escape, which reduces the occupation of shallow traps. The band gap and temperature dependence of the Auger rate demonstrate that the conduction to heavy hole band/splitoff to heavy hole band mechanism generally dominates Auger recombination in undoped low-band gap InxGa1−xAs. With this interpretation, our results give a spin–orbit valence split-off band effective mass of mso=(0.12±0.02)m0.
Analysis of the composition-induced transition from relaxor to ferroelectric state in PbFe2/3W1/3O3–PbTiO3 solid solutions
Authors: Mitoseriu Liliana , Stancu Alexandru , Fedor Cristina , Vilarinho Paula M.
Published: July 2003

The low-field dielectric response of the relaxor-ferroelectric (1−x)PbFe2/3W1/3O3–xPbTiO3 ceramics with various x, was investigated. The permittivity data were analyzed with empirical laws that describe the diffuse phase transitions in relaxors. A change of the character of the phase transition was found with increasing x, from a total diffuse, characteristic of relaxors, to a sharp one, typical of ferroelectrics. The deviations from the Curie–Weiss law of the dielectric constant data in the paraelectric phase were used to calculate a local order parameter within a modified-Landau theory for relaxors. The nonzero values of the local order parameter far above the Curie region indicate the thermal stability of the polar nanoregions in the relaxor state. The temperature dependence of the local order parameter clearly shows the evolution of the system from a short range ordered to a long range ordered ferroelectric, with increasing the PbTiO3 addition.
Factorization method and the supersymmetric monopole harmonics
Authors: Mezincescu G. Andrei , Mezincescu Luca
Published: July 2003

We use the general N=1 supersymmetric formulation of one dimensional sigma models on nontrivial manifolds and its subsequent quantization to formulate the classical and quantum dynamics of the N=2 supersymmetric charged particle moving on a sphere in the field of a monopole. The factorization method is accommodated with the general covariance and it is used to integrate the corresponding system.
Monopoles, affine algebras and the gluino condensate
Authors: Davies N. Michael , Hollowood Timothy J. , Khoze Valentin V.
Published: July 2003

We examine the low-energy dynamics of four-dimensional supersymmetric gauge theories and calculate the values of the gluino condensate for all simple gauge groups. By initially compactifying the theory on a cylinder we are able to perform calculations in a controlled weakly coupled way for a small radius. The dominant contributions to the path integral on the cylinder arise from magnetic monopoles which play the role of instanton constituents. We find that the semi-classically generated superpotential of the theory is the affine Toda potential for an associated twisted affine algebra. We determine the supersymmetric vacua and calculate the values of the gluino condensate. The number of supersymmetric vacua is equal to c2, the dual Coxeter number, and in each vacuum the monopoles carry a fraction 1/c2 of topological charge. As the results are independent of the radius of the circle, they are also valid in the strong coupling regime where the theory becomes decompactified. For gauge groups SU(N), SO(N) and USp(2N) our results for the gluino condensate are in precise agreement with the “weak coupling instanton” expressions (and not with the “strong coupling instanton” calculations). For the exceptional gauge groups we calculate the values of the gluino condensate for the first time.
The geodesic approximation for lump dynamics and coercivity of the Hessian for harmonic maps
Authors: Haskins M. , Speight J. M.
Published: July 2003

The most fruitful approach to studying low energy soliton dynamics in field theories of Bogomol’nyi type is the geodesic approximation of Manton. In the case of vortices and monopoles, Stuart has obtained rigorous estimates of the errors in this approximation, and hence proved that it is valid in the low speed regime. His method employs energy estimates which rely on a key coercivity property of the Hessian of the energy functional of the theory under consideration. In this article we prove an analogous coercivity property for the Hessian of the energy functional of a general sigma model with compact Kähler domain and target. We go on to prove a continuity property for our result, and show that, for the CP1 model on S2, the Hessian fails to be globally coercive in the degree 1 sector. We present numerical evidence which suggests that the Hessian is globally coercive in a certain equivariance class of the degree n sector for n⩾2. We also prove that, within the geodesic approximation, a single CP1 lump moving on S2 does not generically travel on a great circle.
Phenomenology of colloidal crystal electrophoresis
Authors: Medebach Martin , Palberg Thomas
Published: July 2003

We studied the motion of polycrystalline solids comprising of charged sub-micron latex spheres suspended in deionized water. These were subjected to a low frequency alternating square wave electric field in an optical cell of rectangular cross section. Velocity profiles in X and Y direction were determined by Laser Doppler Velocimetry. The observed complex flow profiles are time dependent due to the combined effects of electro-osmosis, electrophoresis, crystal elasticity, and friction of the crystals at the cell wall. On small time scales elastic deformation occurs. On long time scales channel formation is observed. At intermediate times steady state profiles are dominated by a solid plug of polycrystalline material moving in the cell center. At large field strengths the plug shear melts. Mobilities in the shear molten state are on the order of (6.5±0.5) 10−8 m2 V−1 s−1 and connect continuously with those of the equilibrium fluid. The apparent mobility of the plug is much larger than of the fluid and like the mobility of the fluid decreases with increasing particle number density. We qualitatively attribute the accelerated motion of the plug to an incomplete exposure to the electro-osmotic flow profile.
Coulomb focusing and quantum effects on the electron-ion bremsstrahlung processes in plasmas
Authors: Jung Young-Dae
Published: July 2003

