bluuebunny/crossref_metadata_2025 huggingface dataset:
Deflection detection and feedback actuation using a self-excited piezoelectric Pb(Zr,Ti)O3 microcantilever for dynamic scanning force microscopy
Authors: Itoh T. , Lee C. , Suga T.
Published: July 2002

We have developed a dynamic scanning force microscope (SFM) that utilizes microfabricated Pb(Zr,Ti)O3, (PZT) cantilever for lever excitation, deflection sensing and tip-sample spacing control. The lever is a unimorph centilever including a sol–gel derived PZT thin film that has high piezoelectric constants in comparison with sputtered ZnO films. For dynamic operation, the excitation ac voltage signal superimposed on the actuation dc voltage is applied to the PZT layer. The variation of vibration amplitude is detected by measuring the change of current through the layer. By actuating the self-excited cantilever to keep the current constant, we obtain topography images without z actuation of the sample-side scanner. The 200-μm-long PZT microcantilever with the natural resonance frequency of 63.8 kHz has the high actuation sensitivity of 150 nm/V and the maximum range of more than 1.5 μm. Using the SFM, we have obtained the clear cyclic contact images of an evaporated Au film surface.
PtSi–n–Si Schottky-barrier photodetectors with stable spectral responsivity in the 120–250 nm spectral range
Authors: Solt K. , Melchior H. , Kroth U. , Kuschnerus P. , Persch V. , Rabus H. , Richter M. , Ulm G.
Published: July 2002

Front-illuminated PtSi–n–Si Schottky barrier photodiodes have been developed for the ultraviolet and vacuum ultraviolet spectral range. Their spectral responsivity was determined in the 120–500 nm spectral range by use of a cryogenic electrical substitution radiometer operated with spectrally dispersed synchrotron radiation. For wavelengths below 250 nm, the spectral responsivity is about 0.03 A/W, comparable to that of GaAsP Schottky photodiodes. Unlike the GaAsP diodes, the new PtSi–n–Si diodes have a spatially uniform response which is virtually stable after prolonged exposure to short wavelength radiation. Even after a radiant exposure of 150 mJ cm−2 at wavelength 120 nm, the relative reduction in spectral responsivity remains below 0.2%. Due to these features, this type of photodiode is a promising candidate for use as secondary detector standard in the ultraviolet and vacuum ultraviolet spectral ranges.
Thermal annealing of implantation-induced compaction for improved silica waveguide performance
Authors: Johnson C. M. , Ridgway M. C. , Leech P. W.
Published: July 2002

The optimum processing parameters for the fabrication of low-loss waveguides in fused silica by ion implantation and annealing have been determined through a comparison of implantation-induced physical and optical properties. The step height at an implanted/unimplanted boundary resulting from Si implantation was measured as a function of the ion dose (2×1012–6×1016/cm2) and energy (1–9 MeV) and the postimplantation annealing temperature (200–900 °C) and time (0.1–150 min). For a given ion energy (5 MeV), the step height increased for doses <∼1015/cm2 and thereafter, saturated. For a given ion dose (2×1015/cm2), a near-linear trend was evident for step height as a function of ion energy. Isochronal and isothermal annealing both resulted in a nonlinear reduction in step height, typical of a thermally induced process. In contrast to the continual reduction in step height observed during isochronal annealing, the loss coefficient exhibited a distinct minimum of ∼0.15 dB/cm at an intermediate temperature of 500 °C. This feature was consistent with the removal of a specific defect or color center.
Growth of adherent diamond films on optically transparent sapphire substrates
Authors: Singh Rajiv K. , Gilbert D. R. , Laveigne J.
Published: July 2002

The growth of continuous adherent diamond thin films on optically transparent substrates is important for the development of corrosion and erosion resistant infrared windows for many applications. Until now, the growth of adherent diamond films on optically transparent substrates like sapphire has been unsuccessful due to the large thermal mismatch between the film and the substrate and the absence of an interfacial carbide ‘‘glue’’ layer. By employing a low temperature (500–550 °C), low pressure (∼1 Torr) electron–cyclotron–resonance chemical-vapor-deposition process, and utilizing a dispersed–particulate diamond suspension for nucleation, adherent diamond thin films have been fabricated on sapphire substrates. Raman spectroscopy showed that the diamond peak was shifted approximately 6 cm−1 above its equilibrium position, suggesting the presence of very large compressive stresses (∼3.2 GPa) in the film.
A domain size effect in the magnetic hysteresis of NiZn-ferrites
Authors: van der Zaag P. J. , van der Valk P. J. , Rekveldt M. Th.
Published: July 2002

The domain size and the ac-hysteresis of NiZn-ferrites have been investigated as a function of grain size. It is found that the ac-hysteresis or energy dissipation at MHz frequencies is substantially reduced in fine-grained, monodomain NiZn-ferrites, which is attributed to the absence of intragranular domain walls.
Degradation of liquid crystal device performance due to selective adsorption of ions
Authors: Perlmutter S. H. , Doroski D. , Moddel G.
Published: July 2002

Performance degradation in liquid crystal cells was studied by analyzing the time evolution of optical and integrated current vs voltage hysteresis curves as test cells were subjected to a dc bias. We find evidence for permanent increases in liquid crystal mobile ion populations, the primary cause of device performance degradation, and suggest this permanent increase is due to selective adsorption by the alignment layers of ions of a single charge sign combined with the presence of a neutral ionizable species in the liquid crystal.
Generation of coherent terahertz radiation by photomixing in dipole photoconductive antennas
Authors: Matsuura Shuji , Tani Masahiko , Sakai Kiyomi
Published: July 2002

