bluuebunny/crossref_metadata_2025 huggingface dataset:
Schlieren measurements and analysis of concentration field in self-excited helium jets
Authors: Pasumarthi Kasyap S. , Agrawal Ajay K.
Published: November 2003

A low-density gas jet injected into a high-density ambient gas is known to exhibit self-excited global oscillations accompanied by large vortical structures interacting with the flow field. In this study, the formation and evolution of vortices and scalar structure of the flow field are investigated in buoyant helium jets discharged from a vertical tube into quiescent air. This is accomplished by applying the quantitative rainbow schlieren deflectometry technique to optically measure the local helium mole percentage across the whole field. Data were acquired over downstream locations extending from tube exit to about 3.0d (d=31.8 or 19.1 mm is the jet tube inside diameter) at spatial resolution of 0.14 mm and temporal resolution of 16.7 ms. Oscillations at identical frequency were observed throughout the flow field. The evolving flow structure is described by helium mole percentage contours during an oscillation cycle. Instantaneous, mean, and rms concentration profiles are presented to describe interactions of the vortex with the jet flow. Oscillations in a narrow wake region near the jet exit are shown to spread through the jet core near the downstream location of the vortex formation. The effects of jet Richardson number on characteristics of vortex and flow field are investigated and discussed.
A multilevel-based dynamic approach for subgrid-scale modeling in large-eddy simulation
Authors: Terracol M. , Sagaut P.
Published: November 2003

In this paper we present a new dynamic methodology to compute the value of the numerical coefficient present in numbers of subgrid models, by mean of a multilevel approach. It is based on the assumption of a power law for the spectral density of kinetic energy in the range of the highest resolved wave numbers. It is shown that this assumption also allows us to define an equivalent law for the subgrid dissipation, and to obtain a reliable estimation for it through the introduction of a three-level flow decomposition. The model coefficient is then simply tuned dynamically during the simulation to ensure the proper amount of subgrid dissipation. This new dynamic procedure has been assessed here in inviscid homogeneous isotropic turbulence and plane channel flow simulations (with skin-friction Reynolds numbers up to 2000).
Analysis and optimization of the performance of polymorphous silicon solar cells: Experimental characterization and computer modeling
Authors: Poissant Y. , Chatterjee P. , Roca i Cabarrocas P.
Published: November 2003

Hydrogenated polymorphous silicon (pm-Si:H) is a nanostructured silicon thin film produced by plasma enhanced chemical vapor deposition under conditions close to powder formation. It has a lower initial and stabilized density of states, and a hole mobility considerably higher than state-of-the-art a-Si:H, which makes this material an interesting candidate for solar cell applications. In this article, we present experimental studies in conjunction with computer modeling to analyze and explain the relative performances of solar cells in which either a-Si:H or pm-Si:H is used as the intrinsic layer. Our results reveal large differences in the transport and metastability behavior of the two types of solar cells. Moreover, we observe a more damaged p/i interface for the pm-Si:H cells, although the p and n layers have been deposited under identical conditions. As a consequence, the cells fabricated initially with pm-Si:H did not perform better than standard a-Si:H based cells, despite the fact that the model confirmed the better transport properties of pm-Si:H films with respect to a-Si:H. Using insight gained from modeling, the deposition parameters were optimized to ultimately yield pm-Si:H based solar cells with conversion efficiencies higher than a-Si:H based cells in both the annealed and the light stabilized states.
Magnetostriction and surface roughness of ultrathin NiFe films deposited on SiO2
Authors: Hollingworth M. P. , Gibbs M. R. J. , Murdoch S. J.
Published: November 2003

Ni 81 Fe 19 was sputter deposited onto Si single-crystal wafers which had the native oxide layer intact. Dc hysteresis loops were measured using the magneto-optic Kerr effect, and anisotropy fields determined from the susceptibility. Saturation magnetostriction constants were deduced from the change in anisotropy field as the samples were strained, using the Villari effect. The magnitude of the saturation magnetostriction constant λs of these polycrystalline films was observed to decrease as film thickness was reduced, but always remained negative in sign. Surface roughness was measured using an atomic force microscopy. By considering the variation of the magnitude of the saturation magnetostriction constant with a controlled level of surface roughness, we are able to rule out roughness as the cause of changes in λs as films decrease in thickness below 10–15 nm. This is an important further step in the understanding of the contribution of magnetoelasticity in the performance of soft magnetic films in read heads.
Polygonal N-vortex arrays: A Stuart model
Authors: Crowdy Darren
Published: November 2003

A class of exact planar solutions of the Euler equations representing stationary N-polygonal arrays of vortices are found. The solutions are parametrized by two parameters N and Ωmax. N denotes the number of vorticity extrema surrounding the origin; Ωmax denotes the extremal value of this vorticity. Except for a point vortex at the origin, the solutions have everywhere-smooth vorticity distributions and are generalizations of the classic exact solution of Stuart [J. Fluid Mech. 29, 417 (1967)] for an infinite row of smooth vortices. In the limit |Ωmax|→∞, the solutions reduce to the pure point vortex problem considered by Morikawa and Swenson [Phys. Fluids 14, 1058 (1971)]. The new solutions can be understood as “smoothed-out” counterparts to this point vortex problem.
Monolithic multichannel ultraviolet detector arrays and continuous phase evolution in MgxZn1−xO composition spreads
Authors: Takeuchi I. , Yang W. , Chang K.-S. , Aronova M. A. , Venkatesan T. , Vispute R. D. , Bendersky L. A.
Published: November 2003