The electron-ion bremsstrahlung process in a kappa-Maxwellian distribution plasma is investigated. The Coulomb correction effect on the bremsstrahlung emission rate from the kappa-Maxwellian distribution plasma is investigated as a function of the radiation photon energy for T∥>T⊥, T∥=T⊥, and T∥<T⊥, where T∥ and T⊥ are the parallel and perpendicular temperatures, respectively. The results show that for low photon energies the Coulomb focusing effects are found to be less significant for low spectral indices κ when T∥T⊥ are more significant than those for T∥<T⊥. In addition, the plasma screening and quantum effects on the nonrelativistic electron-ion bremsstrahlung process in strongly coupled semiclassical plasmas are also investigated using the effective screened potential model taking into account the plasma screening effect, as well as the quantum-mechanical effect. The analytic form of the bremsstrahlung radiation cross section in strongly coupled semiclassical plasmas is obtained as a function of the radiation photon energy, electron kinetic energy, thermal de Broglie wavelength, and Debye length. It is found that the quantum effect on the bremsstrahlung radiation cross section is slightly increased with increasing radiation photon energy. The quantum effect on the bremsstrahlung cross section is also found to be more important than the plasma screening effect on the bremsstrahlung process in strongly coupled semiclassical plasmas.
Comment on “On the consistency of the collisionless sheath model” [Phys. Plasmas 9, 4427 (2002)]
Authors: Riemann K.-U.
Published: July 2003

Recently, Godyak and Sternberg published a Letter [Phys. Plasmas 9, 4427 (2002)] claiming that the collisionless sheath model found in textbooks is internally inconsistent. The arguments of this paper are based on a misinterpretation of the plasma sheath concept. The conclusions are wrong and must be firmly rejected.
CP
N−1
harmonic maps and the Weierstrass problem
Authors: Grundland A. M. , Zakrzewski W. J.
Published: July 2003

A Weierstrass-type system of equations corresponding to the CPN−1 harmonic maps is presented. The system constitutes a further generalization of our previous construction [J. Math. Phys. 44, 328 (2003)]. It consists of four first order equations for three complex functions which are shown to be equivalent to the CPN−1 harmonic maps. When the harmonic maps are holomorphic (or antiholomorphic) one of the functions vanishes and the system reduces to the previously given generalization of the Weierstrass problem. We also discuss a possible interpretation of our results and show that in our new case the induced metric is proportional to the total energy density of the map and not only to its holomorphic part, as was the case in the previous generalizations.
Fractional charge definitions and conditions
Authors: Goldhaber Alfred Scharff
Published: July 2003

The phenomenon of fractional charge has come to prominence in recent decades through theoretical and experimental discoveries of isolable objects which carry fractions of familiar charge units—electric charge Q, spin S, baryon number B and lepton number L. It is shown here on the basis of a few simple assumptions that all these effects may be described using a generalized version of charge renormalization for locally conserved charges, in which many-body correlations can produce familiar adiabatic, continuous renormalization, and in some circumstances nonadiabatic, discrete renormalization. The fractional charges may be carried either by fundamental particles or by fundamental solitons. This excludes nontopological solitons and also skyrmions: The only known fundamental solitons in three or fewer space dimensions d are the kink (d=1), the vortex (d=2), and the magnetic monopole (d=3). Further, for a charge which is not intrinsically coupled to the topological charge of a soliton, only the kink and the monopole may carry fractional values. The same reasoning enforces fractional local values of B−L for electrically charged elementary particles.
Enhanced inverse bremsstrahlung heating rates in a strong laser field
Authors: Brantov A. , Rozmus W. , Sydora R. , Capjack C. E. , Bychenkov V. Yu. , Tikhonchuk V. T.
Published: July 2003

Test particle studies of electron scattering on ions in an oscillatory electromagnetic field have shown that standard theoretical assumptions of small angle collisions and phase independent orbits are incorrect for electron trajectories with drift velocities smaller than quiver velocity amplitude. This leads to significant enhancement of the electron energy gain and the inverse bremsstrahlung heating rate in strong laser fields. Nonlinear processes such as Coulomb focusing and correlated collisions of electrons being brought back to the same ion by the oscillatory field are responsible for large angle, head-on scattering processes. The statistical importance of these trajectories has been examined for mono-energetic beam-like, Maxwellian and highly anisotropic electron distribution functions. A new scaling of the inverse bremsstrahlung heating rate with drift velocity and laser intensity is discussed.
Correlated scanning Kelvin probe and conductive atomic force microscopy studies of dislocations in gallium nitride
Authors: Simpkins B. S. , Yu E. T. , Waltereit P. , Speck J. S.
Published: July 2003

Scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (C-AFM) have been used to image surfaces of GaN grown by molecular beam epitaxy. Detailed analysis of the same area using both techniques allowed imaging and comparison of both surface potential variations arising from the presence of negatively charged threading dislocations and localized current leakage paths associated with dislocations. Correlations between the charge state of dislocations, conductivity of current leakage paths, and dislocation type were thereby established. Analysis of correlated SKPM and C-AFM images revealed a density of negatively charged features of ∼3×108 cm−2 and a localized current leakage path density of ∼3×107 cm−2, with approximately 25% of the leakage paths spatially correlated with negatively charged dislocation features. Based on correlated topography and previous studies quantifying the densities of edge, screw, and mixed dislocations, our results suggested that dislocations having an edge component behave as though negatively charged while pure screw dislocations are solely responsible for the observed leakage paths and are uncharged.
Carrier kinetics in quantum dots through continuous wave photoluminescence modeling: A systematic study on a sample with surface dot density gradient
Authors: de Sales F. V. , Cruz J. M. R. , da Silva S. W. , Soler M. A. G. , Morais P. C. , da Silva M. J. , Quivy A. A. , Leite J. R.
Published: July 2003

A systematic study is presented of continuous wave (cw) photoluminescence (PL) of self-assembled quantum dots (QDs) grown on GaAs (001) by molecular-beam epitaxy as a function of excitation intensity and QD density. The sample used in this work was grown under nonisotropic indium flux that resulted in a QD density gradient across the sample surface ranging from 0 to 1.8×1011 cm−2. The carrier kinetics in the sample is described by a set of coupled rate equations through which the cw PL data from the GaAs barrier, wetting layer (WL), and QDs were simulated as a function of the excitation intensity and QD density. By comparing the PL data with our simulations we infer that carrier capture into the QD occurs directly from GaAs barrier. Auger and phonon-assisted carrier capture from the WL were found to give negligible contribution. With an increase of the QD density we observe an increase of the nonradiative recombination rates of the barrier and at the WL, which we tentatively correlate with the increase of surface and interface recombination rates. Furthermore, we noticed that the variation of the coefficients used in our model in order to simulate the experimental results over the entire QD density range studied is strongly correlated with the changes in sample morphology. In particular, the behavior of the capture rate as a function of the QD density allowed us to determine a QD capture cross section from the GaAs barrier equal to 1.5×10−11 cm2.
Multiple Si=O bonds at the silicon cluster surface
Authors: Luppi Marcello , Ossicini Stefano
Published: July 2003

A first-principle investigation of the effects of multiple Si=O bonds at the surface of silicon-based clusters with different sizes has been carried out. Total-energy pseudopotential calculations within density functional theory have been applied varying systematically the number of Si=O bonds at the clusters surface. A nonlinear reduction of the energy gap with the Si=O bond number is found. A sort of saturation limit is displayed, providing a consistent interpretation of the photoluminescence redshift observed in oxidized porous silicon samples. Moreover, our results help to clarify the very recent findings on the single silicon quantum dot photoluminescence bandwidth.
Spectral components and their contributions to the 1.5 μm emission bandwidth of erbium-doped oxide glass
Authors: Sun Jiangting , Zhang Jiahua , Luo Yongshi , Lin Jiuling , Song Hongwei
Published: July 2003

In this article, the 1.5 μm emission spectra corresponding to the I13/24→I15/24 transition of Er3+ in oxide glass are studied within the temperature from 11 to 300 K. The spectral components emitting from the lowest and upper Stark levels of the I13/24 state are analyzed and separated. The effect of the spectral components on the 1.5 μm emission bandwidth is investigated. The results indicate that to search a host with higher spontaneous emission probability of the upper Stark levels of the I13/24 state for Er3+ ions is very important to broadening of the 1.5 μm emission band of Er3+. An equivalent model of the four-level system is presented and applied to explain the spectral shape and temperature characteristics of the 1.5 μm emission band. According to the theory of McCumber, we transform the absorption spectrum into the emission spectrum, the shape of which fits well to that measured.
Hydrogen-induced silicon wafer splitting
Authors: Yang Fuqian
Published: July 2003

The wafer splitting in the “smart-cut” process was analyzed by using the linear elasticity theory and thermodynamics under the assumption that the process is controlled by the nucleation and growth of microcracks. It was found that the nucleation size for the initiation of microcracks in the hydrogen-implanted region decreases with the amount of the implanted atomic hydrogen. By considering the annihilation of the implanted atomic hydrogen to form molecular hydrogen, the growth behavior of a microcrack was calculated, from which the maximum crack size was found to be proportional to the square root of the temperature and the amount of the implanted hydrogen. The growth rate of the microcrack increases with temperature and the amount of implanted hydrogen. A lower bound estimate of the amount of molecular hydrogen needed for the growth of the microcrack was established, which is a function of the crack size and fracture toughness of silicon.
Sulfur-passivation mechanism analysis of GaInAsSb photodetectors
Authors: Li Zhihuai , Xia Guanqun , Wu Binhe , Huang Wenkui , Cheng Zongquan
Published: July 2003