We demonstrate the generation of continuous-wave terahertz radiation at frequencies up to 3.5 THz by photomixing in LT-GaAs photoconductors with printed dipole antennas. The expected resonance peak was clearly observed in the radiation spectrum of a 50 μm dipole antenna, and the radiation property was discussed in terms of the antenna impedance. The spectral bandwidth was greater than the electrical bandwidth of the photoconductor determined by the carrier lifetime of the material. The quality of the resonance, however, was not as high as expected because of the imperfect geometry and ohmic loss of the antenna.
Ultraviolet stimulated emission due to biexciton decay process in ZnS-based quantum wells
Authors: Yamada Yoichi , Yoshimura Kazumasa , Fujita Shin-ichi , Taguchi Tsunemasa , Sasaki Fumio , Kobayashi Shunsuke , Tani Toshiro
Published: July 2002

Recombination dynamics of dense excitonic systems in CdxZn1-xS–ZnS quantum-well structures has been studied by means of time-resolved luminescence spectroscopy. Experimental evidence for biexciton formation was obtained. Furthermore, ultraviolet stimulated emission was observed just at the energy position of the biexciton luminescence. It was proposed that the stimulated emission was due to the biexciton decay process.
Is the arsenic incorporation kinetics important when growing GaAs(001), (110), and (111)A films?
Authors: Tok E. S. , Jones T. S. , Neave J. H. , Zhang J. , Joyce B. A.
Published: July 2002

The incorporation coefficients of As2 and As4, obtained from reflection high-energy electron diffraction intensity oscillations in the As-limited growth regime, are compared for the growth of GaAs on (001), (110), and (111)A surfaces by molecular beam epitaxy. The kinetic results are remarkably similar for (110) and (111)A, but very different from those obtained on (001). The incorporation coefficients decrease with increasing temperature for all three surfaces, with the effect being much more dramatic on (110) and (111) A. The low- and temperature-dependent incorporation coefficients on (110) and (111)A explain the need for high As:Ga flux ratios and low substrate temperatures in the preparation of high-quality GaAs epitaxial layers.
SiO
2
film thickness metrology by x-ray photoelectron spectroscopy
Authors: Lu Z. H. , McCaffrey J. P. , Brar B. , Wilk G. D. , Wallace R. M. , Feldman L. C. , Tay S. P.
Published: July 2002

Silicon dioxide films grown by industrial thermal furnace, rapid thermal, and low-pressure thermal methods were measured by x-ray photoelectron spectroscopy, transmission electron microscopy (TEM), spectroscopic ellipsometry, and capacitance–voltage analysis. Based on TEM measurements, the photoelectron effective attenuation lengths in the SiO2 and Si are found to be 2.96±0.19 and 2.11±0.13 nm, respectively. The oxide physical thicknesses (range from 1.5 to 12.5 nm) as measured by all above techniques are in good agreement. The electrical thickness is noted to be slightly thicker than the physical thickness.
The investigation of the relaxation processes in antiferroelectric liquid crystals by electro-optic spectroscopy
Authors: Panarin Yu. P. , Kalinovskaya O. , Vij J. K.
Published: July 2002

Electrooptic spectroscopy of an antiferroelectric liquid crystal is carried out over a range of frequencies from 1 Hz to 100 kHz. In the antiferroelectric SmCA phase two relaxation processes are found, one at the fundamental frequency of a mode and the second at twice the frequency of a different mode. A comparison of the results of the electro-optic spectroscopy with a theoretical study of the motion of the director of an antiferroelectric helix subject to a weak alternating field enables a determination of the origin of the relaxation processes in antiferroelectric phases.
Hydrogen-induced thermal interface degradation in (111) Si/SiO2 revealed by electron-spin resonance
Authors: Stesmans A. , Afanas’ev V. V.
Published: July 2002

Electron-spin resonance (ESR) experiments show that the interface degradation induced in thermal (111) Si/SiO2 by postoxidation annealing (POA) in vacuum—previously isolated by ESR as a permanent creation of Pb (∘Si≡Si3) interface defects—is strongly enhanced (∼6 times) when performed in H2 ambient. It, thus, appears that the H2 POA step, standardly applied to passivate interface states (preexisting Pbs) naturally introduced during oxidation, effectively creates additional defect entities; the process initiates from ∼550 °C onward vis-à-vis ∼640 °C for vacuum. The results unveil the atomic nature of one of the mechanisms of the electrically long-known H-induced POA generation of adverse interface states.
Electronic origin of the stability trend in TiSi2 phases with Al or Mo layers
Authors: Bònoli F. , Iannuzzi M. , Miglio Leo , Meregalli V.
Published: July 2002

Through a tight-binding rigid-band approach we show that changes in the relative stability of the C54, C49, and C40 phases of TiSi2, with electrons per atom ratio, are produced by the corresponding differences in the electronic density of states at the Fermi level. In particular, by increasing this ratio the stable phase evolves from C49 to C54, and then to C40. Our microscopic model provides a straightforward interpretation of very recent experimental findings concerning the sizeable variations in the transition temperature between C49 and C54 TiSi2 in the presence of Al or Mo layers.
Mechanism of photoluminescence of Si nanocrystals fabricated in a SiO2 matrix
Authors: Zhuravlev K. S. , Gilinsky A. M. , Kobitsky A. Yu.
Published: July 2002

The luminescence properties of silicon nanocrystals fabricated by Si ion implantation into a SiO2 matrix and subsequent thermal annealing have been studied. To identify the mechanism of photoluminescence of Si nanocrystals, the dependencies of the steady-state photoluminescence on temperature and excitation power density, and the time-resolved photoluminescence have been investigated. The experimental results point to the mechanism of recombination via the levels of centers which are presumably localized at the silicon nanocrystal–silicon dioxide boundary.
Improved energy transfer in electrophosphorescent devices
Authors: O’Brien D. F. , Baldo M. A. , Thompson M. E. , Forrest S. R.
Published: July 2002

