bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Mg doping on structural defects in AlGaN layers grown by metalorganic chemical vapor deposition
Authors: Cho Hyung Koun , Lee Jeong Yong , Jeon Seong Ran , Yang Gye Mo
Published: July 2002

Influence of Mg doping on structural defects in Al0.13Ga0.87N layers grown on sapphire substrates by metalorganic chemical vapor deposition were studied using transmission electron microscopy. By increasing the Mg source flow rate, the reduction of dislocation density occurred up to the Mg source flow rate of 0.103 μmol/min. While the vertical type inversion domain boundaries (IDBs) were observed in the Al0.13Ga0.87N layers grown with the low Mg source flow rate, the IDBs in the Al0.13Ga0.87N layers grown with the high Mg source flow rate have horizontally multifaceted shapes. The change of polarity by the IDBs of horizontal type also resulted in the 180° rotation of pyramidal defects within the same AlGaN layer.
Low Froude number potential energy resonances in uniform stratification
Authors: Vasholz David P.
Published: July 2002

Internal wave effects generated by a solid body moving uniformly at constant depth through a density-stratified fluid of uniform depth are investigated. In particular an averaged potential energy Γ per unit track length is defined and analyzed under the assumptions of linear theory, uniform stratification, and rigid lid boundary conditions. A general expression is found for Γ in terms of the intrinsic causal structure of the fluid system and the geometric properties of the generating object. It is found that, as the speed of the body decreases to zero, Γ passes through an infinite sequence of infinite-amplitude resonances. A source function approximation is used to model the shape of the generating object and numerical evaluations are made for the case of a sphere. A consideration of the infinite-depth limit allows one to see Γ in terms of the resonance-inducing effects of the rigid lid boundary conditions in conjunction with the relatively smooth and finite “background resonances” associated with pure buoyancy effects.
Effects of rapid thermal annealing on the optical properties of In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells with InGaAs and dielectric capping layers
Authors: Yu Jae Su , Song Jin Dong , Lee Yong Tak , Lim H.
Published: July 2002

We investigated the effects of InGaAs and dielectric (SiO2 and Si3N4) capping layers on the intermixing behavior of InGaAs/InAlAs multiple quantum wells (MQWs) after rapid thermal annealing (RTA). With a fixed RTA time of 45 s, no appreciable shift of photoluminescence (PL) peak energy was observed in the samples annealed up to 800 and 850 °C for the SiO2- and Si3N4-capped MQWs, respectively. But they exhibited a significant improvement of PL intensity without a broadening of the PL spectra after the RTA. After an RTA at 900 °C for 45 s, a blueshift of 17 meV and a redshift of 25 meV were observed for the SiO2- and Si3N4-capped MQWs, respectively. Microstructures of the InGaAs/InAlAs MQWs observed by cross-sectional transmission electron microscopy before and after RTA reveal that the above improvement of PL intensity is associated with the curing of nonradiative recombination centers in the course of the relaxation of vertical composition modulation along the growth direction in the InGaAs well and the InAlAs barrier layers. The influence of dielectric and InGaAs cap layers on the improvement of PL intensity is explained by the ease of the cation and anion vacancy diffusion from the dielectric layers and through the InGaAs cap layer.
Stacking fault band structure in 4H–SiC and its impact on electronic devices
Authors: Miao M. S. , Limpijumnong Sukit , Lambrecht Walter R. L.
Published: July 2002

First principles calculations of the stacking fault (SF) in 4H–SiC indicate the occurrence of an interface band in the gap with maximum depth of 0.2–0.3 eV below the conduction band minimum at the M̄ point. The energy of formation of SFs in 3C–, 4H–, and 6H–SiC on the other hand is found to be of order a few meV/pair. Thus, there is a thermodynamic driving force promoting growth of SF area in an n-type sample. Radiationless recombination of electrons trapped at the SF with holes is proposed to provide sufficient energy to overcome the partial dislocation motion barriers towards formation of additional SF area in a device under forward bias.
Orientation control of liquid crystal droplets dispersed in a polymer matrix
Authors: Nicoletta F. P. , De Filpo G. , Cupelli D. , Macchione M. , Chidichimo G.
Published: July 2002

The encapsulation of monomer molecules in liquid crystal droplets dispersed in a thermoplastic matrix provides a convenient method to control the orientation of liquid crystal directors. The imprint of droplet interfaces is obtained by photopolymerization of monomer molecules by means of a polymerization induced phase separation process performed in a magnetic field. In such a way, uniform orientations of liquid crystal directors can be achieved in cells without any surface treatment. The concentration of monomer molecules is an important parameter, which influences the final electro-optical properties of films.
Spin-glass behavior with short-range antiferromagnetic order in Nd2AgIn3
Authors: Li D. X. , Nimori S. , Shiokawa Y. , Tobo A. , Onodera H. , Haga Y. , Yamamoto E. , Ōnuki Y.
Published: July 2002