Sulfur passivation of GaInAsSb photodetectors was studied in terms of Auger electron spectroscopy and x-ray photoelectron spectroscopy. The experimental results showed that Sb–S and In–S bonds exist on the sulfur-passivated GaInAsSb surfaces. The reverse dark current of the photodetectors was reduced and the peak detectivity reached 2.83×1010 cm Hz1/2 W−1 after passivation.
Theory of the mode stabilization mechanism in concave-micromirror-capped vertical-cavity surface-emitting lasers
Authors: Park Si-Hyun , Park Yeonsang , Jeon Heonsu
Published: July 2003

We have investigated theoretically the transverse mode stabilization mechanism in oxide-confined concave-micromirror-capped vertical-cavity surface-emitting lasers (CMC-VCSELs) as reported by Park et al. [Appl. Phys. Lett. 80, 183 (2002)]. From detailed numerical calculations on a model CMC-VCSEL structure, we found that mode discrimination factors appear to be periodic in the micromirror layer thickness with a periodicity of λ/2. We also found that there are two possible concave micromirror structures for the fundamental transverse mode laser operation. These structures can be grouped according to the thickness of the concave micromirror layer: whether it is an integer or a half-integer multiple of λ/2. The optimal micromirror curvature radius differs accordingly for each case. In an optimally designed CMC-VCSEL model structure, the fundamental transverse mode can be favored as much as 4, 8, and 13 times more strongly than the first, second, and third excited modes, respectively.
Density and atomic number measurements with spectral x-ray attenuation method
Authors: Heismann B. J. , Leppert J. , Stierstorfer K.
Published: July 2003

X-ray attenuation measurements are widely used in medical and industrial applications. The usual results are one- to three-dimensional representations of the attenuation coefficient μ(r). In this paper, we present the ρZ projection algorithm for obtaining the density ρ(r) and atomic number Z(r) with an energy-resolving x-ray method. As input data the algorithm uses at least two measurements μ1,μ2,… with different spectral weightings of the source spectrum S(E) and/or detector sensitivity D(E). Analytically, ρ is a function of μ1−cμ2, c=const, and Z is a function of μ1/μ2. The full numerical treatment yields ρ(μ1,μ2) and Z(μ1,μ2) with S(E) and D(E) as commutative parametric functions. We tested the method with dual-energy computed tomography measurements of an organic sample and a set of chemical solutions with predefined ρ and Z. The resulting images show ρ and Z as complementary information: The density ρ reflects the morphology of the objects, whereas the atomic number Z=number of electrons/atom describes the material distribution. For our experimental setup we obtain an absolute precision of 0.1 for Z and 20 mg/cm3 for ρ. The ρZ projection can potentially lead to these classes of quantitative information for various scientific, industrial, and medical applications.
Transport of carriers in organic light-emitting devices fabricated with a p-phenylenevinylene-derivative copolymer
Authors: Manabe Kaoru , Hu Wenping , Matsumura Michio , Naito Hiroyoshi
Published: July 2003

The majority carriers of polymer-based light-emitting diodes are often considered to be holes. However, we demonstrate that the majority carriers of p-phenylenevinylene-copolymer-based devices, which show very high emission efficiency, are electrons. This conclusion is attained by comparing the current-voltage characteristics of these devices with those of electron-only and hole-only devices. The current due to electrons is proportional to the fourth power of the voltage. This result is interpreted in terms of the space-charge-limited conduction of electrons with field-dependent carrier mobility. The transport of holes in the hole-only devices is also attributed to the same mechanism. The mobility of holes under the normal operational conditions of the devices is estimated to be lower than that of electrons by an order of magnitude. This large difference in the mobility leads to carrier recombination near the interface between the copolymer and the anode, and thus to the high emission efficiency of the light-emitting devices.
Measurements of a component of the piezo-optic tensor of Si by reflectance difference spectroscopy
Authors: Mori Takahiro , Kumagai Naoto , Hanada Takashi , Yao Takafumi , Yasuda Tetsuji
Published: July 2003

The component P̃44(ω) of the piezo-optic tensor of Si was measured by reflectance difference spectroscopy (RDS). Taking advantage of the high sensitivity of RDS, the measurements were carried out under a much lower stress field compared to the previous report using ellipsometry. The spectra measured at 2.1 MPa by RDS agree well with literature data obtained at 2 GPa by ellipsometry.
Hexagonal circle patterns with constant intersection angles and discrete Painlevé and Riccati equations
Authors: Agafonov S. I. , Bobenko A. I.
Published: July 2003

Hexagonal circle patterns with constant intersection angles mimicking holomorphic maps zc and log(z) are studied. It is shown that the corresponding circle patterns are immersed and described by special separatrix solutions of discrete Painlevé and Riccati equations. The general solution of the Riccati equation is expressed in terms of the hypergeometric function. Global properties of these solutions, as well as of the discrete zc and log(z), are established.
Integrable systems and reductions of the self-dual Yang–Mills equations
Authors: Ablowitz M. J. , Chakravarty S. , Halburd R. G.
Published: July 2003

