bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal stability and electrical properties of Zr/Si1−x−yGexCy contacts after rapid thermal annealing
Authors: Aubry-Fortuna V. , Eyal A. , Chaix-Pluchery O. , Barthula M. , Meyer F. , Eizenberg M.
Published: July 2002

In this work, we have investigated the reaction between Zr and SiGeC alloys. Annealings have been performed in a rapid thermal annealing (RTA) furnace at temperatures ranging from 400 to 800 °C for 5 min. The reaction of the metal with the alloy has been investigated by x-ray diffraction and Rutherford backscattering spectrometry. Four crystal x-ray diffraction was performed to measure the residual strain in the epilayer. The analyses indicate that the C49-Zr(Si1−xGex)2 is the final phase of the reaction. For all compositions examined (from 0% up to 33% of Ge), the C49 film has the same Ge content as in the as-deposited Si1−x−yGexCy layer and no Ge segregation has occured. In addition, this thermal treatment leads to only a small strain relaxation in the unreacted epilayer. The presence of C does not modify the reaction and it prevents any strain relaxation. Schottky barrier height measurements have been performed on p-type layers. RTA leads to a slight decrease of the barrier without any degradation of the contact. The C49 film presents a resistivity of about 80 μΩ cm. These results indicate that Zr may be a good candidate for contacts on IV–IV alloys in terms of thermal stability.
Collective effects of interface roughness and alloy disorder in InxGa1−xN/GaN multiple quantum wells
Authors: Zeng K. C. , Smith M. , Lin J. Y. , Jiang H. X.
Published: July 2002

The collective effects of alloy disorder and interface roughness on optical properties of InxGa1−xN/GaN multiple quantum wells (MQWs) have been studied. The results are compared with those of GaN/AlGaN MQWs and InGaN epilayers. InxGa1−xN/GaN MQWs emit a broad and asymmetrical photoluminescence (PL) band, while GaN/AlGaN MQWs and InGaN epilayers emit narrower and Gaussian-shaped PL bands. Furthermore, the decay of excitons at low temperatures in InxGa1−xN/GaN MQWs follows a nonexponential function even at the lower-energy side of the PL spectral peak, while those in GaN/AlGaN MQWs and in InGaN epilayers follow a single exponential function. Both alloy disorder and interface roughness have to be included in order to interpret the PL emission spectrum and the decay dynamics in InxGa1−xN/GaN MQWs. Important parameters of the InxGa1−xN/GaN MQWs, σx, σL, and dτ/dL, denoting the alloy disorder, the interface roughness, and the rate of changing of the exciton decay lifetime with well width, respectively, have been deduced. The method developed here can be used to determine σx, σL, and dτ/dL in any MQW systems with wells being alloy materials.
Direct in situ characterization of Ge surface segregation in strained Si1−xGex epitaxial thin films
Authors: Lam A. M. , Zheng Y.-J. , Engstrom J. R.
Published: July 2002

Low-energy ion scattering spectrometry (LEISS) and x-ray photoelectron spectroscopy (XPS) have been employed to quantify in situ the near-surface composition of strained Si1−xGex epitaxial thin films grown on Si(100) substrates using GeH4 and Si2H6 as sources. The use of LEISS reveals the Ge concentration in essentially the first monolayer, whereas XPS is sensitive to several monolayers. We find that the extent of Ge surface segregation implied by each technique follows the trend: LEISS-Ge%≫XPS-Ge%≫bulk-Ge%. A two-site model (involving surface and bulk states) cannot account for both the XPS and LEISS results, rather a model invoking Ge enrichment in the subsurface layers is required to explain the data.
Microfabrication of nanoscale cluster chains on a patterned Si surface
Authors: Liu Jian , Barnard John C. , Seeger Katrin , Palmer Richard E.
Published: July 2002

We have studied the formation of Cu clusters from Cu atoms deposited onto a Si (111) surface patterned with (2–5 μm width) lines of photoresist. In addition to a thin Cu layer on the exposed Si surface, large (∼150 nm) clusters nucleate at the boundary between the Si and the resist strips, which remain after removal of the resist by dissolution. The results show how it is possible, using the resist to collect deposited atoms, to assemble nanoscale cluster structures with a precision (∼150 nm feature size) which is much better than the resolution of conventional optical lithography (∼μm linewidth).
Charge injection and transport in single-layer organic light-emitting diodes
Authors: Crone B. K. , Campbell I. H. , Davids P. S. , Smith D. L.
Published: July 2002

We present experimental and device model results for the current–voltage characteristics of a series of organic diodes. We consider three general types of structures: electron only, hole only, and bipolar devices. Electron and hole mobility parameters are extracted from the corresponding single carrier structures and then used to describe the bipolar devices. The device model successfully describes the experimental results for: electron only devices as thickness is varied, hole only devices as the contact metals are varied, and bipolar devices are both the thickness and the contact metals are varied.
Excitation of Er3+ emission by host glass absorption in sputtered films of Er-doped Ge10As40Se25S25 glass
Authors: Ramachandran S. , Bishop S. G.
Published: July 2002