We present experimental results of complex susceptibility, static magnetization, magnetic relaxation, specific heat, and electrical resistivity measurements on an intermetallic compound Nd2AgIn3. The results indicated that Nd2AgIn3 undergoes a spin-glass transition at a freezing temperature Tf=12.6 K accompanied with the formation of short-range antiferromagnetic order. The mechanism of formation of spin-glass state in Nd2AgIn3 is different from that in diluted metallic spin-glass or uranium intermetallic compound. The existences of frustrated moments due to the triangles of nearest neighbors in Nd atomic layers and randomly distributed Ruderman–Kittle–Kasuya–Yosida interactions may be responsible for the spin glass state formed in Nd2AgIn3.
Positron-annihilation study of compensation defects in InP
Authors: Shan Y. Y. , Deng A. H. , Ling C. C. , Fung S. , Ling C. D. , Zhao Y. W. , Sun T. N. , Sun N. F.
Published: July 2002

Positron-annihilation lifetime and positron-annihilation Doppler-broadening (PADB) spectroscopies have been employed to investigate the formation of vacancy-type compensation defects in n-type undoped liquid encapsulated Czochrolski grown InP, which undergoes conduction-type conversions under high temperature annealing. N-type InP becomes p-type semiconducting by short time annealing at 700 °C, and then turns into n-type again after further annealing but with a much higher resistivity. Long time annealing at 950 °C makes the material semi-insulating. Positron lifetime measurements show that the positron average lifetime τav increases from 245 ps to a higher value of 247 ps for the first n-type to p-type conversion and decreases to 240 ps for the ensuing p-type to n-type conversion. The value of τav increases slightly to 242 ps upon further annealing and attains a value of 250 ps under 90 h annealing at 950 °C. These results together with those of PADB measurements are explained by the model proposed in our previous study. The correlation between the characteristics of positron annihilation and the conversions of conduction type indicates that the formation of vacancy-type defects and the progressive variation of their concentrations during annealing are related to the electrical properties of the bulk InP material.
Simulation of dislocation damping spectra associated with the collective motion of point defects and kink chain subjected to bulk segregation
Authors: Ogurtani Tarik O. , Güngör Mehmet R. , Oren Ersin Emre
Published: July 2002

A theory of interactive and completely coupled systems of dislocations and mobile point defects is presented. The computer simulations yield an accurate prediction of experimental spectral data in terms of dislocation induced Cottrell relaxation and its gradual conversion into a Cottrell–Koster relaxation peak at high concentrations of spherical point defect. The bulk segregation of point defects to the kinked-dislocation line is also accounted for. Excellent results are obtained in computer simulations of aging, peaking, and finally the stabilization behavior of transient dislocation damping peaks.
High frequency characterization of a GaInAs/InP electronic waveguide T-branch switch
Authors: Lewén R. , Maximov I. , Shorubalko I. , Samuelson L. , Thylén L. , Xu H. Q.
Published: July 2002

We report comprehensive microwave measurements on a T-branch switch; a GaInAs/InP electron waveguide based structure. The study includes a small signal model of the device where limitations of high frequency operation are discussed. An example of large signal operation where the nonlinearity of the device is exploited by operating the T-branch switch as a frequency multiplier is demonstrated.
Flow over a curved wall exiting from an aperture
Authors: Toison Fabrice , Hureau Jacques
Published: July 2002

We consider here a two-dimensional potential liquid flow of a liquid issuing from an aperture and running over a curved bottom, and solve it by mapping the potential plane (f ) onto a unit disk and solving a mixed boundary problem. The iterative process allows us to consider arbitrary curved walls. Our method is first validated by comparison with published and analytical results and is then applied to arbitrary curved walls. Gravity is taken into account.
Characterization of inductively coupled plasma in the ionized physical vapor deposition system
Authors: Chiu K.-F. , Barber Z. H.
Published: July 2002

The ionized physical vapor deposition technique uses a built-in rf coil to generate an inductively coupled plasma (ICP) between the target and substrate holder. For deposition of metallic species, a portion of the depositing atoms is ionized upon passing through the ICP. Since the input energy of the ICP is decoupled from the target, this technique also provides controllable ion bombardment during film growth, in terms of bombarding ion flux and ion energy. An ionized physical vapor deposition system has been studied and fully characterized. The ICP was characterized using a single passive probe method, and the plasma parameters, including electron temperature, plasma potential, and plasma ion density, have been measured. The ionization fraction of the depositing metal flux as a function of deposition parameters has been measured using the single passive probe method and a self-developed parallel-plates method. A simplified one-dimensional model was developed and compared favorably with the measured ionization fractions.
Electro-osmosis in a nanometer-scale channel studied by atomistic simulation
Authors: Freund Jonathan B.
Published: July 2002

An atomistic simulation of an electro-osmotic flow in a 65.3 Å-wide channel is performed to study its physical details and evaluate continuum models. The working fluid is a 0.01 M solution (at midchannel) of Cl− in water. For simplicity and computational efficiency, only negatively charged ions are included. The water is modeled by the SPC/E potential and the Cl− are modeled as point charges plus an established Lennard-Jones potential. The channel walls are fixed lattices of positively charged Lennard-Jones atoms. In one case an appropriate fraction of the wall atoms is given elementary charges; for comparison, another case is simulated with uniformly distributed partial charges on the wall atoms. For the distributed elementary charge case the Cl− concentration at the wall is 80 percent higher than predicted by the Poisson–Boltzmann theory. It is over 100 percent higher for the uniformly charged wall case. In both cases, the waters in the 10 Å closest to the walls are preferentially oriented. Their respective orientations are similar except in the first monolayer. However, the effect of this orientational bias on the permittivity and subsequently the Cl− distribution is shown to be minor by Monte Carlo simulations, which predict an ion distribution in agreement with the dynamic simulation using only ε=80 to model the water. Computed, one-dimensional self-diffusivities of the waters match accepted values greater than 10 Å from the walls, but decay significantly close to the walls. The decay is not monotonic for the wall-normal diffusivity. The atoms near the walls are not fixed in a Stern layer, as typically assumed in models, but the viscosity is found to increase by over a factor of 6 in the 10 Å closest to the wall.
A pistonless Stirling cooler
Authors: Yazaki T. , Biwa T. , Tominaga A.
Published: July 2002