We have fabricated MgxZn1−xO epitaxial composition spreads where the composition across the chip is linearly varied from ZnO to MgO. By using a scanning x-ray microdiffractometer and transmission electron microscopy, we have mapped the phase evolution across the spread. We have discovered a unique growth relationship between cubic and hexagonal MgxZn1−xO where their basal planes are coincident in the phase-separated region of the phase diagram where 0.37⩽x⩽0.6. The continuously changing band gap across the spread is used as a basis for compact broadband photodetector arrays with a range of detection wavelengths separately active at different locations on the spread film. The composition-spread photodetector is demonstrated in the wavelength range of 290–380 nm using the ZnO to Mg0.4Zn0.6O region of the spread.
Optically pumped lead–chalcogenide midinfrared lasers on Si substrates
Authors: Kellermann Klaus , Zimin Dmitri , Alchalabi Karim , Gasser Philippe , Pikhtin N. A. , Zogg Hans
Published: November 2003

Double heterostructure (DH) and quantum well (QW) EuSe/PbSe/Pb1−xEuxSe edge-emitting laser structures on Si substrates are grown by molecular-beam epitaxy. They operate up to 250 K when pumped with 870 nm low-cost laser diodes with peak powers of ∼7 W, and emit up to 200 mW peak output power at ∼5 μm wavelength. Differential quantum efficiencies are up to 20%. The threshold powers are limited by Shockley–Read recombination due to the high dislocation densities (108 cm−2) in the active layers. Nearly similar maximum operation temperatures were observed when employing (111)—instead of (100)—oriented layers, as well when using QW rather than DH structures. Reduction of dislocation densities to 107 cm−2 is feasible and will lead to nearly an order of magnitude lower threshold powers.
Numerical analysis of thermal-slip and diffusion-slip flows of a binary mixture of hard-sphere molecular gases
Authors: Takata Shigeru , Yasuda Shugo , Kosuge Shingo , Aoki Kazuo
Published: November 2003

The thermal-slip (thermal-creep) and the diffusion-slip problems for a binary mixture of gases are investigated on the basis of the linearized Boltzmann equation for hard-sphere molecules with the diffuse reflection boundary condition. The problems are analyzed numerically by the finite-difference method incorporated with the numerical kernel method, which was first proposed by Sone, Ohwada, and Aoki [Phys. Fluids A 1, 363 (1989)] for a single-component gas. As a result, the behavior of the mixture is clarified accurately not only at the level of the macroscopic variables but also at the level of the velocity distribution function. In addition, accurate formulas of the thermal-slip and the diffusion-slip coefficients for arbitrary values of the concentration of a component gas are constructed by the use of the Chebyshev polynomial approximation.
Bipolar cascade lasers with quantum well tunnel junctions
Authors: Yang Rui Q. , Qiu Yueming
Published: November 2003

Bipolar cascade lasers with quantum wells (QWs) in tunnel junctions are proposed and demonstrated with emission wavelength near 2 μm based on an InGaAs/InP material system. These bipolar cascade lasers with QW tunnel junctions exhibited superior performance characteristics such as significantly reduced threshold voltage, increased quantum efficiency (>100%) and operation temperature over similar lasers without a QW in the tunnel junction. These broad-area lasers operated at temperatures up to 240 K in continuous wave mode and up to room temperature in pulsed mode.
Interaction between ferromagnetic/antiferromagnetic systems across a common antiferromagnetic spacer
Authors: Leung C. W. , Blamire M. G.
Published: November 2003

Many aspects of exchange bias between antiferromagnets and ferromagnets remain unclear despite substantial recent research. A principal outstanding issue is the relationship between exchange bias and enhanced coercivity in the ferromagnetic layer. Using a trilayer ferromagnet/antiferromagnet/ferromagnet structure deposited and measured under specific field conditions, we demonstrate that the micromagnetic structure in the antiferromagnet which maintains exchange bias at one interface is sufficiently randomized at the other interface that no net bias is detectable, but it does maintain enhanced coercivity.
Modulation of electrode performance and in situ observation of proton transport in Pt–RuO2 nanocomposite thin-film electrodes
Authors: Park Kyung-Won , Sung Yung-Eun
Published: November 2003

The relationship between the properties of ruthenium oxide and its electrode performance with respect to methanol electro-oxidation was investigated using a Pt–RuO2 nanocomposite thin-film electrode fabricated by a cosputtering system. The performance of the nanocomposite electrode, consisting of a Pt nanophase and a RuO2, was modified by heat treatment at different temperatures (100, 250, and 400 °C) which altered the proton and electron contributions in the electrodes. The transport of protons produced during methanol electro-oxidation in the Pt–RuO2 nanocomposite electrode was directly observed during electrochemical reaction by means of the ectrochromism of RuO2. It is concluded that improved electrical conduction that maintains the transport of protons is responsible for the enhanced electro-oxidation of methanol for 250 °C Pt–RuO2.
Structural studies of bismuth nanocrystals embedded in SiO2 or GeO2 matrices
Authors: Kusz B. , Pliszka D. , Gazda M. , Brusa R. S. , Trzebiatowski K. , Karwasz G. P. , Zecca A. , Murawski L.
Published: November 2003