Photoluminescence and photoluminescence excitation (PLE) spectroscopy have been carried out on the ∼1550 nm I413/2→I415/2Er3+ emission from thin films of Er-doped As40Ge10Se25S25 glass deposited on silicon substrates by radio frequency sputtering. The PLE spectroscopy shows that the Er3+ emission is excited in the 1-μm-thick film by the Urbach absorption edge of the host glass rather than direct absorption by the Er3+ intra-4f shell transitions. This enables the use of a broad range of pump wavelengths and novel pumping geometries. Comparison of the PLE spectra of the Er emission obtained before and after a rapid thermal anneal (RTA) clearly manifests a blue shift in the band gap induced by the RTA process. These results reveal that the broad band Er PLE mechanism discovered recently in Er-doped bulk chalcogenide glasses and attributed to host glass Urbach edge optical absorption and energy transfer mediated by native defects also occurs in sputtered films of Er-doped chalcogenide glass.
Measurement of the AlGaInAs/AlGaAs conduction-band offset using ballistic electron emission spectroscopy
Authors: Bhargava S. , Zheng C. , Ko J. , Chin M. A. , Coldren L. A. , Narayanamurti V.
Published: July 2002

Ballistic electron emission spectroscopy (BEES) has been used to determine the conduction-band offset between a 10-nm-thick Al0.12In0.22Ga0.66As (Q) strained layer and a ternary Al0.2Ga0.8As (T) barrier located beneath the surface. A three-sample process was used so that the known, reproducible Au/GaAs Schottky barrier would be the top layer of all measured structures. BEES thresholds obtained for Au/GaAs, Au/GaAs/Q, and Au/GaAs/Q/T were 0.96±0.02, 0.98±0.04, and 1.08±0.04 meV yielding offsets of ∼20 meV for GaAs/Q and ∼100 meV for Q/T. Under the affect of a high-temperature anneal, the Q/T offset was reduced to ∼40 meV. In addition, a structure employing solely Au/GaAs/AlGaAs was used to study transitivity for the Q/T material system.
Electron diffusion length and lifetime in
p
-type GaN
Authors: Bandić Z. Z. , Bridger P. M. , Piquette E. C. , McGill T. C.
Published: July 2002

We report on electron beam induced current and current–voltage (I–V) measurements on Schottky diodes on p-type doped GaN layers grown by metal organic chemical vapor deposition. A Schottky barrier height of 0.9 eV was measured for the Ti/Au Schottky contact from the I–V data. A minority carrier diffusion length for electrons of (0.2±0.05) μm was measured for the first time in GaN. This diffusion length corresponds to an electron lifetime of approximately 0.1 ns. We attempted to correlate the measured electron diffusion length and lifetime with several possible recombination mechanisms in GaN and establish connection with electronic and structural properties of GaN.
Cation vacancy formation and migration in the AlGaAs heterostructure system
Authors: Mitev P. , Seshadri S. , Guido L. J. , Schaafsma D. T. , Christensen D. H.
Published: July 2002

A simple experimental approach has been employed to obtain thermochemical parameters for cation vacancy formation and migration in the AlGaAs heterostructure system. Cation vacancies are injected into the free surface by annealing under an arsenic-rich ambient. Their presence is detected by monitoring the local rate of Al–Ga interdiffusion at imbedded quantum well markers. The sample is unusually thick allowing us to separately identify contributions from the vapor, epilayer, and substrate phases. The entropies and enthalpies of vacancy formation and migration are (5.2±5.7) kB and (1.8±0.5) eV and (11.3±4.4) kB and (3.3±0.4) eV, respectively.
Tunneling spectroscopy and spectroscopic imaging of granular metallicity of polyaniline
Authors: Yau S.-T. , Barisci J. N. , Spinks G. M.
Published: July 2002

The band-gap electronic structures of two forms of the conducting polymer polyaniline, emeraldine base (EB) and emeraldine salt (ES), have been studied. Tunneling spectroscopy measurements show that ES is metallic in nature with a finite electron density of state at the Fermi energy (EF), while EB behaves as an insulator with a gap at EF in agreement with theoretical calculations. These results allowed us to obtain direct evidence of the granular metallicity of ES by spectroscopic imaging. The spectroscopic images show different spatial distributions of nanometer-sized metallic particles in ES and EB samples, providing important information needed in studying electrical conductivities of conducting polymers.
Anisotropic photoluminescence in incomplete three-dimensional photonic band-gap environments
Authors: Romanov S. G. , Fokin A. V. , De La Rue R. M.
Published: July 2002

The effect of an incomplete three-dimensional photonic band-gap structure upon the wideband photoluminescence from bare opal and opal impregnated with ZnS has been studied by means of angular-resolved photoluminescence. It has been shown that the photoluminescence becomes anisotropic in accordance with the angular dispersion of the stop band. Suppression of the spontaneous emission in the stop-band energy region and amplification of the spontaneous emission at the edge of the stop band have both been demonstrated.
Spatiotemporal correlation of conduction current fluctuations within a hydrodynamic-Langevin scheme
Authors: Shiktorov P. , Starikov E. , Gružinskis V. , González T. , Mateos J. , Pardo D. , Reggiani L. , Varani L. , Vaissiere J. C. , Nougier J. P.
Published: July 2002

We present a hydrodynamic-Langevin scheme to describe electronic noise in unipolar structures and evaluate the cross-correlation functions of conduction current fluctuations entering the transfer impedance method. The theory is developed in terms of microscopic fluctuations of carrier velocity and energy taking place during scattering events. Applications to submicron n+nn+ GaAs structures show that the contribution of stochastic velocity rate prevails over that of the energy rate in determining the cross-correlation function of conduction current fluctuations.
Self- and interdiffusion in AlXGa1−XAs/GaAs isotope heterostructures
Authors: Bracht H. , Haller E. E. , Eberl K. , Cardona M.
Published: July 2002