We demonstrate a prototype acoustic cooler that uses Stirling cycles executed by a traveling wave with high acoustic impedance thermoacoustically induced in a looped tube. The tube has no moving parts, only a pair of stacks sandwiched between two heat exchangers: one amplifies the acoustic power and the amplified wave supplies the driving energy to pump heat directly within the second stack. Because it uses extremely simple hardware consisting of a few parts, the cooling device is potentially a powerful tool for applications such as conventional cooling systems.
Optimization of data acquisition and processing in Carr–Purcell–Meiboom–Gill multiple quantum magic angle spinning nuclear magnetic resonance
Authors: Lefort R. , Wiench J. W. , Pruski M. , Amoureux J.-P.
Published: July 2002

Data acquisition using the Carr–Purcell–Meiboom–Gill (CPMG) train of π pulses has been recently explored in multiple quantum magic angle spinning (MQMAS) nuclear magnetic resonance of half-integer quadrupolar nuclei [T. Vosegaard, F. H. Larsen, H. J. Jakobsen, P. D. Ellis, and N. C. Nielsen, J. Am. Chem. Soc. 119, 9055 (1997)]. Significant increase of sensitivity can be obtained by using this technique at the expense of spectral definition, as the spectrum transforms into a manifold of narrow sidebands. A detailed analysis of the CPMG method, with emphasis on the MQMAS experiment, is presented. The MQ-QCPMG-MAS approach is adapted to spin-5/2 nuclei. Several numerical methods of data treatment are shown that allow for improvement of the definition of the sideband envelope, reconstruction of the standard line shape, and improvement of S/N ratio via optimized filtering. Theoretical and experimental estimates of signal enhancement through proper acquisition and processing of MQ-QCPMG-MAS data are given.
Comparison of low energy BF2+, BCl2+, and BBr2+ implants for the fabrication of ultrashallow P+-N junctions
Authors: Ganguly Swaroop , Lin Li , Kohli Puneet , Li Hong-Jyh , Kirichenko Taras , Srinivasa Raghu , Agarwal Vikas , Banerjee Sanjay
Published: July 2002

We present a comparative study of BCl2+ and BBr2+, and the traditionally used BF2+, as implant species for the formation of ultrashallow P+-N junctions. From “as-implanted” profiles, a large reduction in channeling tail has been observed for the BCl2+ and BBr2+ implants, relative to BF2+ implantation; the depths are reduced by over 200 Å compared to the “equivalent” energy BF2+ implants. After annealing, the 5 keV BCl2+ implanted junctions are shallower than the 5 keV BF2+ junctions by over 150 Å. The 18 keV BBr2+ implants yield junctions as shallow as 100 Å, but suffer from severe doss loss problems—probably caused by enhanced surface scattering and etching for the deceleration mode implant. The 20 keV BBr2+ implants done without deceleration show no dose loss, and result in junction depths identical to those in the 5 keV BF2+ implants. Leakage current measurements indicate an increase of three orders of magnitude for the BCl2+ implanted junctions over BF2+ ones; BBr2+ implanted junctions exhibit lower leakage, comparable to that produced by BF2+. The effect of the different halogen species on B penetration through the gate oxide into the channel region has been studied; the penetration is reduced for BBr2+ implants relative to BF2+ implants. We present comparative device characteristics data from metal-oxide-semiconductor field-effect transistors with source-drain extension regions formed by low energy implants of the three species. We show that the two alternative species might be strong candidates for acceptor implant species for ultrasmall devices; in particular, BBr2+ might be a superior replacement for BF2+.
Crystallographic and magnetic properties of nanocrystalline Fe78Al4Nb5B12Cu1 alloys
Authors: Yoon Sung Hyun , Kim Sung Baek , Lee Hi Min , Kim Chul Sung
Published: July 2002

The effects of crystallographic change induced during the annealing process upon the magnetic properties of nanocrystalline Fe78Al4Nb5B12Cu1 alloy were investigated by using x-ray diffraction, Mössbauer spectroscopy, and macroscopic magnetometry. Special focus was concentrated on the structures of interfacial layer, which is a region between nanocrystallite and amorphous matrix. To examine the differences in the crystallographic and the magnetic properties associated with the thermal treatments, amorphous samples were annealed in two different ways, i.e., flash annealing and conventional annealing. The Mössbauer spectra were analyzed to decompose into three or four components, and revised Vincze method was used to extract the distributions of hyperfine parameters. Interfacial layers were found to have a considerable crystalline order. Flash annealing advances the crystallization process earlier than the conventional annealing. As a result, the fraction of the crystalline phase produced by flash annealing is much higher than that produced by conventional annealing performed at the same temperature. At annealing temperature of as high as 550 °C, second-stage crystallization started and crystalline Fe2B phase was created. The deterioration in soft magnetic properties at this high annealing temperature was attributed this crystallographic change.
Chromium deposition on ordered alumina films: An x-ray photoelectron spectroscopy study of the interaction with oxygen
Authors: Eriksson M. , Sainio J. , Lahtinen J.
Published: July 2002