External quantum efficiencies of up to (5.6±0.1)% at low brightness and (2.2±0.1)% at 100 cd/m2 are obtained from a red electrophosphorescent device containing the luminescent dye 2,3,7,8, 12,13,17,18-octaethyl-21H23H-phorpine platinum(II) (PtOEP) doped in a 4,4′-N,N′-dicarbazolebiphenyl (CBP) host. Due to weak overlap between excitonic states in PtOEP and CBP, efficiency losses due to nonradiative recombination are low. However, energy transfer between the species is also poor. In compensation, a thin layer of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline is used as a barrier to exciton diffusion in CBP, improving the energy transfer to PtOEP. This technique may be applied to improve the efficiency of other electrophosphorescent devices.
Ultraviolet and blue emission from crystalline SiO2 coated with LiNbO3 and LiTaO3
Authors: Siu G. G. , Wu X. L. , Gu Y. , Bao X. M.
Published: July 2002

Crystalline SiO2 (α quartz) coated with LiNbO3 (LN) or LiTaO3 (LT) films emits two strengthened luminescence bands at 368 and 468 nm compared with those of pure α-SiO2, when excited with 280 nm light of a Xe lamp. These two bands strongly depend on the coating: the LN-coated α-SiO2 has a much stronger photoluminescence (PL) than the LT-coated α-SiO2. On the other hand, the two bands are different from those of α-SiO2 in that their excitation spectra do not have any noticeable band in the range 200–320 nm. They could be understood based on excitons in the α-SiO2 surface, which are induced by the photorefractive effect of the coated films. The 368 nm PL band is attributed to the optical transitions of the E′ defect pairs and the 468 nm PL band to the radiative recombination of the self-trapped excitons.
High-reflectivity GaN/GaAlN Bragg mirrors at blue/green wavelengths grown by molecular beam epitaxy
Authors: Langer R. , Barski A. , Simon J. , Pelekanos N. T. , Konovalov O. , André R. , Dang Le Si
Published: July 2002

Highly-reflective GaN/GaAlN quarter-wave Bragg mirrors, designed to be centered at blue/green wavelengths, have been grown by molecular beam epitaxy. The reflectivity for a mirror centered at 473 nm was as high as 93% and the bandwidth reached 22 nm. Detailed x-ray diffraction measurements allowed us to characterize the structural parameters of the Bragg mirrors. We show that, in spite of substantial strain relaxation occurring in our samples, high reflectivity is still possible. In addition, we show that growth interruption at the heterointerfaces is crucial for achieving high reflectivities.
High-Tc edge junctions with a Ga-doped YBa2Cu3O7−δ barrier and interface resistances
Authors: Song I. H. , Lee E.-H. , Kim B. M. , Song I. , Park G.
Published: July 2002

High-Tc ramp-edge junctions with a Ga-doped YBa2Cu3O7−δ barrier have been fabricated in the trilayer geometry of YBa2Cu3O7−δ/YBa2Cu2.79Ga0.21O7−δ/YBa2Cu3O7−δ on LaAlO3 single crystals. Interface resistances of the junctions were drastically reduced by using an in situ Ar plasma cleaning treatment. The Ga-doped YBa2Cu3O7−δ junctions with barrier thickness of 200 and 300 Å clearly exhibited resistively-shunted-junction-like current–voltage characteristics. The critical currents of the Ga-doped junctions were less sensitive to the variation of the barrier thickness compared to those of the other junctions. The increase of the barrier resistivity by Ga-doping and the in situ rf plasma cleaning treatment resulted in an enhancement of the junction reliability and reproducibility.
High oxygen pressure generation of flux-pinning centers in melt-textured YBa2Cu3O7
Authors: Puig T. , Plain J. , Sandiumenge F. , Obradors X. , Rabier J. , Alonso J. A.
Published: July 2002

A strong enhancement of the critical current density, Jcab(H,T), has been generated in melt-textured YBa2Cu3O7/Y2BaCuO5 composites by means of high oxygen pressure (≈100 bar, ∼400 °C). Enhancement factors as high as 180% have been found at low temperatures leading high sample performances at 77 K (Jcab≈1.2×105 A/cm2 at null field). Transmission electron microscopy studies show a high concentration of dendritic-like stacking faults generated as a consequence of the strong nonequilibrium conditions of the transformation from YBa2Cu3O7 to YBa2Cu4O8. The high concentration of 1/6〈031〉 partial dislocations within the (001) planes are proposed as point-defect pinning centers enhancing Jcab(H,T) for the H∥c configuration.
Electron-beam induced growth of Cu nanoparticles in silica glass matrix
Authors: Ito Y. , Jain H. , Williams D. B.
Published: July 2002

The growth of nanometer-scale Cu metal particles in an amorphous SiO2 film was observed under a finely focused, intense electron beam in a dedicated scanning transmission electron microscope. Annular dark-field images show that the particle illuminated by the beam grows at the expense of surrounding Cu particles. Electron energy-loss spectroscopy was used for identifying the oxidation states of particles containing Cu atoms. An electric-field induced diffusion model explains the growth of metal particles in a dielectric medium.
Tantalum carbide ohmic contacts to n-type silicon carbide
Authors: Jang T. , Porter L. M. , Rutsch G. W. M. , Odekirk B.
Published: July 2002