The layer of bismuth nanoclusters embedded in glass matrices and the surface layer of bismuth grains have been obtained by thermal treatment in hydrogen atmosphere of Bi0.33Ge0.67O1.84 and Bi0.57Si0.43O1.72 glass. The thickness and structure of such layers strongly depend on temperature and time of reduction. The structural studies of bismuth nanocrystals embedded in SiO2 or GeO2 matrices were performed with optical microscopy and atomic force microscopy. By the use of a slow-positron beam we monitored the structural changes undergoing in near-to-surface layers after the first steps of isothermal annealing. A simple two-layer model of reduced glasses explains the evolution of the surface layer and electrical properties of the material during the reduction process.
Properties of leaky and degenerate modes in a prism–film coupler with waveguide structure
Authors: Zhang Xi-Jing , Fan Xi-Zhi , Yan Jun , Xu Fei , Lou Xiao-Yuan , He Jing-Liang , Wang Zhen-Lin , Wang Hui-Tian , Ming N. B.
Published: November 2003

A laser beam can be efficiently fed into a planar thin film by means of a prism–film coupler. The properties of the cavity modes, the coupling efficiency, the optimum coupling condition, etc. of the prism–film coupler can be explained using the theory developed by Tien and Ulrich [P. K. Tien and R. Ulrich, J. Opt. Soc. Am. 60, 1325 (1970)]. Based on multiple scattering and multiple wave interference, we propose a theory of the two modes referred to as the leaky and degenerate modes. The properties of the two kinds of modes, such as positions, linewidths, angular spectrum of the leaky mode, and the degenerate mode spectrum are studied in detail. This theory gives an excellent explanation of the underlying origin of light wave propagation in the prism–film coupler.
Effects of dry etching processes on exciton and polariton characteristics in ZnTe
Authors: Sun J. H. , Xie W. B. , Shen W. Z. , Ogawa H. , Guo Q. X.
Published: November 2003

We have employed temperature-dependent reflection spectra to study the effects of reactive ion etching (RIE) on the exciton and polariton characteristics in ZnTe crystals exposed to CH4/H2 gases under different rf plasma powers. Classic exciton-polariton theory has been used to calculate the reflection spectra. By comparing with an as-grown ZnTe crystal and the temperature-dependent behavior, we are able to identify the excitons and RIE-induced polariton structures in these dry etched ZnTe crystals. An increase of the rf plasma power will lead to an increase of defect density in the surface damage layers, resulting in a decrease of the photon energies of the observed exciton and polariton structures.
On the existence of Arnold-stable barotropic flows on a rotating sphere
Authors: Iacono Roberto
Published: November 2003

Application of two well-known finite-amplitude stability conditions by Arnold to two-dimensional flows on a rotating sphere is discussed, in the context of the barotropic vorticity equation. Attention is focused on zonal basic flows with nonmonotonic profiles of absolute vorticity, since for such flows stability would not descend from the Rayleigh–Kuo inflection point theorem. It is shown that global flows of this kind cannot satisfy either of Arnold’s stability conditions.
The evolution of co-rotating vortices in a canonical boundary layer with inclined jets
Authors: Zhang Xin
Published: November 2003

The evolution of co-rotating streamwise vortices in a canonical flat plate turbulent boundary layer (thickness of the boundary layer δ0.99=25 mm) is studied. The vortices are produced by an array of inclined jets (diameter D=14 mm) with the same orientation (skew angle β and pitch angle α of 45°). The focus of the investigation is on the immediate vicinity of the jet exit and downstream locations up to 40 D. The Reynolds number based on the diameter of the jet nozzle ranges from 9700 to 29 000, at various jet speed ratios. The main method of investigation is laser Doppler anemometry. Both mean and statistic data are collected and analyzed. The streamwise vortices are a product of complex fluid flow process, featuring horseshoe vortices in front of the nozzle exit, recirculating flow to the lee side of the nozzle, contra-rotating vortices from the rolling up of vortex sheet around the jet, strong and induced spanwise flow. Two types of streamwise vortices are produced: (a) weak vortices at a jet speed ratio λ of 0.5 located close to the wall and featuring diametrically opposed, secondary, near-wall flows in between the vortices, (b) strong vortices at higher jet speed ratio featuring significant spanwise movement. The vortices are accompanied by high levels of turbulence, with distinct normal and shear stress distributions. Both turbulence production and convection play important roles in defining the normal stress but only the turbulence production is important in determining the shear stress.
Vortex-merger statistical-mechanics model for the late time self-similar evolution of the Kelvin–Helmholtz instability
Authors: Rikanati A. , Alon U. , Shvarts D.
Published: November 2003

The nonlinear growth, of the multimode incompressible Kelvin–Helmholtz shear flow instability at all density ratios is treated by a large-scale statistical-mechanics eddy-pairing model that is based on the behavior of a single eddy and on the two eddy pairing process. From the model, a linear time growth of the mixing zone is obtained and the linear growth coefficient is derived for several density ratios. Furthermore, the asymptotic eddy size distribution and the average eddy life time probability are calculated. Very good agreement with experimental results and full numerical simulations is achieved.
Buoyant mixing of miscible fluids of varying viscosities in vertical tubes
Authors: Debacq M. , Hulin J-P. , Salin D. , Perrin B. , Hinch E. J.
Published: November 2003

Gravity-induced mixing of two fluids in long vertical tubes is studied experimentally as a function of the density contrast characterized by the Atwood number At (10−5 to 0.2), the fluid viscosity ν (1 to 16×10−6 m2 s−1) and the tube diameter d (2 to 44 mm). At low density contrasts, a stable counterflow is observed over a large fraction of the tube and its region of existence increases at high viscosities and small tube diameters. For larger density contrasts, the flow is either convective or turbulent and the mean concentration profile C̄(x,t) follows a diffusive spreading law characterized by a diffusivity D. An unexpected increase of D and of the characteristic velocity Vf of random fluid motions is observed when ν increases. This results from the coarser mixing in more viscous fluids which increases local density contrasts and buoyancy forces. Dimensionless plots of the diffusion coefficient D/ν as a function of the Reynolds number of the flow indicate a transition between two different diffusive regimes. Scaling arguments are put forward to account for the dependence of Vf and of the characteristic diffusion length in the convective–diffusive regime.
Stability of a stationary hydraulic jump in an upward sloping channel
Authors: Defina Andrea , Susin Francesca Maria
Published: November 2003