We have studied metallic and oxidized chromium layers on thin ordered alumina films grown on a NiAl(110) substrate using x-ray photoelectron spectroscopy. The interaction between the chromium layers and the substrate has been characterized after deposition at room temperature and after oxidation at 300 and 700 K. Our results indicate partial oxidation of the deposited chromium with the fraction of oxidized Cr decreasing with increasing Cr coverage. Oxidation of the chromium layers at room temperature using O2 results in Cr3+ species on the surface. These oxidized chromium species can be reduced by heating the sample to 700 K for 5 minutes. Oxidation at 700 K results in chromium species that cannot be thermally reduced. Our results do not indicate formation of Cr6+ species although such are present in impregnated catalysts.
Frequency sweeping of phase space structures
Authors: Eremin D. Yu. , Berk H. L.
Published: July 2002

Holes and clumps, spontaneously formed in the nonlinear development of a single mode driven by a weakly destabilizing kinetic distribution function in a dissipative medium, were expected to persist for an appropriate collisional time scale. However, Fokker–Planck calculations for the nonlinear system abruptly break down in the course of the calculation for some initial conditions. It is found that this arises because an adiabatic description of phase space structures at zero collisionality does not necessarily lead to continual adiabatic frequency sweeping. Criteria are found for a class of initial distribution functions that determine whether adiabatic frequency sweeping continues indefinitely, or suddenly terminates. An unresolved bifurcation problem is also observed under certain conditions.
Ag photodoping into crystalline ZnSe
Authors: Lee Hyun-Yong , Song Joon-Suk , Makino Hisao , Yao Takafumi
Published: July 2002

We have investigated the possibility of Ag photodoping into crystalline (c-) ZnSe grown by molecular-beam epitaxy. A Ag/c-ZnSe sample was illuminated with a 325 nm HeCd laser at 10 K and its photoluminescence (PL) characteristics were measured in real time. Depending on the illumination time, there are considerable changes in the intensities of the deep-level peaks (2.059 and 2.345 eV) as well as the near-band-edge peak (2.798 eV). The emission band at 2.059 eV is almost independent of PL-excitation power, while the green luminescence band at 2.345 eV exhibits distinctly a power-dependent blueshift with increasing excitation power, which can be assigned as a donor-to-acceptor pair transition (AgiAgZn). In this letter, a model to explain Ag photodoping into c-ZnSe is proposed on the basis of the PL results. We believe that Ag, as an activator for luminescence, can be photodoped into c-ZnSe even though the extent and kinetics of the Ag doping are different from those of the amorphous chalcogenide.
Secondary electron emission in a mixed gas for application to the plasma display panel
Authors: Uhm Han S. , Choi Eun H. , Lim Jae Y.
Published: July 2002

The secondary electron emission from the surface of the MgO crystal is investigated for a mixed gas in connection with applications to the plasma display panel. The experimental data indicate that the coefficient γ of the secondary emission for the neon gas mixed with a small amount of xenon is almost the same as the coefficient for the xenon-only gas. This observation agrees very well with theoretical predictions.
A dipolar vortex in a magnetized pair plasma containing nonuniform flows
Authors: Vranješ J. , Pandey B. P. , Shukla P. K. , Poedts S.
Published: July 2002

The linear and nonlinear properties of electromagnetic waves, which are excited by a nonuniform electron current almost parallel to the magnetic field lines, are studied in a pair (electron–positron) plasma. The strongly nonlinear stage of weakly interacting modes is controlled by a vector-product type nonlinearity. The latter is responsible for the formation of a dipolar vortex whose salient features are described analytically.
Planar microwave discharges with active control of plasma uniformity
Authors: Yasaka Yasuyoshi , Koga Kazuya , Ishii Nobuo , Yamamoto Tetsuya , Ando Makoto , Takahashi Masaharu
Published: July 2002

Planar microwave discharges using a multi-slotted planar antenna are investigated. The enhancement of the microwave fields near the plasma resonance is observed in accordance with the theory of the resonant absorption. By operating the antenna with the azimuthally rotating fields, highly uniform overdense plasmas can be produced without being affected by eigenmode structures of surface waves. As a result, the radial distribution of the ion saturation current can be controlled by tailoring the power radiation profile from the antenna. It is also demonstrated that the active control of the plasma uniformity is possible by simply changing the vertical position of the antenna.
Anisotropic ferroelectric properties of epitaxially twinned Bi3.25La0.75Ti3O12 thin films grown with three different orientations
Authors: Lee Ho Nyung , Hesse Dietrich
Published: July 2002