Many integrable equations are known to be reductions of the self-dual Yang–Mills equations. This article discusses some of the well known reductions including the standard soliton equations, the classical Painlevé equations and integrable generalizations of the Darboux–Halphen system and Chazy equations. The Chazy equation, first derived in 1909, is shown to correspond to the equations studied independently by Ramanujan in 1916.
Time-resolved measurements of the shock-compression response of Mo+2Si elemental powder mixtures
Authors: Vandersall Kevin S. , Thadhani Naresh N.
Published: July 2003

The shock-compression response of Mo+2Si elemental powder mixtures was investigated using instrumented experiments in the velocity range of 500 m/s to 1 km/s. The experiments employed polyvinyl difluoride stress gauges placed at the front and rear surfaces of the powder mixtures to determine the crush strength, densification history, and shock-induced reaction initiation characteristics. Experiments performed on ∼58% dense Mo+2Si powder mixtures at input stresses less than 4 GPa showed characteristics of powder densification and dispersed propagated wave stress profiles with rise time >∼40 ns. At input stress between 4 and 6 GPa, the powder mixtures showed a shock-compression response following the Hugoniot of the solid-density mixture. In the stress regime of 6–7 GPa, shock-induced melting of silicon was observed, which appears to inhibit a shock-induced chemical reaction on the time scale of the time-resolved measurements. The results of the present work on Mo-Si, taken in conjunction with prior work on the Nb-Si and Ti-Si systems, illustrate that premature melting of silicon and its capillary flow can limit the deformation and mixing between reactants, thereby inhibiting the initiation of “shock-induced” chemical reactions.
Plane-wave model for electromagnetic power absorption in biological tissues
Authors: Soldea D. F. , Razansky D. , Einziger P. D.
Published: July 2003

The phenomenon of electromagnetic power absorption in biological tissues has recently become of increased scientific and public interest, particularly in the areas of cellular communication and hyperthermia systems. Electromagnetic power absorption in biological tissues is a well-known phenomenon. Its evaluation requires, in general, a solution of the three-dimensional frequency-dependent wave equation in complex configurations, which may necessitate quite massive analytical and numerical efforts. Herein, we focus on an elementary plane-wave model. The model establishes a tight estimate on the optimal (maximal/minimal) power absorption in realistic microwave configurations, in particular, cellular communications safety assessments. Furthermore, the absorption efficiency as well as the source impedance are obtained via explicit closed-form expressions, leading to an explicit maximal power absorption criterion for highly lossy tissues. The results depend continuously and explicitly on the physical parameters of biological tissues as well as on the plane-wave incidence angle and excitation source location. They are shown to be closely related to electromagnetic specific absorption rate estimations in biological tissues. In view of the well-known analogy between plane waves and rectangular waveguide modes, our results can be applied to optimization schemes of microwave heating and waveguide based therapeutic hyperthermia systems.
Morphological evolution of InGaN/GaN quantum-well heterostructures grown by metalorganic chemical vapor deposition
Authors: Ting S. M. , Ramer J. C. , Florescu D. I. , Merai V. N. , Albert B. E. , Parekh A. , Lee D. S. , Lu D. , Christini D. V. , Liu L. , Armour E. A.
Published: July 2003

The morphological evolution of InGaN/GaN multiple-quantum wells (MQWs) grown by metalorganic chemical vapor deposition has been examined by atomic force microscopy and cross-sectional transmission electron microscopy. We have determined that GaN barrier growth at low temperature (∼800 °C) in a H2-free environment results in a microstructure that consists of not only V-defects, but also inclusions embedded within V-defects that originate at the first InGaN-to-GaN growth interface. Propagation of the inclusions results in progressive deterioration of the surface morphology and reduced MQW thermal stability as quantum-well periods are added. Raising the GaN barrier growth temperature to 900 °C or adding H2 suppresses inclusion propagation entirely and preserves two-dimensional step-flow growth mode, resulting in superior morphology and higher thermal stability.
Dependence of type II “W” mid-infrared photoluminescence and lasing properties on growth conditions
Authors: Canedy C. L. , Bewley W. W. , Kim C. S. , Kim M. , Vurgaftman I. , Meyer J. R.
Published: July 2003

We have studied how the photoluminescence (PL) and lasing characteristics of mid-IR type II “W” [InAs/Ga1−xInxSb/InAs/AlAsySb1−y] structures depend on the molecular beam epitaxy growth conditions. The growth temperature that yields the highest PL intensities and narrowest PL lines is found to be in the ≈480–510 °C range, which is considerably higher than the temperatures used in previous type II antimonide laser growths. Also contrary to earlier reports is our observation that using migration-enhanced epitaxy to force an InSb-like interface bond type is detrimental to the laser performance. The PL intensity at 78 K is found to correlate closely with the PL intensities and linewidths at all temperatures, as well as the lasing linewidths, thresholds, and efficiencies. Even though the tested laser structures were grown at a nonoptimal lower substrate temperature of 425 °C, they nonetheless yielded much better morphology, better-resolved x-ray features, stronger PL intensities, narrower PL linewidths, lower lasing thresholds (<200 W/cm2 at 78 K), lower internal losses (<5 cm−1 at 78 K), and longer Shockley–Read lifetimes (>20 ns at 78 K) than any previous antimonide lasers grown on the present Riber 32P molecular beam epitaxy system.
Ion energy distribution and optical measurements in high-density, inductively coupled C4F6 discharges
Authors: Benck Eric C. , Goyette Amanda , Wang Yicheng
Published: July 2003