We report self- and interdiffusion studies between 800 and 1160 °C in buried Al71GaAs/Al69GaAs/71GaAs and AlAs/71GaAs isotope heterostructures. Ga diffusion at Al71GaAs–Al69GaAs interfaces was found to decrease with increasing Al content. Al–Ga interdiffusion at AlGaAs–GaAs and AlAs–GaAs interfaces reveals a concentration dependent interdiffusion coefficient. The temperature dependence of Ga and Al diffusion in GaAs and of Ga diffusion in AlGaAs is described by a single activation enthalpy in the range of 3.6±0.1 eV, but with different pre-exponential factors. The experimentally observed higher Al diffusion in GaAs compared to Ga self-diffusion as well as the decreasing Ga diffusion with increasing Al content is explained.
The impact of supersaturated carbon on transient enhanced diffusion
Authors: Rücker H. , Heinemann B. , Bolze D. , Kurps R. , Krüger D. , Lippert G. , Osten H. J.
Published: July 2002

Transient enhanced diffusion of B is strongly suppressed in C-rich Si. We show that the physical mechanism for this suppression is out-diffusion of C from regions of high C concentration. B doping superlattices with background C concentrations between 1018 and 1020 cm−3 were grown by molecular beam epitaxy and implanted with BF2 ions. The measured dependence of transient B diffusion on the C profile is explained by coupled diffusion for C and Si point defects. The proposed model is supported by the observation of transient enhanced diffusion of C.
Phase separation in a two-dimensional Co–Cr alloy
Authors: Kahng S.-J. , Choi Y. J. , Park J.-Y. , Kuk Y.
Published: July 2002

Two-dimensional phase separation in a Co–Cr random binary alloy on a W(110) surface was confirmed with ultra-high-vacuum scanning tunneling microscopy. With 1.1 ML Co and 0.1 ML Cr co-deposited at room temperature, the film shows a kinetically limited structure. Upon annealing, phase separation between Co and Cr-rich phases of ∼100 Å size was observed. Evidence of compositional inhomogeneity was observed in Cr-rich phases but not in Co-rich phases.
Cathodoluminescence enhancement in porous silicon cracked in vacuum
Authors: Rams J. , Mendez B. , Craciun G. , Plugaru R. , Piqueras J.
Published: July 2002

An increase of the cathodoluminescence (CL) signal of porous silicon (PS) cracked in vacuum of up to three orders of magnitude has been achieved. Under high electron-beam currents, the samples cracked in interconnected pieces of tens of microns, exposing new surfaces to the electron beam. This treatment enhances the radiative intensity in PS associated with a broadband peaked at 720 nm, which is highly stable while the sample is kept in vacuum. Cross-sectional CL observations show that most of the light is generated in the top surface of the porous layer. The spectral depth dependence of the emitted light reveals a relatively weak blue emission in the region closer to the substrate.
Optimum irradiance distribution of concentrated sunlight for photovoltaic energy conversion
Authors: Benı́tez Pablo , Mohedano Rubén
Published: July 2002

The irradiance distribution on a concentration photovoltaic cell that produces maximum conversion efficiency has been found with the tools of Variational Calculus. The analysis is two dimensional and can be applied to a comb-like double busbar solar cell illuminated by a line-focus concentrator. The optimum distribution is, in general, nonuniform, and depends on the internal parameters of the solar cell: the higher the contribution of the grid to the global cell series resistance, the lower the uniformity of the optimum irradiance distribution. In practical cases, the efficiency for uniform illumination is close to that of the optimum, but in the latter the irradiance close to the busbar may be noticeable higher than the average.
Polarization independent phase demodulation using photorefractive two-wave mixing
Authors: Delaye Philippe , Blouin Alain , Drolet Denis , Monchalin Jean-Pierre , de Montmorillon Louis-Anne , Roosen Gérald
Published: July 2002

We describe both theoretically and experimentally a polarization independent interferometric adaptive photodetector based on photorefractive two-wave mixing. The configuration is based on the simultaneous recording of two independent gratings in a single photorefractive crystal. Applied to the detection of ultrasonic signals, this interferometric photodetector operates with depolarized beams issued from multimode fibers and gives a detection limit close to the ultimate.
Ultrasonic characterization of plastic foams via measurements with static pressure variations
Authors: Ayrault Christophe , Moussatov Alexei , Castagnède Bernard , Lafarge Denis
Published: July 2002

A method for ultrasonic characterization of plastic foams by changing the static pressure of air that saturates the foam has been proposed. The method is based on high frequency asymptotic expressions of the standard Johnson–Allard equivalent fluid model. It is shown, both experimentally and theoretically, that the real part of squared acoustical refractive index and logarithm of the transmission coefficient depend linearly on the inverse of the square root of applied static pressure. These linear relations provide a simple and convenient way to determine experimentally the constitutive parameters. The method is illustrated with industrial open-cell foams. Advantages, limitations, and perspectives are discussed.
Effect of depolarization of scattered evanescent waves on particle-trapped near-field scanning optical microscopy
Authors: Gu Min , Ke Pu Chun
Published: July 2002

The degree of polarization of the scattered evanescent wave is measured with a laser-trapped particle for different incident angles. It is found that depolarization under s polarized beam illumination is stronger than that under p polarized beam illumination. As a result, the contrast of the evanescent wave interference pattern imaged in a particle-trapped near-field scanning optical microscope is improved approximately by a factor of 3 with a parallel analyzer under s polarized beam illumination. The phase shift of scattered evanescent waves under s and p polarized beam illumination is determined from the measured evanescent wave interference pattern.
Evolution of Coulomb blockade spectra in parallel coupled quantum dots
Authors: Adourian A. S. , Livermore C. , Westervelt R. M. , Campman K. L. , Gossard A. C.
Published: July 2002