A simple theoretical criterion to predict stability of a stationary hydraulic jump in a rectangular channel is presented. It is shown that stability condition depends on the Froude number of the flow, as well as on the bottom slope and friction along the channel. In particular, it is found that in an upward sloping channel the jump may either be unstable (i.e., if the jump is slightly displaced from its equilibrium location, then it moves further away from it) or stable (i.e., it returns to its original location). This result improves previous theoretical findings reported in the literature.
Disorder/order phase transition in C60 thin films studied by surface photovoltage spectroscopy
Authors: Katz E. A. , Faiman D. , Mishori B. , Shapira Yoram , Isakina A. , Strzhemechny M. A.
Published: November 2003

The electronic properties of C60 thin films have been studied using surface photovoltage (SPV) spectroscopy at 120–300 K. Temperature variations of the SPV spectra are correlated with temperature-dependent x-ray diffraction patterns of the same samples, which indicate the first-order phase transition at Tc=250 K. Absolute values of both the band-to-band and band (or band tail) -to-band tail SPV signals are shown to exhibit a clear minimum at Tc=250 K in contrast with the well-known increasing background of the SPV intensity with decreasing temperature. The energy positions of the thresholds of the band-to-band and band (or band tail) -to-band tail regions in the SPV spectra also exhibit nonmonotonic behavior with a mirror symmetry and distinct extrema near Tc. On the basis of the presented results, we discuss possible reasons behind the effect of rotational and orientational states of C60 molecules on the electronic structure of the C60 fullerite and charge carrier transport in C60 thin films.
Thermal expansion and stress development in the first stages of silicidation in Ti/Si thin films
Authors: Chenevier B. , Chaix-Pluchery O. , Gergaud P. , Thomas O. , La Via F.
Published: November 2003

The structural evolution in fiber-textured Ti/Si thin films has been investigated by in situ x-ray diffraction, in a temperature range preceding the formation of silicide compounds. At low temperature, thermoelastic behavior of the metallic film was observed. Abnormal thermal behavior of both 002 and 101 diffraction profiles was observed at 360 and 450 °C, which could be understood in terms of Si diffusion, first in Ti grain boundaries, then, into the grains. By combining ex situ strain studies with analysis of the Si local environment in the whole Ti silicide family from Ti5Si3 to TiSi2, it was possible to determine the stress-free lattice parameters of annealed films. They are significantly higher than the bulk parameters, and indicate that nearly 4.5 at. % Si is present in the Ti grains.
Long wavelength infrared negative luminescent devices with strong Auger suppression
Authors: Nash G. R. , Ashby M. K. , Lindle J. R. , Gordon N. T. , Bewley W. W. , Meyer J. R. , Giess J. , Haworth L. , Ashley T.
Published: November 2003

We have investigated the negative luminescent properties of HgCdTe photodiodes having peak emission at a wavelength of 7.6 μm, as a function of temperature and applied bias. Under reverse bias at room temperature, intrinsic Auger processes are almost fully suppressed. The internal negative luminescence efficiency of ≈85% is nearly independent of temperature in the 240–300 K range, and at 296 K corresponds to an apparent temperature reduction of 43 K. The small currents needed to drive these diodes (as little as 0.7 A/cm2 at 296 K), together with their long-wavelength luminescence, make them suitable for a range of device applications.
Photoluminescence of ZnxCdyMg1−x−ySe alloys as a manifestation of the breakdown of “common-anion rule”
Authors: Zhou X. , Gu Y. , Kuskovsky Igor L. , Neumark G. F. , Zeng L. , Tamargo M. C.
Published: November 2003

We have systematically studied the optical properties of ZnxCdyMg1−x−ySe alloys using photoluminescence (PL) and photoluminescence excitation. It is shown that, at low temperatures, PL is dominated by excitons localized by potential fluctuations, which become stronger with increasing Mg concentration. Such potential fluctuations are discussed in terms of a large valence band offset in Zn(Cd)Se/MgSe systems, which serves as a manifestation of the breakdown of “common-anion rule”.
Electroreflectance of surface-intrinsic-n+-type-doped GaAs by using a large modulating field
Authors: Lin Y. C. , Wang K. Q. , Wang D. P. , Huang K. F. , Huang T. C.
Published: November 2003

It is known that electroreflectance of surface-intrinsic-n+-type-doped GaAs has exhibited many Franz–Keldysh oscillations to enable the application of fast Fourier transform to separate the heavy- and light-hole transitions. However, each peak still contains two components, which belong to F+δF/2 and F−δF/2, respectively, where F is the electric-field strength in the undoped layer and δF is the modulating field of applied ac voltage (Vac). In order to resolve the heavy- and light-hole transitions, δF was kept much smaller than F in the previous works. In this work, we have used a larger Vac and, hence, a larger δF, to further separate the peaks. The peaks can be divided into two groups which belong to F+δF/2 and F−δF/2, respectively. The peak belonging to the heavy-hole transition and F−δF/2 can be singled out to compare with the Airy function theory.
Current conduction mechanism of Pt/GaN and Pt/Al0.35Ga0.65N Schottky diodes
Authors: Kim Jong Kyu , Jang Ho Won , Lee Jong-Lam
Published: November 2003