Epitaxially twinned (001)- , (118)- , and (104)-oriented La-substituted Bi4Ti3O12 (BLT) ferroelectric films have been grown by pulsed laser deposition on (001)- , (011)- , and (111)-oriented SrTiO3 single-crystal substrates, respectively, covered with SrRuO3. Well-defined (001)-oriented BLT films were grown at a substrate temperature as low as 600 °C. By x-ray diffraction characterization it has been found that the low-index three-dimensional epitaxial orientation relationship BLT(001)‖SrRuO3(001)‖SrTiO3(001); BLT[11̄0]‖SrRuO3[100]‖SrTiO3[100] is valid for all epitaxially twinned BLT thin films grown on SrRuO3-covered SrTiO3 substrates in spite of their different orientations. The (104)-oriented BLT films showed an about 1.5 times higher remanent polarization (2Pr=31.9 μC/cm2) than the (118)-oriented BLT films (2Pr=20.7 μC/cm2), while (001)-oriented BLT films revealed only a small polarization component (2Pr=1.1 μC/cm2), thus demonstrating the ferroelectric anisotropy.
Thermal instability explanation of similar density limits in gas fueled, DIII-D H-mode shots with different operating conditions
Authors: Stacey W. M. , Petrie T. W. , Leonard A. W.
Published: July 2002

Recent experiments on DIII-D [J. L. Luxon, F. Batty, C. Baxi et al., Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] examined the effect of different operating conditions (“open” and “closed” divertor geometry, active pumping, fueling location) on the maximum achievable density in gas fueled H-mode (high confinement mode) discharges. Several phenomena observed at these higher densities (≈0.8 the Greenwald density)—degradation in energy confinement, detachment of the core plasma from the divertor plate, multifaceted asymmetric radiation from edge formation—are found to be correlated with the predicted onset of various thermal instabilities in the plasma edge or divertor regions. The similarity of the maximum achievable densities under the different operating conditions can be related to a similarity of edge thermal instability characteristics.
Si dangling-bond-type defects at the interface of (100)Si with ultrathin layers of SiOx, Al2O3, and ZrO2
Authors: Stesmans A. , Afanas’ev V. V.
Published: July 2002

Electron spin resonance on (100)Si/SiOx/ZrO2 and (100)Si/Al2O3/ZrO2 stacks with nm-thin dielectric layers reveals the Si dangling-bond-type centers Pb0, Pb1 as prominent defects at the (100)Si/dielectric interface. This Pb0, Pb1 fingerprint indicates that, while gratifying for the Si/SiOx/ZrO2 case, the as-deposited (100)Si/Al2O3 interface is basically Si/SiO2-like. The interfaces are in an enhanced (unrelaxed) stress state, characteristic of low-temperature Si/SiO2 growth. Based on the Pb0, Pb1 criterion, standard thermal Si/SiO2 interface properties may be approached by appropriate annealing (⩾ 650 °C) in vacuum in the case of Si/SiOx/ZrO2. Yet, O2 ambient is required for Si/Al2O3, indicating that the initial abruptness of the interface prevents thermal adaptation to occur until an additional SiOx interlayer grows. A minimal SiOx interlayer thickness (≳0.5 nm) appears requisite.
Self-organized nanostructures in surface chemical reactions: Mechanisms and mesoscopic modeling
Authors: Hildebrand M.
Published: July 2002

Nanoscale patterns can form in reactive adsorbates on catalytic surfaces as a result of attractive lateral interactions. These structures can be described within a mesoscopic theory that is derived by coarse graining the microscopic master equation thus providing a link between microscopic lattice models and reaction–diffusion equations. Such mesoscopic models allow to systematically investigate mechanisms responsible for the formation of nanoscale nonequilibrium patterns in reactive condensed matter. We have found that stationary and traveling nanostructures may result from the interplay of the attractive lateral interactions and nonequilibrium reactions. Besides reviewing these results, a detailed investigation of a single reactive adsorbate in the presence of attractive lateral interactions and global coupling through the gas phase is presented. Finally, it is outlined how a mesoscopic theory should be constructed for a particular scanning tunneling microscopy experiment [the oxidation of hydrogen on a Pt(111) surface] in order to overcome the failure of a corresponding reaction–diffusion model to quantitatively reproduce the experiments.
Environmental and thermal aging of Au/Ni/p-GaN ohmic contacts annealed in air
Authors: Wang S. H. , Mohney S. E. , Birkhahn R.
Published: July 2002

In recent years, Au/Ni/p-GaN ohmic contacts annealed in air have been studied extensively because they provide low specific contact resistances and high transparency. In this article we focus on the environmental and thermal degradation that we have observed in these contacts. When the contacts were not protected from the environment, degradation of the contacts always occurred over a period of days, and after sitting 30 days in the laboratory at room temperature, the current–voltage characteristics of the originally ohmic contacts were nonlinear and the contacts were significantly more resistive. To pinpoint the degradation mechanism, samples were stored at room temperature in dry nitrogen, dry oxygen, dry air, air saturated with water vapor, nitrogen gas saturated with water vapor, or vacuum. These experiments revealed that water vapor was the cause of the room temperature degradation. Since no change in the sheet resistance of the p-type GaN was observed upon aging, four point probe measurements and x-ray photoelectron spectroscopy depth profiles were carried out to determine the interaction between water vapor and the p-type NiO present in the annealed contact metallization. The measurements indicated that hydroxyl groups were incorporated in the NiO, leading to a reduction in its conductivity and presumably a decrease in its hole concentration. Aging studies at 200 °C further revealed poor thermal stability of the contacts not only in water vapor but also in nitrogen gas and vacuum, and this degradation was again linked to a degradation in the conductivity of the NiO component of the contact metallization.
Vibrational relaxation in CO+He collisions: Sensitivity to interaction potential and details of quantum calculations
Authors: Krems Roman V.
Published: July 2002