We report low-temperature conductance measurements in the Coulomb blockade regime on two nominally identical tunnel-coupled quantum dots in parallel defined electrostatically in the two-dimensional gas of a GaAs/AlGaAs heterostructure. We find that the Coulomb blockade spectra of such devices exhibit two distinct sets of peaks, each of which behaves differently with varying interdot tunnel conductance and with temperature. The results conform to recent theories regarding the role of interdot quantum charge fluctuations, and provide evidence for the possible role of inelastic cotunneling between dots at finite interdot conductances.
Thickness dependence of leakage current in BaBi2Ta2O9 thin films
Authors: Foschini C. R. , Longo E. , Varela J. A. , Desu S. B.
Published: July 2002

BaBi 2 Ta 2 O 9 thin films having a layered structure were fabricated by metalorganic solution deposition technique. The films exhibited good structural, dielectric, and insulating properties. The room temperature resistivity was found to be in the range of 1012–1014 Ω cm up to 4 V corresponding to a field of 200 kV/cm across the capacitor for films annealed in the temperature range of 500–700 °C. The current-voltage (I–V) characteristics as a function of thickness for films annealed at 700 °C for 1 h, indicated bulk limited conduction and the log(I) vs V1/2 characteristics suggested a space-charge-limited conduction mechanism. The capacitance–voltage measurements on films in a metal–insulator–semiconductor configuration indicated good Si/BaBi2Ta2O9 interface characteristics and a SiO2 thickness of ∼5 nm was measured and calculated.
Dielectric properties of alkoxy-derived Sr2Nb2O7 thin films crystallized via rapid thermal annealing
Authors: Shoyama Masashi , Tsuzuki Akihiro , Kato Kazumi , Murayama Norimitsu
Published: July 2002

The low temperature crystallization of ferroelectric Sr2Nb2O7 thin films on the Pt/TiO2/SiO2/Si(100) substrate was attained by rapid thermal annealing followed by each coating step using Sr2Nb2O7 precursor solution. X-ray diffraction results indicated that the crystallization of the Sr2Nb2O7 phase was initiated at 550 °C, and the Sr2Nb2O7 films exhibited the (0k0) orientation. The dielectric constant and loss value of the films annealed at 650 and 750 °C are 48, 0.022 and 50, 0.007, respectively, at room temperature (100 kHz).
Octahedral cation site disorder effects on magnetization in double-perovskite Sr2FeMoO6: Monte Carlo simulation study
Authors: Ogale Abhijit S. , Ogale S. B. , Ramesh R. , Venkatesan T.
Published: July 2002

Monte Carlo simulation studies are performed to examine the implications of octahedral cation (Fe, Mo) site disorder for magnetization in the double-perovskite Sr2FeMoO6. Correlations between the near-neighbor cation distributions and the spin distributions are identified to gain insight into the spin arrangement within, and on the periphery of a given transition element cation cluster. It is shown that the drop in the magnetic moment is nearly linear with the increase in the mis-site defect concentration for the case of randomly created defects. Implications of the concomitant presence of mis-site defects and oxygen vacancies are also analyzed.
Polarization analysis of diffracted orders from a birefringence grating recorded on azobenzene containing polymer
Authors: Lagugné Labarthet F. , Rochon P. , Natansohn A.
Published: July 2002

Polarization analyses of transmitted and first-diffracted orders have been performed on photoinduced gratings in azopolymers. The polarization states of diffracted orders depend strongly on irradiation time because of the contribution from both a phase grating at short time and a surface relief grating at longer time. These results are nicely explained using a Jones’ matrix approach which predicts the polarization states of transmitted and diffracted orders of polarization holographic gratings.
The influence of Mg doping on the formation of Ga vacancies and negative ions in GaN bulk crystals
Authors: Saarinen K. , Nissilä J. , Hautojärvi P. , Likonen J. , Suski T. , Grzegory I. , Lucznik B. , Porowski S.
Published: July 2002

Gallium vacancies and negative ions are observed in GaN bulk crystals by applying positron lifetime spectroscopy. The concentration of Ga vacancies decreases with increasing Mg doping, as expected from the behavior of the VGa formation energy as a function of the Fermi level. The concentration of negative ions correlates with that of Mg impurities determined by secondary ion mass spectrometry. We thus attribute the negative ions to MgGa−. The negative charge of Mg suggests that Mg doping converts n-type GaN to semi-insulating mainly due to the electrical compensation of ON+ donors by MgGa− acceptors.
Gain characteristics of InAs/GaAs self-organized quantum-dot lasers
Authors: Harris L. , Ashmore A. D. , Mowbray D. J. , Skolnick M. S. , Hopkinson M. , Hill G. , Clark J.
Published: July 2002

We report a study of the gain characteristics of a self-organized InAs/GaAs quantum-dot laser. Using the Hakki–Paoli technique, we are able to determine the spectral form of the modal gain, its dependence upon current, and the differential gain. A quasiperiodic modulation of the below-threshold gain is observed. This modulation is shown to be responsible for the form of the lasing spectra, which consist of groups of lasing modes separated by nonlasing spectral regions. Possible mechanisms for this behavior are discussed.
Carrier energy relaxation by means of Auger processes in InAs/GaAs self-assembled quantum dots
Authors: Morris D. , Perret N. , Fafard S.
Published: July 2002