Tantalum carbide contacts with and without Au, Pt, and W/WC overlayers on n-type 6H–SiC (0001) were ohmic after annealing at temperatures between 800 and 1075 °C. Specific contact resistivities (SCRs) were calculated from current–voltage measurements of transmission line model patterns at temperatures ranging from 20 to 400 °C in air. The minimum SCRs at room temperature on SiC (2.3×1019 cm−3) for TaC and for TaC with Pt and Au overlayers were 2.1×10−5, 7.4×10−6, and 1.4×10−6 Ω cm2, respectively. The SCRs for both the Au/TaC/SiC (5.3×10−7Ω cm2) and the Pt/TaC/SiC (7.5×10−7 Ω cm2) samples decreased with measurement temperature to 200 and 400 °C, respectively, while the latter samples showed reversibility after heating to 400 °C. W/WC/TaC/SiC samples showed the best stability after annealing at 400 °C for 144 h in vacuum. Changes in the electrical characteristics were correlated with increases in O incorporation in the contacts as a result of annealing. Investigation of the TaC/SiC interface by transmission electron microscopy indicated that there was little or no reaction between the materials.
Improved contact performance of GaN film using Si diffusion
Authors: Lin C. F. , Cheng H. C. , Chi G. C. , Bu C. J. , Feng M. S.
Published: July 2002

In this letter, we investigate a metalization process for reducing the contact resistance on undoped GaN layers. The Si metal source was diffused successfully into the GaN films by using SiOx/Si/GaN/Al2O3 structures. By using a high-temperature annealing process, we diffused and activated the Si atoms into the GaN film. This caused a heavy doped n-type GaN layer to be formed near the GaN surface. Under high temperatures, such as a diffusion process at 1000 °C, the as-deposited Ni/Al/Ti contact had good ohmic properties and a low specific contact resistivity (ρc) of 1.6×10−3 Ω cm2. Rapid thermal annealing the contact at 800 °C for 30 s caused the ρc to decrease rapidly to 5.6×10−7 Ω cm2. The Ni/Al/Ti contact characteristics on the GaN films diffused at various temperatures are also discussed.
Growth mode mapping of SrTiO3 epitaxy
Authors: Lippmaa M. , Nakagawa N. , Kawasaki M. , Ohashi S. , Koinuma H.
Published: July 2002

We have mapped the growth mode of homoepitaxial SrTiO3 thin films as a function of deposition rate and substrate temperature during pulsed laser deposition. The transition from layer by layer growth to step flow growth was mapped by making 260 depositions, 3 monolayers each, on a single substrate. The growth mode was determined by time-resolved reflection high-energy electron diffraction. An atomically smooth surface was regenerated after each deposition by annealing the sample at temperatures above 1200 °C. The depositions were performed at an oxygen pressure of 10−6 Torr and covered a temperature range from 900 to 1380 °C. The effective activation energies of surface migration on Ti- and Sr-terminated surfaces were determined from the mapping results.
Locally enhanced Raman spectroscopy with an atomic force microscope
Authors: Anderson Mark S.
Published: July 2002

An atomic force microscope (AFM) tip has been used to selectively produce surface enhanced Raman scattering (SERS) for localized Raman spectroscopy. Spectra of thin films, undetectable with a Raman microprobe spectrometer alone, were readily acquired in contact with a suitably gold-coated AFM tip. Similarly, an AFM tip was used to remove sample layers at the nanometer scale and subsequently served as a SERS substrate for ultratrace analysis. This work demonstrates the interface of an AFM with a Raman spectrometer that provides increased sensitivity, selectivity, and spatial resolution over a conventional Raman microprobe. An AFM guiding the SERS effect has the potential for targeted single molecule spectroscopy.
Growth and fabrication with single-walled carbon nanotube probe microscopy tips
Authors: Cheung Chin Li , Hafner Jason H. , Odom Teri W. , Kim Kyoungha , Lieber Charles M.
Published: July 2002

Single-walled carbon nanotube (SWNT) probe microscopy tips were grown by a surface growth chemical vapor deposition method. Tips consisting of individual SWNTs (1.5–4 nm in diameter) and SWNT bundles (4–12 nm in diameter) have been prepared by design through variations in the catalyst and growth conditions. In addition to high-resolution imaging, these tips have been used to fabricate SWNT nanostructures by spatially controlled deposition of specific length segments of the nanotube tips.
Free energy screening of small ligands binding to an artificial protein cavity
Authors: Banba Shinichi , Brooks Charles L.
Published: July 2002

The λ-dynamics simulation method was used to study the binding of 10 five-member ring heterocycle derivatives to an artificial cavity created inside cytochrome C peroxidase by mutagenesis. Application of λ dynamics using a multiple topology approach resulted in trapping in local minima. To extend the method to these cases, a new restraining potential was devised and added to the extended Hamiltonian. Two approximations were introduced in order to estimate the binding free energy within small simulation times using this potential: (a) The entropy terms related to the restraining potential are assumed to cancel, due to the similarity of the ligands. (b) The restraining potential calculated from the coordinates of the environmental atoms during a λ-dynamics simulation is assumed to be equal to that of the average coordinates. Relatively short λ-dynamics simulations with this restraining potential successfully yielded reasonable estimates of the binding affinity of the ligands as compared with both experimental data and free energy perturbation calculations. Long time λ-dynamics simulations with a ten-ligand system revealed that better ligands tend to have small statistical errors, which is appropriate for screening out the plausible ligands from all candidates. Furthermore, short time λ-dynamics simulations with ten identical ligands demonstrated that sufficient precision was achieved for putative discovery of tight binding ligands or guests.
Direct numerical simulation of turbulent thermal boundary layers
Authors: Kong Hojin , Choi Haecheon , Lee Joon Sik
Published: July 2002