Electrical properties of Pt/AlxGa1−xN Schottky diodes and chemical bonding states of AlxGa1−xN surface were examined simultaneously to investigate the change in the current transport mechanisms of the Pt/AlxGa1−xN diodes with increasing Al mole fraction. The Pt/GaN diodes showed electrical properties given by the thermionic-emission theory, while the Pt/Al0.35Ga0.65N showed a nonideal Schottky behavior. The oxygen donors were predominantly incorporated at the surface of AlxGa1−xN with increasing Al mole fraction, causing the surface to be heavily doped n type. Consequently, the current transport in the Pt/Al0.35Ga0.65N diodes was dominated by the field emission of electrons through the Schottky barrier, leading to the nonideal Schottky behavior.
Electrical characterization of InGaAs/InP quantum wells by scanning capacitance microscopy
Authors: Maknys K. , Douhéret O. , Anand S.
Published: November 2003

In this work, cross-sectional scanning capacitance microscopy (SCM) is used to investigate InGaAs/InP quantum wells. It is demonstrated that SCM is indeed capable of detecting the electrons in the quantum wells and that the SCM signal shows a systematic trend for the different wells. Clear dips in the dC/dV signal are observed at the InGaAs quantum wells and imply carrier densities higher than the surrounding barriers. It is also shown that the depletion regions in the barriers adjacent to the wells can be resolved. The results show that geometric tip-averaging effects significantly influence the imaging of electrons in quantum wells and limit the lateral resolution.
Transfer of small negative atmospheric ions
Authors: Maruyama Toshiro , Katayama Takashi
Published: November 2003

The transfer of small negative atmospheric ions in air and from air to water has been studied. The mean lifetime of ions was experimentally obtained from the concentration distribution of ions in a round free jet. It was about 30 s in relative humidity of 63%–75%, indicating that ions generated at one source in a room could not be homogeneously spread throughout the room by conventional convective flow of air. Rapid dissipation of the ions occurs during diffusion in stationary air. However, most of the ions that enter the mouth of a human body reach the lungs, because of the high convective velocity of breathing. The irradiation of ions onto water decreased the electric conductivity of the water. This finding suggests that the transfer of ions into water in a human body breaks the network of H2O molecules and enhances the formation of hydrogen bonds between a biopolymer and water.
Transmission electron microscopy investigations of damage induced by high energy helium implantation in 4H–SiC
Authors: Beaufort M. F. , Pailloux F. , Declémy A. , Barbot J. F.
Published: November 2003

In 4H–SiC, damage created by helium implantation at high fluence (5×1016 ions cm−2) and high energy (1.6 MeV) was studied using different techniques of electron microscopy all along the ion path. Around the end of range, conventional transmission electron microscopy was used to observe the fine microstructure of defects in the as-implanted and 1500 °C annealed samples. No bubbles were found in the as-implanted sample while numerous cavities with different shape, size and density are present after annealing. The amorphous-crystalline (a/c) transition region was checked by high resolution transmission electron microscopy. The strain profile, determined using the large angle convergent beam electron diffraction, shows a strong correlation with the nuclear stopping curve given by SRIM simulation. The dilatation of the c axis measured all along the ion path is in agreement with the value of the observed swelling. Relaxation volumes for interstitial and vacancy do not cancel. After annealing the cavity parameters are found to be dependent on both the helium density and the as-implanted microstructure.
Optical emission from SiOx (x=1.2–1.6) nanoparticles irradiated by ultraviolet ozone
Authors: Dan Lu , Wu X. L.
Published: November 2003

We have investigated the photoluminescence (PL) of SiOx (x=1.2–1.6) powder with nanoparticle sizes of 5–15 nm irradiated by ultraviolet ozone. A blue PL band was observed with a large intensity. The peak position of this band shows a redshift with increasing irradiation time and its intensity has a maximum in the sample with an irradiation time of 60 min. PL excitation spectral examinations reveal that this broad PL band arises from optical transition of the self-trapped excitons at the surfaces of SiOx nanoparticles, which are induced by ultraviolet ozone irradiation. Fourier transform infrared absorption result and energy-dispersive x-ray fluorescence analysis confirm the existence of oxygen interstitials and oxygen vacancies, which provides a basis for forming the self-trapped excitons. This work improves the understanding of the blue-emitting property in Si/oxygen-related nanomaterials.
Temperature dependence and temperature limits of magnetic shape memory effect
Authors: Heczko Oleg , Straka Ladislav
Published: November 2003

We study the temperature dependence and low and high temperature limits of the magnetic shape memory effect (MSME) in five-layered tetragonal Ni–Mn–Ga martensite. Using a simple model we show that, additionally to the limits posed by transformation to austenite or an intermartensitic transformation, the temperature dependence of the magnetic anisotropy, tetragonality of the lattice, and twinning stress play important role when considering the temperature limits of the MSME. With decreasing temperature, the lattice distortion and magnetic anisotropy increase, but saturate in a low temperature region. The twinning stress does not saturate and its temperature dependence has exponential-like character that increases rapidly in the low temperature region. The model predicts that the low-temperature limit of the MSME is 165 K for Ni49.7Mn29.1Ga21.2 composition. This agrees very well with the value of 173 K determined from direct measurements. The high temperature limit is transformation to austenite at 315 K. The interval of the MSME existence is therefore 173–315 K. Quasistatic measurements of the MSME in the range 203–313 K up to the 1.15 T magnetic field show that the onset of the MSME shifts to a higher field and that a maximum field-induced strain increases with decreasing temperature.
Photoluminescence study of AlGaN-based 280 nm ultraviolet light-emitting diodes
Authors: Yasan A. , McClintock R. , Mayes K. , Kim D. H. , Kung P. , Razeghi M.
Published: November 2003