Carrier relaxation processes are investigated in self-assembled InAs/GaAs quantum dots using time-resolved photoluminescence spectroscopy. The quantum-dot photoluminescence rise time has been measured as functions of carrier excitation density and excitation wavelengths. The measured relaxation time is about 32 ps at low excitation density and decreases by 1 over the excitation density from about 3 W/cm2, under nonresonant laser excitation. The threshold of this density-dependent regime occurs at a slightly higher density as the excitation wavelength increases and it disappears when the photon pumping energy is below the wetting layer barrier energy. These results clearly establish the regime where Auger processes become the dominant carrier relaxation mechanism in these self-assembled quantum dots.
Electrical properties of ZrO2 gate dielectric on SiGe
Authors: Ngai T. , Qi W. J. , Sharma R. , Fretwell J. , Chen X. , Lee J. C. , Banerjee S.
Published: July 2002

We report the electrical properties of a high dielectric constant (high-k) material, ZrO2, deposited directly on SiGe, without the use of a Si buffer layer or a passivation barrier. ZrO2 thin films of equivalent oxide thickness (EOT) down to 16.5 Å were deposited on strained SiGe layers by reactive sputtering. Results indicate that ZrO2 films on SiGe have good interfacial properties and low leakage currents. Sintering in forming gas at 350 °C for 1 h could further improve the film quality. Although threshold voltage stability and dielectric dispersion become a concern for thick ZrO2 films, thin ZrO2 films of EOT less than 20 Å exhibit excellent electrical properties making them a good candidate for SiGe applications.
Fabrication of circular optical structures with a 20 nm minimum feature size using nanoimprint lithography
Authors: Li Mingtao , Wang Jian , Zhuang Lei , Chou Stephen Y.
Published: July 2002

We demonstrated the fabrication of Fresnel zone plates with a 75 nm minimum feature size and circular gratings with a 20 nm minimum linewidth in polymethyl methacrylate using nanoimprint lithography, and in metals by means of a lift-off technique. Observation of sharp Moiré patterns indicated the high fidelity of nanoimprint lithography in pattern duplication. Our results showed that nanoimprint lithography is a promising technology for patterning integrated optics.
Improved brightness, efficiency, and stability of sputter deposited alternating current thin film electroluminescent ZnS:Mn by codoping with potassium chloride
Authors: Waldrip Karen E. , Lewis J. S. , Zhai Q. , Davidson M. R. , Holloway P. H. , Sun S. S.
Published: July 2002

Electroluminescent ZnS:Mn thin films have been codoped with KCl via an ex situ deposition and diffusion process. The brightness, efficiency, and stability of sputter deposited ZnS:Mn alternating current thin film electroluminescent devices have been greatly improved over untreated devices. Potassium chloride doping is shown to have a modest fluxing effect on the grain size of ZnS:Mn films, particularly in the region of the film where nucleation occurs. However, improved electroluminescence is postulated to be strongly influenced by doping control of electric fields and injected charge.
A cross-correlation spectroscopy in subterahertz region using an incoherent light source
Authors: Morikawa O. , Tonouchi M. , Hangyo M.
Published: July 2002

A subterahertz (sub-THz) spectroscopic system using a multimode laser diode and photoconductive antennas (PA) has been proposed. It employs a random fluctuation of the light intensity to produce the subterahertz radiation from the emitter PA and also to trigger the detector PA. The signal is obtained as the cross correlation between the sub-THz radiation amplitude and laser light intensity. The decrease in the amplitude and phase delay of the radiation due to transmission from a sample can be calculated from the signal in a broad spectral region of sub-THz. This system is applied to the measurement of the complex refractive indices of Si wafers. The obtained dispersion of the refractive indices is well explained by the Drude model.
Step-by-step excimer laser induced crystallization of a-Si:H
Authors: Lengsfeld P. , Nickel N. H. , Fuhs W.
Published: July 2002

Amorphous silicon films (a-Si:H) with a hydrogen content of 10 at. % were crystallized employing a step-by-step crystallization method. Structural changes during the sequential crystallization process were monitored by Raman spectrometry. Initially, at low laser fluences EL, a two-layer system is created. Independent of the thickness of the a-Si:H layer explosive crystallization of a thin surface layer is observed at EL⩾100 mJ/cm2 confirming recent theoretical results. Crystallization is accompanied by dehydrogenation. In completely crystallized poly-Si a residual H concentration of up to 5 at. % was observed.
Effect of Ar ion beam channeling on AlGaN/GaN heterostructures during the ion beam etching process
Authors: Breitschädel O. , Hsieh J. T. , Kuhn B. , Scholz F. , Schweizer H.
Published: July 2002

The effect of Ar+ ion beam etching of AlGaN/GaN heterostructures at a bias voltage of 250 V was investigated with respect to different ion incident angles. The samples were measured before and after etching with respect to mobility, sheet electron concentration, and sheet resistance. We found a pronounced dependency of the electrical characteristics after etching on the ion incident angle. Especially at zero degree, the mobility of the two-dimensional electron gas (2DEG), which is located at the AlGaN/GaN interface, decreases dramatically after etching. The sheet resistance increases in the same way. At larger ion incidence angles, the effect vanished. We attribute this behavior predominantly to channeling of the ions through the AlGaN layer down to the 2DEG. An annealing step after etching shows improvement of the electrical characteristic. These results show that gate-recessed GaN field effect transistors can be limited in their device performance by etch-process-induced ion channeling effects. The results also show how etch-induced defects in the gate recess step can minimized.
Nanopatterning of copper (111) vicinal surfaces by oxygen-induced mesoscopic faceting
Authors: Vollmer S. , Birkner A. , Lukas S. , Witte G. , Wöll Ch.
Published: July 2002