In this paper, a method of generating realistic turbulent temperature fluctuations at a computational inlet is proposed and direct numerical simulations of turbulent thermal boundary layers developing on a flat plate with isothermal and isoflux wall boundary conditions are carried out. Governing equations are integrated using a fully implicit fractional-step method with 352×64×128 grids for the Reynolds number of 300, based on the free-stream velocity and the inlet momentum thickness, and the Prandtl number of 0.71. The computed Stanton numbers for the isothermal and isoflux walls are in good agreement with power-law relations without transient region from the inlet. The mean statistical quantities including root-mean-square temperature fluctuations, turbulent heat fluxes, turbulent Prandtl number, and skewness and flatness of temperature fluctuations agree well with existing experimental and numerical data. A quadrant analysis is performed to investigate the coherence between the velocity and temperature fluctuations. It is shown that the behavior of the wall-normal heat flux is similar to that of the Reynolds shear stress, indicating close correlation between the streamwise velocity and temperature. The effect of different thermal boundary conditions at the wall on the near-wall turbulence statistics is also discussed.
Failure mechanism of Ta diffusion barrier between Cu and Si
Authors: Laurila Tomi , Zeng Kejun , Kivilahti Jorma K. , Molarius Jyrki , Suni Ilkka
Published: July 2002

The reaction mechanisms in the Si/Ta/Cu metallization system and their relation to the microstructure of thin films are discussed on the basis of experimental results and the assessment of the ternary Si–Ta–Cu phase diagram at 700 °C. With the help of sheet resistance measurements, Rutherford backscattering spectroscopy, x-ray diffraction, a scanning electron microscope, and a transmission electron microscope, the Ta barrier layer was observed to fail at temperatures above 650 °C due to the formation of TaSi2, the diffusion of Cu through the silicide layer, and the resulting formation of Cu3Si precipitates. However, in order for the TaSi2 phase to form first, the Ta diffusion barrier layer must be thick enough (e.g., 50–100 nm) to prevent Cu diffusion into the Si substrate up to the temperature of TaSi2 formation (∼650 °C). Independent of the Ta layer thickness, Cu3Si was present as large nodules, whereas the TaSi2 existed as a uniform layer. The resulting reaction structure was found to be in local equilibrium on the basis of the assessed Si–Ta–Cu phase diagram at 700 °C, and therefore no further reactions were expected. The role of oxygen was also found to be important in the reactions and it seems to have a strong effect on the thermal stability of the barrier layer.
Ferroelastic switching and elastic nonlinearity in 〈001〉-oriented Pb(Mg1/3Nb2/3)O3–PbTiO3 and Pb(Zn1/3Nb2/3)O3–PbTiO3 crystals
Authors: Viehland D. , Powers J. , Ewart L. , Li J. F.
Published: July 2002

The strain versus electric field (ε−E) and strain versus stress (ε−σ) response characteristics of 〈001〉-oriented crystals of Pb(BI1/3BII2/3)O3–PbTiO3 have been investigated. Under zero field, a ferroelastic switching has been observed under moderate uniaxial prestress levels. The magnitude of the elastic strain switched was ∼0.4%. In addition, an elastic softening was observed during switching, where the elastic constant decreased from ∼1010 to ∼3×109 N/m2. At higher stresses, the elastic constant stiffened to ∼5×1010 N/m2, which upon application of dc electrical bias softened to ∼1010 N/m2. The importance of ferroelastic switching and elastic nonlinearity is discussed.
Colloidal electrolyte friction and sedimentation: A mode-coupling approach
Authors: Kollmann Markus , Nägele Gerhard
Published: July 2002

Using a simplified mode-coupling scheme (MCS) for multicomponent Brownian dispersions, we calculate the effect of mobile salt and counterions on the tracer diffusion and sedimentation of a suspended colloidal macroion. In this scheme, potential forces and also the leading far-field hydrodynamic interactions (HI) between macroion and small electrolyte ions forming its ion atmosphere are accounted for on the basis of the many-body Smoluchowski equation. The static pair correlation functions, which are needed as input, are treated in a first step within the Debye–Hückel approximation. In this way, analytical results are derived for the time-dependent friction function, the long-time tracer diffusion coefficient, and the sedimentation velocity of the macroion as functions of the electrolyte concentration, electrolyte ion mobilities, and macroion charge. Onsager’s limiting law for simple electrolytes is recovered from the MCS in the limit of vanishing macroion radius. The results of our theoretical treatment are compared with experimental data on electrolyte friction and predictions of other existing theories. Good agreement with experimental data on tracer diffusion and with Booth’s theory of electrolyte friction is observed. The inclusion of HI between macroion and microions is shown to be essential for a proper description of the electrolyte friction effect.
The effect of nonstoichiometry (δ) on the magnetic properties of (Mg0.22Mn0.07Fe0.71)3−δO4 ferrite
Authors: Kang Sun-Ho , Yoo Han-Ill
Published: July 2002

The effect of nonstoichiometry (δ) on the magnetic properties of (Mg0.22Mn0.07Fe0.71)3−δO4 ferrite was investigated. The nonstoichiometry was controlled via a solid state electrochemical method to encompass from δ=+0.005 to −0.006. Initial permeability was found to be affected significantly by nonstoichiometry. From the natural resonance frequency in the magnetic loss spectra and initial permeability at low frequencies, magnetocrystalline anisotropy constant (K1) of (Mg0.22Mn0.07Fe0.71)3−δO4 ferrite was calculated as a function of nonstoichiometry, which increased from −1.03×104 to −8.60×103 ergs/cm3 with decreasing nonstoichiometry from +0.005 to −0.006. The effect of nonstoichiometry on the initial permeability of (Mg0.22Mn0.07Fe0.71)3−δO4 ferrite is discussed in terms of saturation magnetization and magnetic anisotropy.
Molecular beam epitaxial growth of atomically smooth scandium nitride films
Authors: Al-Brithen Hamad , Smith Arthur R.
Published: July 2002