We investigated optical properties of single quantum well AlGaN-based UV 280 nm light-emitting diodes using temperature-dependent photoluminescence (PL) measurement. We found an “S-shaped” temperature dependence of the peak energy. From the Arrhenius plot of integrated PL intensity, we speculate that dislocations as well as thermal emission of carriers out of the quantum well are responsible for the PL quenching behavior. Also a second nonradiative channel with much lower activation energy was found, the origin of which we believe to be quenching of the bound excitons.
Highly sensitive optical monitoring of molecular film growth by organic molecular beam deposition
Authors: Goletti C. , Bussetti G. , Chiaradia P. , Sassella A. , Borghesi A.
Published: November 2003

Reflectance anisotropy spectroscopy (RAS) has been employed to study in situ the growth of thin α-sexithiophene films by organic molecular beam deposition onto an organic substrate. A large anisotropy can be detected by following the line shape evolution of the RAS spectrum; in addition, the signal variation at a fixed wavelength is used to monitor the film growth. The signal intensity scales with the deposited thickness, demonstrating a very high sensitivity of RAS to less than 1/50 of a monolayer. Evidence of the advantages of RAS to monitor in real time the growth of molecular films and to probe in situ their properties is therefore obtained.
Optical parametric chirped-pulse amplification in periodically poled KTiOPO4 at 1053 nm
Authors: Jovanovic Igor , Schmidt Jason R. , Ebbers Christopher A.
Published: November 2003

We have demonstrated a high-gain preamplifier for Nd:glass-based chirped-pulse amplification systems using optical parametric chirped-pulse amplification (OPCPA) in periodically poled potassium titanyl phosphate (PPKTP) in a collinear quasi-phase-matched configuration. The absence of birefringent walk-off enables high conversion efficiency and high beam quality using <1 mJ of pump energy. PPKTP has a potential to replace traditional angularly sensitive beta-barium borate preamplifiers for nearly degenerate OPCPA at wavelengths near 1 μm.
Strained Si/strained Ge dual-channel heterostructures on relaxed Si0.5Ge0.5 for symmetric mobility p-type and n-type metal-oxide-semiconductor field-effect transistors
Authors: Lee Minjoo L. , Fitzgerald Eugene A.
Published: November 2003

By growing heterostructures that combine a surface strained Si layer with a buried strained Ge layer on Si0.5Ge0.5, we have fabricated metal-oxide-semiconductor field-effect transistors with mobility enhancement factors over bulk Si of 1.7–1.9 for electrons and 10–12 for holes. While high hole mobility can be attained in strained Si/strained Ge heterostructures grown on Si0.3Ge0.7, we have found the electron mobility in similarly grown heterostructures to be limited by defect scattering in the Si cap. Reducing the Ge content of the virtual substrate to Si0.5Ge0.5 and optimizing the strained Si and strained Ge layer thicknesses allowed the realization of devices where the p-channel mobility as a function of inversion density actually matches or exceeds the n-channel mobility.
Focusing of millimeter-wave radiation beyond the Abbe barrier
Authors: Pimenov A. , Loidl A.
Published: November 2003

A hemispherical focusing system for millimeter waves is presented. Focusing of the beam to a spot of ∼0.3λ was obtained in the frequency range from 100 to 200 GHz. Compared to the scanning near-field optical spectroscopy, the system allows simultaneous imaging of extensive subwavelength objects. As an example, a double slit of size d=1 mm was clearly resolved using a radiation with a wavelength λ=2 mm. Using future optically dense lenses, a resolution of the order λ/100 seems to become possible.
Nanoscale domains and local piezoelectric hysteresis in Pb(Zn1/3Nb2/3)O3-4.5%PbTIO3 single crystals
Authors: Bdikin I. K. , Shvartsman V. V. , Kholkin A. L.
Published: November 2003

High-resolution domain studies have been performed in Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 (PZN-PT) single crystals via piezoresponse force microscopy (PFM). Irregular domain patterns with the typical sizes 20–100 nm have been observed on the (001)-oriented surfaces of unpoled samples. On the contrary, (111) crystal cuts exhibit normal micron-sized regular domains with the domain boundaries directed along allowed crystallographic planes. The existence of nanodomains in (001)-oriented crystals is tentatively attributed to the relaxor nature of PZN-PT where small polar clusters may form under zero-field-cooling conditions. These nanodomains are considered as the nuclei of the opposite polarization state and can be responsible for the smaller coercive field for this particular crystal cut. However, local piezoelectric hysteresis performed by PFM on the nanometer scale indicates similar switching behavior of PZN-PT for both investigated crystallographic orientations.
Defect-mediated and resonant optical excitation of Er3+ ions in silicon-rich silicon oxide
Authors: Kuritsyn D. , Kozanecki A. , Przybylińska H. , Jantsch W.
Published: November 2003

Sensitization of the 4I13/2–4I15/2 Er3+ luminescence at 1.54 μm in silicon-rich silicon oxide (SRSO) is studied in the blue-green range. We show that defects due to excess Si in silica act as luminescence sensitizers. We also suggest that there exist two types of Er centers—isolated ones and others—strongly coupled to defects. In SRSO competition of direct, resonant excitation of Er3+ and indirect processes via defects is observed. Enhancement of the Er emission for off-resonant excitation does not seem to compensate losses in the direct channel of excitation to the 2H11/2 state of Er3+. We suggest that the emission efficiency of Er3+ is limited by distance-dependent transfer rate and little spectral overlap of the interacting states.
Terahertz pulsed imaging with 1.06 μm laser excitation
Authors: Baker C. , Gregory I. S. , Tribe W. R. , Bradley I. V. , Evans M. J. , Withers M. , Taday P. F. , Wallace V. P. , Linfield E. H. , Davies A. G. , Missous M.
Published: November 2003