We report the preparation of periodic nanostripes on vicinal copper surfaces. For the investigated Cu(n,n,n−1) surfaces [n=4,3,2], an oxygen-induced mesoscopic faceting of the regular monoatomic stepped surfaces into periodic nanostripes consisting of Cu(111) and Cu(110)–O(2×1) facets is observed. The width and thermal stability of these nanostripes increase with the terrace length of the initial vicinal surfaces. Stripe widths of 50, 20, and 12 nm were obtained for Cu(443), Cu(332), and Cu(221), respectively. Whereas on Cu(221) the nanostripes disappear above 450 K, they are stable up to 800 K on Cu(443). For the latter surface, the nanostructures are found to be unusually stable and could be observed ex situ by atomic force microscopy under ambient conditions.
Self-organized type-II In0.55Al0.45As/Al0.50Ga0.50As quantum dots realized on GaAs(311)A
Authors: Liu H. Y. , Zhou W. , Ding D. , Jiang W. H. , Xu B. , Liang J. B. , Wang Z. G.
Published: July 2002

Self-organized In0.55Al0.45As/Al0.50Ga0.50As quantum dots are grown by the Stranski–Krastanow growth mode using molecular beam epitaxy on the GaAs(311)A substrate. The optical properties of type-II InAlAs/AlGaAs quantum dots have been demonstrated by the excitation power and temperature dependence of photoluminescence spectra. A simple model accounting for the size-dependent band gap of quantum dots is given to qualitatively understand the formation of type-II In0.55Al0.45As/Al0.50Ga0.50As quantum dots driven by the quantum-confinement-induced Γ→X transition. The results provide new insights into the band structure of InAlAs/AlGaAs quantum dots.
Strong surface disorder and loss of N produced by ion bombardment of GaN
Authors: Kucheyev S. O. , Williams J. S. , Jagadish C. , Li G. , Pearton S. J.
Published: July 2002

The damage buildup in wurtzite GaN films under light (12C) and heavy (197Au) ion bombardment at temperatures from −196 to 550 °C is studied by Rutherford backscattering/channeling spectrometry. A strong surface peak of lattice disorder in addition to the expected damage peak in the region of the maximum of nuclear energy loss has been observed for all implant conditions of this study. Capping of GaN with SiOx and SixNy layers prior to implantation somewhat reduces but does not eliminate surface disordering. This suggests that nitrogen loss is not the main reason for the observed enhanced surface disorder, but, rather, the GaN surface acts as a strong sink for migrating point defects. However, pronounced loss of N during ion bombardment is observed for high dose implantation when the near-surface region is amorphized. Moreover, after amorphization, annealing at temperatures above about 400 °C leads to complete decomposition of the near-surface layer.
Schwarzschild perturbations die in time
Authors: Friedman John L. , Morris Michael S.
Published: July 2002

Smooth perturbations of Schwarzschild black holes whose initial data has compact support outside the horizon are shown to die in time along the trajectory of the asymptotically timelike Killing vector tα. A gravitational or other zero-rest-mass perturbation of a Schwarzschild black hole can be expressed in terms of radial derivatives of a scalar field Φ that satisfies a wave equation with positive potential. A theorem due to Wilcox is used to show that the pointwise limit as t→±∞ of Φ and all its derivatives vanishes. This result strengthens previous work that bounds the perturbation in time.
Momentum transfer in the combined release and radiation effects satellite plasma injection experiments: The role of parallel electric fields
Authors: Delamere P. A. , Stenbaek-Nielsen H. C. , Swift D. W. , Otto A.
Published: July 2002

Optical observations of the combined release and radiation effects satellite (CRRES) plasma injection experiments have revealed signatures of the coupling processes between the injected and ambient plasmas. The elongation of the ion cloud along the geomagnetic field indicated the presence of parallel electric fields. A dense core of ions polarized and E×B drifted along the satellite trajectory for 6–10 s. In an effort to understand the momentum coupling between the two plasma populations, a three-dimensional hybrid code is used. The simulation, which neglected electron inertia, showed a very efficient transfer of momentum via Alfvén waves and the skidding ion core was stopped within 2 s. To reconcile the difference between the observed and simulated skidding times, we propose that the cloud must have been decoupled from the ambient plasma via parallel electric fields. The parallel fields may be associated with inertial Alfvén waves propagating in filamentary current layers at the edges of the ion cloud, or due to a variety of possible plasma instabilities. The results of this study can be applied to coupling processes in other geophysical and astrophysical systems such as cometary environments, Io’s plasma torus, and the auroral regions of the ionosphere and magnetosphere.
Simulation of the third law free energies of face-centered-cubic and hexagonal-close-packed Lennard-Jones solids
Authors: Somasi Sweta , Khomami Bamin , Lovett Ronald
Published: July 2002

On a thermodynamic path proposed by Lutsko, Wolf, and Yip, a solid is transformed at fixed density into an ideal gas. The accuracy of molecular dynamics simulation results for the free energy of solids using this path is compromised by the presence of a singularity on the path and by the nonergodic behavior of the ideal gas. We present calculations of the third law free energies of the face-centered-cubic (fcc) and hexagonal-close-packed (hcp) solids of argon (modeled as a Lennard-Jones system) to illustrate how the singularity and the nonergodic dynamics introduce errors. Strategies for removing the singularity and the nonergodic dynamics are presented. Finally, an analytic expansion around the ideal state is developed to provide an alternate route to the free energy of those states for which simulation estimates give large statistical uncertainties. The quantitative errors in these new approaches to the simulation of the free energy of a solid are given.
9,9′-Bianthryl and its van der Waals complexes studied by rotational coherence spectroscopy: Structure and excited state dynamics
Authors: Fujiwara Takashige , Fujimura Yo , Kajimoto Okitsugu
Published: July 2002