High quality scandium nitride films have been grown on magnesium oxide (001) substrates by molecular beam epitaxy using a rf plasma source for nitrogen. Both reflection high energy electron diffraction and x-ray diffraction confirm that these films have (001)-orientation. Atomic force microscopy reveals a surface morphology consisting of large plateaus and pyramids. The plateaus are found to be atomically smooth and have a 1×1 surface structure, as revealed by in situ scanning tunneling microscopy.
Higher order decouplings of the dilated electron propagator with applications to P Be−2, P Mg−2 shape and S Be+2 (1s−1) Auger resonances
Authors: Venkatnathan Arun , Mahalakshmi S. , Mishra Manoj K.
Published: July 2002

The full third order (Σ3), quasi-particle third order (Σq3) and Outer Valence Green’s Function decouplings of the bi-orthogonal dilated electron propagator have been implemented for the first time and results from their application to P Be−2, P Mg−2 shape and S Be+2 (1s−1) Auger resonances are presented and compared with energies and widths obtained using the zeroth order (Σ0), quasi-particle second order (Σq2) and second order (Σ2) decouplings. The energies and widths from third order decoupling for shape resonances are close to those obtained using second order self-energy approximants. The energy and width calculated using the third order decoupling for Auger resonances provide better agreement with experimental results, with the much more economic quasi-particle third order decoupling being just as effective. The differences between FDAs from different decouplings are analyzed to elicit the role of correlation and relaxation in the formation and decay of shape and Auger resonances.
Investigations of amplitude and phase excitation profiles in femtosecond coherence spectroscopy
Authors: Kumar Anand T. N. , Rosca Florin , Widom Allan , Champion Paul M.
Published: July 2002

We present an effective linear response approach to pump–probe femtosecond coherence spectroscopy in the well-separated pulse limit. The treatment presented here is based on a displaced and squeezed state representation for the nonstationary states induced by an ultrashort pump laser pulse or a chemical reaction. The subsequent response of the system to a delayed probe pulse is modeled using closed form nonstationary linear response functions, valid for a multimode vibronically coupled system at arbitrary temperature. When pump–probe signals are simulated using the linear response functions, with the mean nuclear positions and momenta obtained from a rigorous moment analysis of the pump induced (doorway) state, the signals are found to be in excellent agreement with the conventional third-order response approach. The key advantages offered by the moment analysis-based linear response approach include a clear physical interpretation of the amplitude and phase of oscillatory pump–probe signals, a dramatic improvement in computation times, a direct connection between pump–probe signals and equilibrium absorption and dispersion lineshapes, and the ability to incorporate coherence associated with rapid nonradiative surface crossing. We demonstrate these aspects using numerical simulations, and also apply the present approach to the interpretation of experimental amplitude and phase measurements on reactive and nonreactive samples of the heme protein myoglobin. The role played by inhomogeneous broadening in the observed amplitude and phase profiles is discussed in detail. We also investigate overtone signals in the context of reaction driven coherent motion.
Correct dissociation behavior of radical ions such as H2+ in density functional calculations
Authors: Chermette H. , Ciofini I. , Mariotti F. , Daul C.
Published: July 2002

In this contribution it is shown that the unphysical dissociation energy curves of dimeric ions bearing a small odd number of electrons as obtained with DFT calculations can be cured by a posteriori corrections. The self-interaction error, which is known to be at the origin of the unphysical dissociation behavior, is corrected by a Slater’s transition state calculation. A very satisfactory dissociation energy curve is obtained for He2+. However for H2+, it is also necessary to introduce fractional occupation numbers to obtain a good description of the system.
Magnesium incorporation in GaN grown by molecular-beam epitaxy
Authors: Ptak A. J. , Myers T. H. , Romano L. T. , Van de Walle C. G. , Northrup J. E.
Published: July 2002

A pronounced dependence of Mg incorporation on surface polarity was observed in a series of Mg step-doped epitaxial GaN layers grown by rf-plasma-assisted molecular-beam epitaxy. Incorporation was studied for both (0001), or Ga-polarity and (0001̄) or N-polarity orientations. Up to a factor of 30 times more Mg was incorporated in Ga-polarity layers under certain conditions, as determined by secondary ion mass spectrometry. Measurements indicate surface accumulation of Mg occurs during growth, with stable accumulations of close to a monolayer of Mg on the Ga-polarity surface. The presence of atomic hydrogen during growth significantly increased incorporation of Mg without also incorporating potentially compensating hydrogen.
Emphasizing the exchange-correlation potential in functional development
Authors: Menconi Giuseppina , Wilson Philip J. , Tozer David J.
Published: July 2002