The present work investigates the sensitivity of cross sections for vibrational relaxation in He+CO collisions to details of quantum coupled-states calculations and the choice of vibrational wave functions used for evaluation of the coupling matrix elements. It is illustrated that the J-labeled coupled-states (CS) approach provides results in close agreement with accurate close coupling calculations and experimental measurements while the l-labeled approximation leads to cross sections underestimating the accurate data by a large factor. It is shown that centrifugal distortion of vibrational wave functions enhances cross sections for vibrational relaxation by approximately an energy independent factor. It is found that vibrationally inelastic cross sections are more sensitive to high-order terms in the Legendre expansion of the interaction potential when the J-labeled CS approach and the accurate vibrational wave functions with centrifugal distortion are used for the calculations.
Energy transfer and relaxation in europium-activated Y2O3 after excitation by ultraviolet photons
Authors: Tallant D. R. , Seager C. H. , Simpson R. L.
Published: July 2002

We present persistence data from the 5D manifolds of europium (3+)-activated Y2O3, after excitation by ultraviolet photons. For europium concentrations below 1 at. %, the persistence is largely consistent with multiphonon relaxation. For europium concentrations at and above 1 at. %, the persistence shows evidence for energy transfer interactions between europium activators. Interactions involving one activator in an upper (5D3, 5D2 or 5D1) manifold and another in a ground state (7F) manifold affect the kinetics of the relaxation of the upper 5D manifolds but do not degrade phosphor efficiency. Interactions involving two activators in excited manifolds ultimately dissipate, by phonon emission, excitation that might be emitted as photons. The interactions involving two activators in excited manifolds appear to be related to both concentration quenching and the reduction of phosphor efficiency at high excitation density.
Evidence for correlated double layers, bipolar structures, and very-low-frequency saucer generation in the auroral ionosphere
Authors: Newman D. L. , Goldman M. V. , Ergun R. E.
Published: July 2002

Recent observations by the Fast Auroral SnapshoT satellite have provided high-time-resolution measurements of three interrelated phenomena in the downward current region of the auroral ionosphere: Intense parallel electric fields (double layers) localized to tens of Debye lengths; drifting localized bipolar field structures interpreted in terms of electron phase-space holes; and intense quasi-electrostatic whistler emissions (very-low-frequency saucers) originating on the same field lines as the bipolar structures. Numerical simulations and theoretical modeling suggest how these observations may be related.
A self-organized critical transport model based on critical-gradient fluctuation dynamics
Authors: Garcia L. , Carreras B. A. , Newman D. E.
Published: July 2002

A one-dimensional transport model based on critical-gradient fluctuation dynamics is presented. This model has the characteristic properties of a self-organized critical (SOC) system. As the source increases and for an input flux above a threshold value, a dynamical transition spontaneously takes place. A high-gradient edge region forms. The width of this region increases with increasing value of the particle source. Transport dynamics in this edge region self-organizes to be very close to marginal stability, while the core remains at the subcritical gradient that is typical of a SOC system.
Thermal conduction in doped single-crystal silicon films
Authors: Asheghi M. , Kurabayashi K. , Kasnavi R. , Goodson K. E.
Published: July 2002

This work measures the thermal conductivities along free-standing silicon layers doped with boron and phosphorus at concentrations ranging from 1×1017 to 3×1019 cm−3 at temperatures between 15 and 300 K. The impurity concentrations are measured using secondary ion mass spectroscopy (SIMS) and the thermal conductivity data are interpreted using phonon transport theory accounting for scattering on impurities, free electrons, and the layer boundaries. Phonon-boundary scattering in the 3-μm-thick layers reduces the thermal conductivity of the layers at low temperatures regardless of the level of impurity concentration. The present data suggest that unintentional impurities may have strongly reduced the conductivities reported previously for bulk samples, for which impurity concentrations were determined from the electrical resistivity rather than from SIMS data. This work illustrates the combined effects of phonon interactions with impurities, free electrons, and material interfaces, which can be particularly important in semiconductor devices.
Observation of saturated polarization and dielectric anomaly in magnetoelectric BiFeO3 thin films
Authors: Palkar V. R. , John J. , Pinto R.
Published: July 2002

We report here synthesis of phase-pure magnetoelectric BiFeO3 thin films with controlled oxygen stoichiometry on Pt/TiO2/SiO2/Si substrate using pulsed-laser deposition technique. Saturated ferroelectric hysteresis loop has been observed in phase-pure and highly resistive BiFeO3 thin films. The dielectric response study of the films with temperature indicates an anomaly in dielectric constant ε(T), in the vicinity of Néel temperature (∼380 °C). This anomaly in ε(T) is explained as an influence of vanishing magnetic ordering on electric ordering of magnetoelectric BiFeO3 sample.
On the complete solution of the Sturm–Liouville problem (d2X/dx2)+λ2X=0 over a closed interval
Authors: Sosov Yuriy , Theodosiou Constantine E.
Published: July 2002