We report the operation of a pulsed terahertz (THz) imaging system that uses a 120 fs pulse-width laser operating at 1.06 μm, and photoconductive InGaAs emitters and detectors. THz images are presented of biological and nonbiological specimens. The signal-to-noise ratio and frequency range of this system are compared to those obtained from existing pulsed imaging systems based on 0.8 μm excitation.
Mega-electron-volt ion beam induced anisotropic plasmon resonance of silver nanocrystals in glass
Authors: Penninkhof J. J. , Polman A. , Sweatlock Luke A. , Maier Stefan A. , Atwater Harry A. , Vredenberg A. M. , Kooi B. J.
Published: November 2003

30 MeV Si ion beam irradiation of silica glass containing Ag nanocrystals causes alignment of Ag nanocrystals in arrays along the ion tracks. Optical transmission measurements show a large splitting of the surface plasmon resonance bands for polarizations longitudinal and transversal to the arrays. The splitting is in qualitative agreement with a model for near-field electromagnetic plasmon coupling within the arrays. Resonance shifts as large as 1.5 eV are observed, well into the near-infrared.
Digital alloy interface grading of an InAlAs/InGaAs quantum cascade laser structure studied by cross-sectional scanning tunneling microscopy
Authors: Offermans P. , Koenraad P. M. , Wolter J. H. , Beck M. , Aellen T. , Faist J.
Published: November 2003

We have studied an InGaAs/InAlAs quantum cascade laser structure with cross-sectional scanning tunneling microscopy. In the quantum cascade laser structure digital alloy grading was used to soften the barriers of the active region. We show that due to alloy fluctuations, softening of the barriers occurs even without the digital grading.
Low and anisotropic barrier energy for adatom migration on a GaAs (110) surface studied by first-principles calculations
Authors: Ishii Akira , Aisaka Tsuyoshi , Oh Ji-Won , Yoshita Masahiro , Akiyama Hidefumi
Published: November 2003

We determined potential-energy surfaces for Ga and As adatoms on a GaAs (110) surface by first-principles calculations in order to understand the epitaxial growth mechanism. We found small migration barrier energies for Ga and As, which explain the long atom-migration length suggested by experiments. We also found that Ga migration is one dimensional and As migration is two dimensional, and that, for both Ga and As adatoms, the sites near As of the topmost layer are stable while those near Ga are unstable.
90 W continuous-wave diode edge-pumped microchip composite Yb:Y3Al5O12 laser
Authors: Dascalu T. , Pavel N. , Taira T.
Published: November 2003

High-power continuous-wave (cw) operation of a diode edge-pumped microchip composite Yb:Y3Al5O12 (YAG) laser is reported. Up to 90 W were obtained from a 400-μm-thick Yb:YAG/YAG structure with a 10 at. % Yb:YAG square core of 2×2 mm2 area; the slope efficiency and optical-to-optical efficiency with respect to the pump power were 40% and 28%, respectively. Measurements of the optical phase distortions induced by pumping gives a focus shift bellow 0.05 m and shows the absence of astigmatic effects, indicating the axial heat flow in this pumping configuration.
Band gap tailoring of Nd3+-doped TiO2 nanoparticles
Authors: Li W. , Wang Y. , Lin H. , Ismat Shah S. , Huang C. P. , Doren D. J. , Rykov Sergey A. , Chen J. G. , Barteau M. A.
Published: November 2003

Undoped and Nd3+-doped TiO2 nanoparticles were synthesized by chemical vapor deposition in order to tailor the band gap of TiO2. The doping reduced the band gap. The band gap was measured by ultraviolet-visible light absorption experiments and by near-edge x-ray absorption fine structure. The maximum band gap reduction was 0.55 eV for 1.5 at. % Nd-doped TiO2 nanoparticles. Density functional theory calculations using the generalized gradient approximation with the linearized augmented plane wave method were used to interpret the band gap narrowing. The band gap narrowing was primarily attributed to the substitutional Nd3+ ions which introduced electron states into the band gap of TiO2 to form the new lowest unoccupied molecular orbital.
High resistivity annealed low-temperature GaAs with 100 fs lifetimes
Authors: Gregory I. S. , Baker C. , Tribe W. R. , Evans M. J. , Beere H. E. , Linfield E. H. , Davies A. G. , Missous M.
Published: November 2003

We demonstrate the critical effect of postgrowth annealing temperature on the properties of low-temperature-grown GaAs. By using annealing temperatures substantially below the 500–600 °C commonly reported, GaAs with high resistivity and with carrier lifetimes as short as 100 fs can be routinely obtained. We discuss the optimum, but different, anneal conditions required for terahertz photoconductive emitters and detectors, and illustrate their use in a continuous-wave system.
Temperature dependence of the c-axis mobility in 6H-SiC Schottky diodes
Authors: Roccaforte F. , La Via F. , Raineri V. , Mangano F. , Calcagno L.
Published: November 2003