Exchange-correlation functionals are determined by constraining the potentials of flexible functional forms to be as parallel as possible to asymptotically vanishing ab initio exchange-correlation potentials. No thermochemical or gradient information is explicitly included in the fitting procedure. A range of spatial weightings is considered and the functionals are assessed by comparing with experiment and with the HCTH functional [J. Chem. Phys. 109, 6264 (1998)], which was determined by fitting to both potentials and to thermochemical and gradient data. Optimal thermochemistry, structures, and polarizabilities are simultaneously achieved by emphasizing an intermediate spatial region in the fit; an optimal functional is presented. The thermochemistry of this functional is less accurate than HCTH, although the structures of the fitting molecules are significantly improved. The mean absolute bond length error for 40 of the fitting molecules is 0.006 Å, a factor of 2 improvement over HCTH. The bond lengths of 16 diatomic radicals absent from the fitting data are also improved. For the difficult molecules FOOF, FNO2, O3, FO2, Cr(CO)6, and Ni(CO)4, the results are variable. The new functional improves the polarizabilities of 14 small molecules, compared to HCTH. It also improves electronic excitation energies to Rydberg states of N2, H2CO, and C6H6, although the errors remain significant, reflecting the incorrect asymptotic potential. To obtain optimal nuclear shielding constants, it is necessary to emphasize regions closer to the nuclei; a second functional is presented which gives improved shieldings compared to HCTH. By considering the dominant occupied-virtual excitation contributions to the paramagnetic shieldings in CO and H2O, analogies are drawn between our results and those of a recently proposed method for improving density functional shielding constants.
A survey of dusty plasma physics
Authors: Shukla P. K.
Published: July 2002

Two omnipresent ingredients of the Universe are plasmas and charged dust. The interplay between these two has opened up a new and fascinating research area, that of dusty plasmas, which are ubiquitous in different parts of our solar system, namely planetary rings, circumsolar dust rings, the interplanetary medium, cometary comae and tails, as well as in interstellar molecular clouds, etc. Dusty plasmas also occur in noctilucent clouds in the arctic troposphere and mesosphere, cloud-to-ground lightening in thunderstorms containing smoke-contaminated air over the United States, in the flame of a humble candle, as well as in microelectronic processing devices, in low-temperature laboratory discharges, and in tokamaks. Dusty plasma physics has appeared as one of the most rapidly growing fields of science, besides the field of the Bose–Einstein condensate, as demonstrated by the number of published papers in scientific journals and conference proceedings. In fact, it is a truly interdisciplinary science because it has many potential applications in astrophysics (viz. in understanding the formation of dust clusters and structures, instabilities of interstellar molecular clouds and star formation, decoupling of magnetic fields from plasmas, etc.) as well as in the planetary magnetospheres of our solar system [viz. Saturn (particularly, the physics of spokes and braids in the B and F rings), Jupiter, Uranus, Neptune, and Mars] and in strongly coupled laboratory dusty plasmas. Since a dusty plasma system involves the charging and dynamics of massive charged dust grains, it can be characterized as a complex plasma system providing new physics insights. In this paper, the basic physics of dusty plasmas as well as numerous collective processes are discussed. The focus will be on theoretical and experimental observations of charging processes, waves and instabilities, associated forces, the dynamics of rotating and elongated dust grains, and some nonlinear structures (such as dust ion-acoustic shocks, Mach cones, dust voids, vortices, etc). The latter are typical in astrophysical settings and in several laboratory experiments. It appears that collective processes in a complex dusty plasma would have excellent future perspectives in the twenty-first century, because they have not only potential applications in interplanetary space environments, or in understanding the physics of our universe, but also in advancing our scientific knowledge in multidisciplinary areas of science.
A direct comparison between terahertz time-domain spectroscopy and far-infrared Fourier transform spectroscopy
Authors: Han P. Y. , Tani M. , Usami M. , Kono S. , Kersting R. , Zhang X.-C.
Published: July 2002

We perform a comprehensive comparison between terahertz (THz) time-domain spectroscopy and conventional far-infrared Fourier transform spectroscopy, including radiation source, detector, signal to noise ratio, bandwidth, availability, applications, and their own uniqueness. In terms of signal to noise ratio, THz time-domain spectroscopy is advantageous at low frequencies under 3 THz, while Fourier transform spectroscopy works better at frequencies above 5 THz. In addition, we provide a detailed discussion of the unique features of THz time-domain spectroscopy and its application to dynamic and time-resolved processes.
Continuous-wave pump-enhanced singly resonant optical parametric oscillator pumped by an extended-cavity diode laser
Authors: Lindsay I. D. , Petridis C. , Dunn M. H. , Ebrahimzadeh M.
Published: July 2002

A continuous-wave pump-enhanced singly resonant optical parametric oscillator (OPO) directly pumped by a grating-stabilized external-cavity diode laser operating at ≈810 nm is described. The OPO was based on periodically poled LiNbO3 and could be tuned over 1.06–1.19 μm at the signal and 2.58–3.44 μm at the idler. The OPO threshold was typically 25–30 mW over the observed tuning range. Up to 4 mW of one-directional idler output was obtained for 62 mW of external-cavity diode laser pump power. The high stability of the external-cavity diode-laser pump source allowed locked, single-mode OPO operation for periods of greater than 1 h.
Thermoelectric properties of heavily GaP- and P-doped Si0.95Ge0.05
Authors: Yamashita Osamu
Published: July 2002

The Seebeck coefficient S, the electrical resistivity ρ and the thermal conductivity κ of Si0.95Ge0.05 samples doped with 0.4 at. % P and/or 0.5–2.0 mol % GaP, which were prepared by a conventional arc melting method, were measured as functions of GaP content and temperature T in the range from 323 to 1208 K. When multidoped with P and GaP, Ga tends to segregate more strongly with Ge to the grain boundaries than P, while when doped with GaP alone, both P and Ga segregate equally strongly with Ge. For multidoped samples, the S values at 323 K have a minimum at 1.0 mol % GaP and then increase with additional GaP, while the values of ρ and κ decrease monotonically with increasing GaP content. The optimum additional content of GaP that gives the largest thermoelectric figures of merit (ZT=S2T/κρ) for multidoped n-type Si0.95Ge0.05 samples was 1.5 mol %, which is slightly less than the 2.0 mol % of GaP added to Si0.8Ge0.2 alloy by hot pressing. The ZT value for multidoped Si0.95Ge0.05 with an optimum content of GaP increases linearly with temperature, and at 1073 K is 18% higher than that obtained previously for Si0.95Ge0.05 doped with only 0.4 at. % P. At 1173 K the ZT value is 1.16, which corresponds to 95% of that obtained previously at the corresponding temperature for Si0.8Ge0.2 alloy doped with 2.0 mol % GaP.
Analysis of the Na2 23Σg+→a3Σu+ continua: Potentials and transition moment function
Authors: Ivanov V. S. , Sovkov V. B. , Li Li , Lyyra A. M. , Whang T. J. , Magnier S.
Published: July 2002