The structure and excited state dynamics of jet-cooled 9,9′-bianthryl (BA) and its 1:1 van der Waals (vdW) complexes with Ne, Ar, and H2O were studied using rotational coherence spectroscopy (RCS). For a free BA molecule, the magnitude and persistence of the recurrent transient appearing in the time-correlated single photon counting (TCSPC) measurement was found to be dependent on the torsional level of BA, indicating the rotational constant changes with the torsional energy level. The RCS–TCSPC measurement of the BA–Ar and BA–H2O complexes in the S1 state showed no coherent transients. However, the pump–probe time-resolved fluorescence depletion (TRFD) detected the weak J-type transient. Those facts imply the loss of coherence in the BA vdW complexes due to the excited-state dynamics, which coincides with the analysis of the laser-induced fluorescence excitation and dispersed fluorescence spectra. The structure of the ground-state 1:1 BA complex with Ne, Ar, and H2O was determined based on the RCS transients observed in the TRFD measurement with the help of a minimum energy structure calculation using atom–atom pairwise potentials. The rapid dephasing in the excited state was demonstrated by the magic angle TRFD detection near t=0. The dominant dephasing process for the rare-gas complexes is ascribed to intramolecular vibrational energy redistribution (IVR) which is accelerated by significant coupling between the torsional vibration and the low-lying vdW vibrations. IVR process for the H2O complex accompanies the rapid conversion to the charge-transfer state, which is also responsible for the loss of excited-state coherence.
Quaternion formulation of diffusion quantum Monte Carlo for the rotation of rigid molecules in clusters
Authors: Benoit David M. , Clary David C.
Published: July 2002

A quaternion formulation is used to derive an algorithm for performing calculations on molecular clusters using the quantum diffusion Monte Carlo method. It is assumed that the monomers in the cluster rotate and translate as rigid bodies. The algorithm is tested on the water dimer and the benzene–water cluster. Comparison with dissociation energies and rotational constants obtained with other methods illustrates the accuracy of the algorithm.
Sintering, structure, and mechanical properties of nanophase SiC: A molecular-dynamics and neutron scattering study
Authors: Chatterjee Alok , Kalia Rajiv K. , Nakano Aiichiro , Omeltchenko Andrey , Tsuruta Kenji , Vashishta Priya , Loong Chun-Keung , Winterer Markus , Klein Sylke
Published: July 2002

Structure, mechanical properties, and sintering of nanostructured SiC (n-SiC) are investigated with neutron scattering and molecular-dynamics (MD) techniques. Both MD and the experiment indicate the onset of sintering around 1500 K. During sintering, the pores shrink while maintaining their morphology: the fractal dimension is ∼2 and the surface roughness exponent is ∼0.45. Structural analyses reveal that interfacial regions in n-SiC are disordered with nearly the same number of three- and fourfold coordinated Si atoms. The elastic moduli scale with the density as ∼ρμ, where μ=3.4±0.1.
Relaxation and electron transfer dynamics in bare and DTDCI sensitized MoS2 nanoclusters
Authors: Chikan V. , Waterland M. R. , Huang J. M. , Kelley D. F.
Published: July 2002

The trapping dynamics of photogenerated electrons and holes in MoS2 nanoclusters has been studied using time resolved emission polarization and absorption spectroscopies. These results are compared to absorption kinetics obtained on MoS2 nanoclusters with adsorbed DTDCI (DTDCI≡diethylthiodicarbocyanine iodide) dye. The results indicate that emission from the MoS2 band edge state is polarized, while emission from trapped electrons and holes is unpolarized. This polarization difference is used to obtain the electron and hole trapping times and values of 275 ps and 42 ps, respectively, are obtained. Decays having the same time constants are observed in the transient absorption results. The results obtained on the MoS2/DTDCI system show that electron injection occurs with a time constant of 12 ps. These kinetics also show a 225 ps decay component which is assigned to electron trapping and reverse electron transfer. The 225 ps decay time along with the 275 ps trapping time indicates that reverse electron transfer from the conduction band has a time constant of about 1.2 ns. Following trapping, reverse electron transfer from deep traps is slow.
AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors on SiC substrates
Authors: Khan M. Asif , Hu X. , Tarakji A. , Simin G. , Yang J. , Gaska R. , Shur M. S.
Published: July 2002

We report on AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOS-HFETs) grown over insulating 4H–SiC substrates. We demonstrate that the dc and microwave performance of the MOS-HFETs is superior to that of conventional AlGaN/GaN HFETs, which points to the high quality of SiO2/AlGaN heterointerface. The MOS-HFETs could operate at positive gate biases as high as +10 V that doubles the channel current as compared to conventional AlGaN/GaN HFETs of a similar design. The gate leakage current was more than six orders of magnitude smaller than that for the conventional AlGaN/GaN HFETs. The MOS-HFETs exhibited stable operation at elevated temperatures up to 300 °C with excellent pinch-off characteristics. These results clearly establish the potential of using AlGaN/GaN MOS-HFET approach for high power microwave and switching devices.
High power solid state single-frequency pulsed laser for Doppler velocimeters
Authors: Ludmirsky A. , Moshe E. , Horovitz Y. , Fraenkel M. , Henis Z. , Goldberg I. B.
Published: July 2002