Hexafluoro-1,3 butadiene (C4F6) is a potential etching gas with a very low global warming potential for the manufacturing of semiconductors, unlike commonly used fluorocarbon gases such as CF4 and c−C4F8. We report ion energy distributions, relative ion intensities and absolute total ion current densities measured at the edge of an inductively coupled gaseous electronics conference radio-frequency reference cell for discharges generated in pure C4F6 and C4F6:Ar mixtures. In addition, the ratio of radical densities relative to CF measured using submillimeter absorption spectroscopy and optical emission spectroscopy measurements are presented. These measurements of the C4F6 plasmas were made for several different gas pressures (0.67, 1.33, and 2.66 Pa) and gas mixture ratios (25%, 50%, 75%, and 100% C4F6 volume fractions).
Laser ablation of Mg, Cu, and Pb using infrared and ultraviolet low-fluence lasers
Authors: Qaisar M. S. , Pert G. J.
Published: July 2003

The modified two-dimensional hydrodynamic code POLLUX has been used to simulate the ablation of magnesium, copper, and lead targets in the early stage of expansion (<100 ns) by 30 ns, 0.248 μm KrF excimer and 1.064 μm Nd–YAG lasers at fluences of 3–10 J/cm2. The results of magnesium ablation using KrF laser with different fluences are compared and found to be in reasonably good agreement with the experimental results. A comparative study of laser ablation for magnesium, copper, and lead has been made by simulation using Nd–YAG lasers at fluence 10 J/cm2. The temporal evolution of surface temperature of the target, ablated vapor mass, and ionization fraction of plasma plume is studied for each material. The simulated temporal evolution and spatial distribution of electron density, electron temperature, and axial velocity of ablated plume of each material are studied and the results are compared. It is found that the plume expansion follows adiabatic behavior and is slowed down with increasing atomic mass. In addition to that, the effect of laser wavelength on ablation has also been investigated by comparing ultraviolet KrF excimer and infrared Nd–YAG lasers. It is found that shorter-wavelength lasers favor ablation and plasma screening is more effective for longer-wavelength lasers.
Stopping powers of polycarbonate for 0.36–5.94-MeV protons and 1.0–24.0-MeV α particles
Authors: Räisänen J. , Trzaska W. H. , Alanko T. , Lyapin V. , Porter L. E.
Published: July 2003

The stopping powers of polycarbonate for protons of energy 0.6–5.9 MeV and α particles of energy 2.0–24.0 MeV have been measured in a transmission experiment employing thin-foil targets. The essence of the novel experimental method employed in the present measurements is to record both projectile energy and time of flight while constantly alternating measurements with and without the target foil in place. The accuracies of the proton data and α-particle data range from 2.0% to 3.0% and 2.2% to 2.7%, respectively. All of the resulting measurements were analyzed in terms of modified Bethe–Bloch theory in order to extract values of the target mean excitation energy (I) and Barkas-effect parameter (b). The composite results for the two projectiles are that I=71.52 eV and b=1.13, the former value lying about one standard deviation below the additivity value and the latter value lying about one standard deviation below the expected value of 1.4±0.1. Previous measurements of the stopping power of polycarbonate for Li7 ions were analyzed by the same method, but with the inclusion of an effective charge parameter (λ). The proton and α-particle data, with a few lower-energy points included, were analyzed for a three-parameter fit, also. Results for the three projectiles studied indicate both internal consistency and agreement with expectations based on modified Bethe–Bloch theory.
Characterization of ultrathin insulating Al2O3 films grown on Nb(110)/sapphire(0001) by tunneling spectroscopy and microscopy
Authors: Dietrich Ch. , Boyen H.-G. , Koslowski B.
Published: July 2003

Various structural as well as chemical properties of ultrathin Al2O3 films prepared on Nb(110)/sapphire(0001) were analyzed. For this purpose, in a first step, 40-nm-thick Nb(110) films are grown epitaxially by sputtering on top of sapphire(0001). The Nb(110) films are (1×1) reconstructed and exhibit the epitaxial relations Nb(110)‖Al2O3(0001), Nb[001]‖Al2O3[1_010] and equivalents as determined by x-ray diffraction. In a second step, a 1-nm-thick Al film is evaporated on top of the Nb(110) and oxidized ex situ in an oxygen plasma resulting in a polycrystalline Al2O3 film of thickness ∼1.3 nm. This oxide film was characterized by scanning tunneling microscopy and spectroscopy (STS) as well as by x-ray photoelectron spectroscopy. Additionally, the ultrathin Al-oxide films were tested as insulators in large area metal-insulator-metal (MIM) or superconductor-insulator-metal tunneling contacts. According to these experiments, the niobium film underneath is completely protected from oxidation, while the Al film is fully oxidized without pinholes over areas of some mm2. The tunneling barrier heights range from 1.0 to 1.5 eV according to MIM tunneling and STS.
Hot-carrier charge trapping and trap generation in HfO2 and Al2O3 field-effect transistors
Authors: Kumar Arvind , Fischetti Massimo V. , Ning Tak H. , Gusev Evgeni
Published: July 2003