We discuss the sets of orthogonal functions that form solutions to the Sturm–Liouville problem for the equation d2X/dx2+λ2X=0 and for the general unmixed boundary conditions over a closed interval of the variable x. The conditions for the presence of the solutions with λ2⩽0 are specifically considered and their necessity for completeness of a set of eigenfunctions. Their implications are discussed for three examples from mathematical physics, showing that although for some problems the solutions, corresponding to the negative values of λ2, may reflect physically unusual boundary conditions, their presence is necessary in the general solution for the drift diffusion equation where they may represent stationary or growing in time solutions.
Enstrophy dynamics of stochastically forced large-scale geophysical flows
Authors: Blömker Dirk , Duan Jinqiao , Wanner Thomas
Published: July 2002

Enstrophy is an averaged measure of fluid vorticity. This quantity is particularly important in rotating geophysical flows. We investigate the dynamical evolution of enstrophy for large-scale quasi-geostrophic flows under random wind forcing. We obtain upper bounds on the enstrophy, as well as results establishing its Hölder continuity and describing the small-time asymptotics.
Growth of single-crystal SiO2 film on Ni(111) surface
Authors: Kundu Manisha , Murata Yoshitada
Published: July 2002

We grew a single-crystal SiO2 film on a Ni(111) surface. Atomic hydrogen during initial growth of an amorphous SiO2 film and an ambient oxygen atmosphere during annealing of this film were essential in forming this single-crystal SiO2 film. The film grew with a (√3×√3) R30° crystallographic orientation, and its structure was the high-temperature phase of quartz, i.e., β quartz. In addition, a polycrystalline silicide phase was formed at the SiO2/Ni(111) interface.
Cathodoluminescence from BN buried layers by high-dose ion implantation
Authors: Barbadillo L. , Cervera M. , Hernández M. J. , Rodrı́guez P. , Piqueras J. , Martı́nez O. , Avella M. , Jiménez J.
Published: July 2002

Boron, nitrogen, and carbon ions were co-implanted in silicon wafers, and subsequently annealed. Infrared spectra show the formation of BN-rich buried layers. The presence of a band at 1375 cm−1 characteristic of boron nitride in a hexagonal configuration has been observed. Traces of the cubic phase formation were detected in some cases. Implanted samples exhibit a broad emission band about 550 nm.
Coupled quasidiabatic potential energy surfaces for LiFH
Authors: Jasper Ahren W. , Hack Michael D. , Truhlar Donald G. , Piecuch Piotr
Published: July 2002

We present high-level ab initio calculations for the global adiabatic potential energy surfaces of the ground state (X̃ 2A′) and several excited states (Ã 2A′, B̃ 2A″, C̃ 2A′, D̃ 2A′, and Ẽ 2A″) of LiFH, including the valleys leading to Li+HF and LiF+H. The ab initio calculations were carried out using the multireference singles and doubles configuration interaction method with 99 reference configuration state functions (CSFs) for the A′2 states and 39 reference CSFs for the A″2 states. The basis set consisted of 140 contracted Gaussian functions, including specifically optimized diffuse functions, and calculations were performed on a dense grid of ∼3500 nuclear geometries which allowed us to construct an accurate analytic representation of the two lowest-energy LiFH potential energy surfaces. An analytic 2×2 quasidiabatic potential energy matrix was obtained by fitting physically motivated functional forms to the ab initio data for the two lowest-energy adiabatic states and explicitly including long-range interactions. The newly presented LiFH fit is compared to several ground-state LiFH fits and one excited-state LiFH fit that have appeared in the literature.
Effect of growth temperature on strain barrier for metalorganic vapor phase epitaxy grown strained InGaAs quantum well with lattice matched InGaAsP barriers
Authors: Sharma T. K. , Arora B. M. , Kumar S. , Gokhale M. R.
Published: July 2002

InGaAs strained quantum well (SQW) samples with lattice matched InGaAsP quaternary barriers are grown on GaAs substrates by metalorganic vapor phase epitaxy. These SQW samples are characterized using photoluminescence, photomodulated reflectance, surface photovoltage spectroscopy and high-resolution x-ray diffraction techniques. The results based on numerical calculations are used to identify the various transitions seen in the spectra. The effect of growth temperature on the indium content of the InGaAs SQW with lattice matched InGaAsP quaternary barriers is studied. Contrary to the reported higher value of indium incorporation in InGaAs SQW with GaAs (In-free) barriers when the growth is performed at low temperatures, we find that the indium content of the InGaAs SQW with InGaAsP (In-based) quaternary barriers decreases if the SQW is grown at lower growth temperatures. A possible explanation for this behavior is provided.
Compact plasma accelerator device
Authors: Foster John E.
Published: July 2002