In this work, the temperature dependence of the mobility along the c axis in silicon carbide (6H-SiC) was determined from the I–V characteristics of Schottky diodes. This procedure used series resistance measurements with Norde’s method in Schottky diodes for extracting the mobility values in the epitaxial layer. For a dopant concentration of 3×1015 cm−3, at room temperature a value of 61 cm2 V−1 s−1 was found, which decreased to 24 cm2 V−1 s−1 at 448 K. In the temperature range 298–448 K, a dependence of the mobility as T−2.1 was determined. This result suggests that the lattice scattering, together with other mechanisms such as scattering with defects in the material, rule the carrier transport in this temperature range.
Atomistic modeling of deactivation and reactivation mechanisms in high-concentration boron profiles
Authors: Aboy Maria , Pelaz Lourdes , Marqués Luis A. , Barbolla Juan , Mokhberi Ali , Takamura Yayoi , Griffin Peter B. , Plummer James D.
Published: November 2003

We use kinetic nonlattice Monte Carlo atomistic simulations to investigate the physical mechanisms for boron cluster formation and dissolution at very high B concentrations, and the role of Si interstitials in these processes. For this purpose, high-dose, low-energy B implants and theoretical structures with fully active box shaped B profiles were analyzed. Along with the theoretical B profile, different Si interstitial profiles were included. These structures could be simplifications of the situation resulting from the regrowth of preamorphized or laser annealed B implants. While for B concentrations lower than 1020 cm−3, B clusters are not formed unless a high Si interstitial concentration overlaps the B profile, our simulation results show that for higher B concentrations, B clusters can be formed even in the presence of only the equilibrium Si interstitial concentration. The existence of a residual concentration of Si interstitials along with the B boxes makes the deactivation faster and more severe.
Effects of surface treatments on isolation currents in AlGaN/GaN high-electron-mobility transistors
Authors: Moser N. A. , Gillespie J. K. , Via G. D. , Crespo A. , Yannuzzi M. J. , Jessen G. H. , Fitch R. C. , Luo B. , Ren F. , Gila B. P. , Onstine A. H. , Abernathy C. R. , Pearton S. J.
Published: November 2003

Interdevice isolation currents in mesa-isolated AlGaN/GaN high-electron-mobility transistors are found to exhibit thermally activated behavior, with an activation energy of ∼1.5 eV. This value is largely independent of surface cleaning processes or the type of passivation film (SiNX, Sc2O3, MgO) used to reduce the current collapse phenomena in the devices. However, the magnitude of the isolation current is a strong function of the surface treatment employed. The lowest isolation currents for conditions under which current collapse is mitigated are obtained using Sc2O3 passivation layers.
Enhancement of Curie temperature in Ga1−xMnxAs/Ga1−yAlyAs ferromagnetic heterostructures by Be modulation doping
Authors: Wojtowicz T. , Lim W. L. , Liu X. , Dobrowolska M. , Furdyna J. K. , Yu K. M. , Walukiewicz W. , Vurgaftman I. , Meyer J. R.
Published: November 2003

The effect of modulation doping by Be on the ferromagnetic properties of Ga1−xMnxAs is investigated in Ga1−xMnxAs/Ga1−yAlyAs heterojunctions and quantum wells. Introducing Be acceptors into the Ga1−yAlyAs barriers leads to an increase of the Curie temperature TC of Ga1−xMnxAs, from 70 K in undoped structures to over 100 K with the modulation doping. This increase is qualitatively consistent with a multiband mean field theory simulation of carrier-mediated ferromagnetism. An important feature is that the increase of TC occurs only in those structures where the modulation doping is introduced after the deposition of the magnetic layer, but not when the Be-doped layer is grown first. This behavior is expected from the strong sensitivity of Mn interstitial formation to the value of the Fermi energy during growth.
The electrical turn-on characteristics of vertical-cavity surface-emitting lasers
Authors: Liu Yang , Choquette Kent D. , Hess Karl
Published: November 2003

We present a detailed comparison of the electrical turn-on characteristics of 980 nm vertical-cavity surface-emitting lasers (VCSELs) with simulations using a recently extended laser simulator. It is shown that the three recombination mechanisms, spontaneous emission, Shockley–Read–Hall recombination, and Auger recombination, result in distinctly different exponential current–voltage dependencies below threshold. Therefore, information can be extracted about the relative strength of the recombination rates due to those processes, and their relative contributions to the threshold current can be assessed. We show that for the VCSELs studied in this work, spontaneous emission is the dominant contribution to the threshold current.
Continuous-wave laser action at λ=1064.3 nm in proton- and carbon-implanted Nd:YAG waveguides
Authors: Domenech M. , Vázquez G. V. , Cantelar E. , Lifante G.
Published: November 2003

This work reports continuous laser oscillation at λ=1064.3 nm at room temperature in Nd:YAG planar waveguides fabricated by two different techniques: proton implantation with a multi-implant of energies around 1 MeV and carbon implantation with a single-implant at an energy of 7 MeV. Threshold powers of 11 and 22 mW and slope efficiencies of 7% and 9% were achieved in the proton- and carbon-implanted guides, respectively. The laser outputs show a very high stability operating in cw regime at room temperature.
Gallium nitride nanowires doped with silicon
Authors: Liu Ji , Meng Xiang-Min , Jiang Yang , Lee Chun-Sing , Bello Igor , Lee Shuit-Tong
Published: November 2003

High-quality GaN nanowires doped with silicon have been synthesized by hot-filament chemical vapor deposition on Au-coated Si (100) wafers. The GaN was systematically characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence (PL). The GaN nanowires had a uniform concentration of 3% Si, a uniform diameter around 10 nm, and a hexagonal wurtzite structure grown along the [001] direction. The intense PL peak of GaN nanowires at 344 nm showed a distinct blueshift from the bulk bandgap emission, revealing a clear quantum confinement effect. The growth of GaN nanowires is discussed in terms of the oxide-assisted metal-catalyst vapor–liquid–solid model.