We present a comprehensive experimental study of hot-carrier trap generation and charging effects in high-κ dielectrics using field-effect transistors fabricated with HfO2 and Al2O3 gate insulator stacks and polycrystalline silicon gates. The experiments utilize substrate injection of hot carriers generated either optically or by direct injection in the dark from a forward-biased p–n junction. Comparison of charge-trapping measurements taken using these two techniques on n-channel field-effect transistors (nFETs) and p-channel field-effect transistors (pFETs) finds that enhanced charge trapping occurs when hot holes are present (in the light or in the dark in pFETs but only under illumination in nFETs). A fundamental understanding of the conditions for hot-carrier damage in nFETs is obtained by studying the dependence on light wavelength, temperature, and substrate bias. In particular, the wavelength dependence reveals that the hot-carrier damage depends on a combination of the electron and photon energies. Study of the time dependence of the gate current indicates the buildup of positive charge in the dielectric during stressing. The density of interface traps generated by hot-carrier stressing is estimated using the capacitance–voltage characteristic, and charge transfer experiments to probe the existence of slow states are performed. Finally, the experimental findings are discussed in the context of a speculative picture in which hot holes act as a precursor to damage in the oxide.
Magnetic resonance investigation of Mn2+ in ZnO nanocrystals
Authors: Zhou Huijuan , Hofmann Detlev M. , Hofstaetter Albrecht , Meyer Bruno K.
Published: July 2003

Electron paramagnetic resonance measurements were carried out to probe the structure of Mn2+ in ZnO nanocrystals with different surface conditions, modified by an annealing process. Changes in the spectra by the annealing treatment indicate the existence of three Mn2+ centers. The first is a sextet signal SI with g=2.0028 and a hyperfine coupling constant |A|=74.6×10−4 cm−1 and is present in all samples. Its hyperfine splitting is similar to Mn-doped ZnO single crystals, and therefore it is assigned to isolated substitutional Mn2+ ions. The second spectrum SII with g=2.001 and |A|=89×10−4 cm−1 decreases upon annealing. It is attributed to Mn2+ in strongly distorted environment, i.e., Mn2+ in Zn(OH)2 which is present as a surface shell of the ZnO nanoparticles. The superimposed broad background is associated with Mn–Mn clusters or dipole interactions, and it increases in intensity upon annealing.
Properties of Co-, Cr-, or Mn-implanted AlN
Authors: Frazier R. M. , Stapleton J. , Thaler G. T. , Abernathy C. R. , Pearton S. J. , Rairigh R. , Kelly J. , Hebard A. F. , Nakarmi M. L. , Nam K. B. , Lin J. Y. , Jiang H. X. , Zavada J. M. , Wilson R. G.
Published: July 2003

AlN layers grown on Al2O3 substrates by metalorganic chemical vapor desposition were implanted with high doses (3×1016 cm−2, 250 keV) of Co+, Cr+, or Mn+. Band-edge photoluminescence intensity at ∼6 eV was significantly reduced by the implant process and was not restored by 950 °C annealing. A peak was observed at 5.89 eV in all the implanted samples. Impurity transitions at 3.0 and 4.3 eV were observed both in implanted and unimplanted AlN. X-ray diffraction showed good crystal quality for the 950 °C annealed implanted samples, with no ferromagnetic second phases detected. The Cr- and Co-implanted AlN showed hysteresis present at 300 K from magnetometry measurements, while the Mn-implanted samples showed clear loops up to ∼100 K. The coercive field was <250 Oe in all cases.
Effects of the trapping dust particles on the sheath structure in radio-frequency discharges
Authors: Wang Dezhen , Sheng Mingyu
Published: July 2003

The spatial distributions of the trapping dust particles under the interaction of the electrostatic, gravitational, ion-drag, and pressure gradient forces in a radio-frequency sheath are investigated with a self-consistent model. The effects of the trapping dust particles on the ion flux, ion mean velocity, ion and time-averaged electron densities, and time-averaged local potential in the sheath are studied in detail. The distribution for the particles with a finite temperature has a thermal broadening. The region where dust particles trapped is wider for a higher dust temperature or a smaller particulate radius. Due to the collection and scattering of ions by dust particles, the ion flux in the sheath with dust suspending is not constant but decreased. The ion and time-averaged electron densities in the region of trapping particles are also decreased. The time-averaged local sheath potential in the region of the trapping dust particles is even more negative and the local ion mean velocity is increased, which are qualitatively consistent with the experimental measurements of Arnas et al. [Phys. Plasmas 7, 4418 (2000)].
Creation and quenching of interference-induced emitter-quantum wells within double-barrier tunneling structures
Authors: Zhao Peiji , Woolard Dwight L. , Gelmont Boris L. , Cui Hong-Liang
Published: July 2003