The repulsive a 3Σu+ state potential of the Na2 molecule is determined by applying the IPA (inverse perturbation approach) of V. S. Ivanov and V. B. Sovkov [Chem. Phys. 213, 295 (1996)] to the analysis of the Na2 2 2Σg+ (v=15, N=27, and v=5, N=39)→a 3Σu+ continua observed in the work of T. J. Whang et al. [J. Mol. Spectrosc. 160, 411 (1993)]. The resulting a 3Σu+ state potential is close to the ab initio potential within the experimental error, but its right branch deviates from the RKR–Le Roy–Bernstein potential function of the a 3Σu+ state shallow well reported in E.-J. Friedman-Hill and R. W. Field [J. Chem. Phys. 96, 2444 (1992)]. The transition moment function M(R) of the 2 3Σg+−a 3Σu+ transition is also obtained from the analysis of the spectra and compared to the ab initio one. The same 2 3Σg+ (v=15, N=27)→a 3Σu+ experimental spectrum is used for investigating abilities of the WKB nodes method, the FCF phase method, and the method of the accurate equation for the potential in terms of the transition amplitudes. They were developed earlier by us in order to determine the bound state potential function from the structure of the bound-free continuous spectrum alone without using information about the ro-vibrational term values. This attempt resulted in very reliable potential curves.
Effects of sulfur treatment on electrical and optical performance of InGaN/GaN multiple-quantum-well blue light-emitting diodes
Authors: Huh Chul , Kim Sang-Woo , Kim Hyun-Soo , Kim Hyun-Min , Hwang Hyunsang , Park Seong-Ju
Published: July 2002

The results of the sulfur treatment of multiple-quantum-well (MQW) light-emitting diodes (LEDs) with (NH4)2S and (NH4)2S+t-C4H9OH solutions prior to the deposition of a light-transmitting p-electrode metal are presented. The room-temperature I–V curves showed that the forward voltages of MQW LEDs treated with the two sulfur solutions decrease by 0.12 and 0.35 V at 20 mA, respectively, compared to the untreated MQW LED, as the result of an improvement in p-Ohmic contact characteristics. The relative light-output power and external quantum efficiency of MQW LEDs increased by a factor of 1.28 for the (NH4)2S treated sample and 2.23 for the (NH4)2S+t-C4H9OH treated sample compared to the untreated sample. In addition, the reverse leakage current characteristic of MQW LEDs was reduced as a result of sulfur treatment. This can be attributed to the passivation of surface and sidewall damages formed after the dry-etching process for a reliable pattern transfer. The present results indicate that the sulfur treatment greatly improves the electrical and optical performance of MQW LEDs.
Silicon carbide and silicon carbide:germanium heterostructure bipolar transistors
Authors: Roe K. J. , Katulka G. , Kolodzey J. , Saddow S. E. , Jacobson D.
Published: July 2002

In this letter, we report on heterostructure bipolar transistors (HBTs) based on silicon carbide (SiC) and a silicon carbide:germanium (SiC:Ge) alloy. The SiC:Ge base alloy was formed by the ion implantation of Ge into p-type 4H–SiC and subsequent annealing. HBT mesa structures were fabricated using a reactive ion etching process. The incorporation of Ge was found to increase the gain and the Early voltage of the devices. A common-emitter current gain (β) of greater than 3 was measured for the SiC:Ge HBTs. Homojunction SiC transistors were fabricated as a reference using the same process (except no Ge in the base region) and exhibited a β of 2.2. The transistors exhibited high breakdown voltages (>50 V without passivation), that typify SiC-based devices. These results indicate that SiC:Ge is a promising material for use in SiC-based heterostructure devices.
Origin of the first sharp diffraction peak in the structure factor of disordered network-forming systems: Layers or voids?
Authors: Massobrio Carlo , Pasquarello Alfredo
Published: July 2002

The origin of the first sharp diffraction peak (FSDP) in the total structure factor of disordered network-forming glasses and liquids is frequently interpreted either as resulting from crystalline-like layers or from correlations between basic structural units (clusters) and interstitial voids. By applying appropriate criteria to configurations of liquid SiO2 and GeSe2 obtained by first-principles molecular dynamics, we find that neither interpretation holds. Despite the appearance of a FSDP in both liquids, there is no evidence for the occurrence of crystalline-like layers. By contrast, cluster–void correlations do not only occur for these liquids, but also for a model system in which the FSDP is absent.
Sensing metabolites using donor–acceptor nanodistributions in fluorescence resonance energy transfer
Authors: Rolinski O. J. , Birch D. J. S. , McCartney L. J. , Pickup J. C.
Published: July 2002

Before fluorescence sensing techniques can be applied to media as delicate and complicated as human tissue, an adequate interpretation of the measured observables is required, i.e., an inverse problem needs to be solved. Recently we have solved the inverse problem relating to the kinetics of fluorescence resonance energy transfer (FRET), which clears the way for the determination of the donor–acceptor distribution function in FRET assays. In this letter this approach to monitoring metabolic processes is highlighted and the application to glucose sensing demonstrated.