A compact plasma accelerator concept based on plasma production at a magnetic cusp was designed and fabricated. Plume and discharge characteristics of the device were documented using a Faraday probe and a retarding potential analyzer. The discharge current variations with increasing discharge voltage were nonlinear with the discharge current increasing rapidly with voltage. The device demonstrated the capability of generating ion beamlets (∼80 eV) with downstream peak current densities comparable to that of higher power ion thruster devices (7 mA/cm2). In general, the device appeared to operate best at very low flow rates. High propellant utilization fractions (ionization fractions) were measured below 1 SCCM of Xe flow (88% at 0.48 SCCM). Floating potential measurements made downstream of the device were used to estimate the downstream electric field. These measurements, which were used to qualitatively assess beam neutralization, indicated beam neutralization does occur downstream of the device. Based on the measurements, the compact plasma accelerator concept could potentially be used as a low energy ion source for propulsion applications or for low energy plasma/materials processing applications.
A new approach to static force-free electromagnetic fields
Authors: Willis Barton L.
Published: July 2002

In the early 1950s, Lundquist showed the significance of electromagnetic fields with a vanishing Lorentz force density to magnetohydrostatic fluids. Today, these fields are known as force-free electromagnetic fields. Later that decade, Lüst and Schlüter demonstrated that cosmic magnetic fields are force-free and Chandrasekhar and Kendall then constructed a large class of force-free fields whose electric charge density field is also vanishing. In this article, we constructed force-free fields without assuming that the electric charge density vanishes, and in some cases we established a connection between force-free fields and nonlinear Schrödinger equations.
Monitoring of hydrocarbon vapor diffusion in air using a thermal wave interferometer
Authors: Lima J. A. P. , da Silva M. G. , Massunaga M. S. O. , Marı́n E. , Vargas H. , Miranda L. C. M.
Published: July 2002

A discussion on the use of thermal wave interference (TWI) for the monitoring of the transients of hydrocarbon in air is presented. The thermal wave signal was modeled using the logarithm-mixing model for the thermal diffusivity of a two-phase gas system in which the hydrocarbon vapor concentration in the air-filled TWI cell is a varying function of time. The time varying hydrocarbon vapor concentration was described assuming the simple Fick’s model for mass diffusion of the hydrocarbon vapor in the stagnant air column of the TWI cell. The transient TWI signal amplitude data fitting yielded two parameters, namely, the saturation concentration and the characteristic diffusion time. From the corresponding values of the diffusion time the hydrocarbon mass diffusivities were straightforwardly obtained. The obtained values for the hydrocarbon mass diffusivities were found to be in good agreement with the ones reported in the literature.
TiC nanoisland formation on 6H–SiC(0001)Si
Authors: Platow W. , Oh Jaehwan , Nemanich R. J. , Sayers D. E. , Hartman J. D. , Davis R. F.
Published: July 2002

Spontaneous formation of titanium carbide nanoislands on silicon carbide substrates has been studied with scanning tunneling microscopy and x-ray absorption near-edge spectroscopy. Scratch-free and atomically flat 6H–SiC(0001)Si substrates were prepared by high temperature hydrogen etching. The surfaces were subsequently cleaned by in situ ultrahigh vacuum annealing. Titanium carbide nanoislands were formed by titanium deposition and annealing at 950 °C. The average width (10–50 nm) and separation of the Ti islands was controlled by varying the titanium coverage (0.1–0.3 nm) and the annealing time (1–20 min). At the lowest coverage, the islands were uniformly distributed over the surface, while at higher coverage the islands tended to collect at the substrate step edges.
The dilute A4 model, the E7 mass spectrum and the tricritical Ising model
Authors: Seaton K. A. , Batchelor M. T.
Published: July 2002

The exact perturbation approach is used to derive the (seven) elementary correlation lengths and related mass gaps of the two-dimensional dilute A4 lattice model in regime 2− from the Bethe Ansatz solution. This model provides a realization of the integrable φ(1,2) perturbation of the c=710 conformal field theory, which is known to describe the off-critical thermal behavior of the tricritical Ising model. The E7 masses predicted from purely elastic scattering theory follow in the approach to criticality. Universal amplitudes for the tricritical Ising model are calculated.
A criterion for the breakdown of vibrational equilibrium in expansions
Authors: Bird G. A.
Published: July 2002

The ratio of the mean time between collisions of a typical molecule to the scale time of the density change of an element moving with the flow has long been used as a criterion for the breakdown of translational equilibrium in gaseous expansions. The pressure tensor becomes anisotropic when this parameter ceases to be very small in comparison with unity. It is proposed in this paper that a breakdown parameter for an internal mode is obtained simply by multiplying the translational parameter by the relaxation collision number for that mode. Emphasis is placed on the breakdown of vibrational equilibrium because this is of practical significance in many applications. For steady flows, the vibrational breakdown parameter is the product of a constant of order unity, the relaxation collision number, the speed ratio or Mach number, and the Knudsen number based on the scale length of the density variation. The Mach number term becomes nonlinear if the scale length of the area variation is employed in place of the density scale length. The direct simulation Monte Carlo (DSMC) method is used to make a series of calculations that support the use of this parameter to predict the breakdown of vibrational equilibrium in expansions.
Heat interaction in a resonance tube
Authors: Alexeev A. , Goldshtein A. , Gutfinger C.
Published: July 2002

Gas oscillations in a resonance tube are investigated experimentally and numerically. We found that for large amplitudes, heat interaction with the tube wall affects strongly gas flow for frequencies within the resonance band. A proposed model, accounting for heat interaction, shows good agreement with the experimental data.