The initial creation and subsequent quenching of the emitter quantum well within double-barrier resonant tunneling structures (RTSs) is the key process that explains the origin of the hysteresis and plateau-like structure of the I–V characteristics. This fundamental process, which evolves out of quantum-mechanical interference, defines the basic mechanism that can lead to intrinsic high-frequency oscillations. This article presents numerical results, derived from a coupled Wigner–Poisson model, that illustrate the underlying mechanisms responsible for the creation and disappearance of the emitter-quantum well. Additional theoretical results are also given that demonstrates how subband state coupling, between the emitter-quantum well (EQW) and the main-quantum well (MQW) defined by the double-barrier heterostructure, leads to the hysteresis and instability behavior. This article will reveal how the quantum interference that develops between the incident and reflected electron wave function (i.e., from the first barrier) leads to the formation of an emitter-quantum well. An analysis is also performed to define the effects of EQW–MQW subband coupling on the current–density verses voltage (I–V) characteristics and the overall I–V dependence on the initial charging states of the individual wells. In particular, this analysis is used to show how the EQW is formed and quenched and how it influences the time-dependent behavior of the structure when it is subject to forward- and backward-bias sweeps of the applied bias voltages. This article provides fundamental quantum-mechanical explanations for the complicated time-dependent processes within double-barrier RTSs and provides insight into the hysteresis and intrinsic oscillation behavior.
Energy-band structure in strained silicon: A 20-band k⋅p and Bir–Pikus Hamiltonian model
Authors: Richard Soline , Aniel Frédéric , Fishman Guy , Cavassilas Nicolas
Published: July 2003

A strain Bir–Pikus Hamiltonian Hst, associated with a 20-band sp3s* k⋅p Hamiltonian Hkp, is used to describe the valence band and the first two conduction bands all over the Brillouin zone. Because the local (in k space) deformation potentials Ⅺu and Ⅺd used in pseudopotential method are unusable in k⋅p theory, we show that taking into account the Bir–Pikus parameters (a, b) of the Brillouin zone center in the Hst Hamiltonian allows one to describe the dispersion relation in the whole Brillouin zone. The method is applied to strained Si on a relaxed Si1−xGex alloy. The values of the energy band gap, and of the Δ2−4 conduction band splitting between the four equivalent in-plane valleys Δ4 and the two valleys along the growth direction Δ2 are in very good agreement with those reported in other publications. The small value of the spin-orbit splitting in silicon is taken explicitly into account. We show that the valence band splitting is consequently not proportional to the stress.
Current–voltage relations for the dual channel double heterostructure optoelectronic switch
Authors: Opper H. , Taylor G. W.
Published: July 2003

A theoretical model for an optoelectronic modulation doped thyristor is developed. The thyristor incorporates two modulation doped interfaces and as such it forms a technology base for both p- and n-type modulation doped transistors. The thyristor current–voltage (I–V) relations are developed based upon the thermionic current flow over the modulation doped junctions providing the feedback to initiate switching and upon the feedback current between quantum wells which stabilizes the device in the on state. The dependence of the carrier densities in the quantum wells upon bias are shown to be a unique property of the device. The I–V dependence is shown as a function of the critical device dimensions which are key design parameters for optoelectronic integrated circuits. The validity of the model and the structure is confirmed by comparison to the measured I–V characteristic.
High resolution absorption spectrum of jet-cooled OCS between 64 000 and 91 000 cm−1
Authors: Cossart-Magos Claudina , Jungen Martin , Xu Rong , Launay Françoise
Published: July 2003

The absorption spectrum of jet-cooled OCS was photographed from 190 to 110 nm at a resolution limit of 0.0008 nm. No band maximum was observed between 190 and 156 nm, i.e., below 64 000 cm−1. Ab initio calculations of the electronic energies and transition moments were carried out, including spin–orbit interaction, in the frozen core approximation. Rydberg states considered have ionic core X̃ 2Π and principal effective quantum number n*=2–5.5, electronic angular momentum l=0–5. Up to n*=3.5, l=0–2, calculations were also done in the coupled electron pair approximation. It is shown that in OCS, like in N2O, CO2 or CS2, npπ 1Σ+ states are at lower energy than npσ1,3Π. From the doublet structure shown by the corresponding transition origin bands, the rotational constant of the 4pσ1,3Π and 4pπ 3Σ− states was deduced to be B0′=0.1940(5) cm−1. Transitions involving excitation to ns or nd Rydberg orbitals, allowed in the less-symmetric molecules, were calculated to have relative intensities respectively two and three orders of magnitude greater in OCS than in N2O. The ns series could be assigned only to medium intensity or weak bands. In contrast, ndδ1Π transitions were assigned to relatively strong bands and could be followed up to n about 20. Previous low-resolution absorption and electron-impact spectra are reviewed. Most of the present assignments agree with those of resonance enhanced multiphoton ionization spectra and satisfactory assignments are obtained for the 15 unassigned electronic origins observed by Morgan et al. [J. Chem. Phys. 105, 2141 (1996)].