A high power solid state single-frequency pulsed oscillator/amplifier laser system developed for Doppler velocimeters is described. The oscillator is based on a continuous wave diode pumped Nd:YAG single frequency laser, operating at a wavelength of 1064 nm. The output of the laser is chopped into pulses of 100 ns to 10 μs duration and 10 Hz repetition rate by a Pockels cell and polarizers. The pulses are amplified by a double pass flash-pumped Nd:YAG amplifier to kW power levels. The amplified laser pulses are frequency doubled with high efficiency to 532 nm in a periodically poled KTiOPO4 crystal. This ladder system is useful for many velocimeter applications and its utilization in an optically recording velocity interferometer system is described.
Activation of aluminum implanted at high doses in 4H–SiC
Authors: Bluet J. M. , Pernot J. , Camassel J. , Contreras S. , Robert J. L. , Michaud J. F. , Billon T.
Published: July 2002

We report an investigation of the electrical activation of aluminum implanted at high dose in 4H–SiC. We show that at reasonably high temperature implantation and annealing conditions, one activates about 37.5% of the implanted species. Of course, the final (concentration-dependent) activation ratio differs slightly from this average value but varies only between 0.5 and 0.25 when the targeted concentration increases from 3.33×1018 to 1021 cm−3. Provided a standard mobility can be maintained, this results in fairly low sheet resistance. The best (lowest) value obtained in this work is 15 mΩ cm at 700 K (95 mΩ cm at room temperature) for a 190-nm-thick layer implanted with 1021 atoms cm−3. In MESA-etched p–n junctions with a 100 μm diameter, this resulted in a typical on-resistance of 1.5 mΩ cm2, mainly limited by the substrate and n− epitaxial layer.
Fluctuon effects in ferroelectric polarization switching
Authors: Molotskii M. , Kris R. , Rosenman G.
Published: July 2002

The perplexing problem of the extremely large activation energy for nucleation Uac∼106 eV, which, as was shown by Landauer, appears in the Merz thermal-fluctuational theory of ferroelectric switching, is examined. Such enormous values of Uac are a result of the combined contribution of the energy of the depolarization field and the surface energy of the nucleus. While the depolarizing field term can be reduced by a redistribution of free charges, the surface energy term alone is too large (greater than 104 eV) for thermal-fluctuation-aided nucleation to be possible. This conclusion contradicts the numerous observations of a rapid increase in the number of repolarization nuclei with temperature, which suggest that the actual activation energy is on the order of an electron volt. This article is devoted to the problem of how the surface energy of a nucleus can be compensated. It is assumed that repolarization nuclei form by a fluctuon mechanism, and it is shown how this assumption can help in resolving the conflict between theory and experiment. It is suggested that upon application of an external electric field, electrons tunnel into the region where the spontaneous polarization vector fluctuates. This fluctuation region maintains its stability owing to the field of the electrons localized within the region. This leads to the appearance of a multielectron fluctuon, which is a bound state of electrons and a fluctuation and can be regarded as a repolarization nucleus. The electronic structure of these nuclei is calculated with density-functional theory. The calculations show that fluctuon effects can fully compensate the surface energy term. The assumption that repolarization nuclei have a fluctuon nature is shown to account for the principal laws governing the switching process, including low activation energy values. The results of possible experiments that could verify the proposed mechanism are predicted.
Simultaneous measurement of electron and hole mobilities in polymer light-emitting diodes
Authors: Martens H. C. F. , Huiberts J. N. , Blom P. W. M.
Published: July 2002

The transport properties of electrons and holes in a poly(p-phenylene vinylene)-based polymer light-emitting diode (PLED) have been investigated. Using admittance spectroscopy, we are able to simultaneously obtain the electron and hole mobility, μ=μ0 exp(γE), in a single PLED. The dynamics of the electron and hole transport are separated in the frequency domain as a result of the different transit times. At room temperature, we find μ0=5.6×10−11 m2/V s and γ=5.0×10−4 (m/V)1/2 for holes. For electrons μ0=1.0×10−12 m2/V s and γ=8.0×10−4 (m/V)1/2 are obtained.
Characterizations of the canonical phase observable
Authors: Lahti Pekka , Pellonpää Juha-Pekka
Published: July 2002

In this paper we investigate various properties of phase observables which could serve to determine the canonical phase observable among the family of all phase observables. We also show that any contractive weighted shift operator defines a unique phase observable, and we characterize phase observables that give the most accurate phase distribution in coherent states in the classical limit.
The G-generalized Hitchin systems and Prym varieties
Authors: Kjiri M.
Published: July 2002

In this article, we consider the generalized Hitchin systems, introduced by Bottacin and Markman, for an arbitrary reductive complex group G; they have the structure of Lagrangian fibrations Pr→U by generalized Prym varieties over sets U parametrizing families of Weyl-invariant curves. The G-generalized Hitchin systems satisfy a rank two condition and one can find invariant varieties X which distinguish these systems. It is then shown that there is a correspondence between these integrable systems and the varieties X=KΣ[D]⊗h̃, which are equipped with an appropriate two form with values in the Cartan subalgebra 𝔥.
Construction of multisymplectic schemes of any finite order for modified wave equations
Authors: Sun Ya Juan , Qin Meng Zhao
Published: July 2002

In this paper, we analyze several symplectic discretizations of linear wave equations with periodic boundary conditions, and show the discretizations are multisymplectic of arbitrary finite order for wave equations when the periodic boundary condition vanishes.
Nonlocal regularization for non-Abelian gauge theories for arbitrary gauge parameter
Authors: Basu Anirban , Joglekar Satish D.
Published: July 2002

We study the nonlocal regularization for the non-Abelian gauge theories for an arbitrary value of the gauge parameter ξ. We show that the procedure for the nonlocalization of field theories established earlier by the original authors, when applied in that form to the Faddeev–Popov effective action in a linear gauge cannot lead to a ξ-independent result for the observables. We then show that an alternate procedure which is simpler can be used and that it leads to the S-matrix elements (where they exist) independent of ξ.