bluuebunny/crossref_metadata_2025 huggingface dataset:
High-pressure x-ray diffraction of icosahedral Zr–Al–Ni–Cu–Ag quasicrystals
Authors: Jiang J. Z. , Saksl K. , Rasmussen H. , Watanuki T. , Ishimatsu N. , Shimomara O.
Published: July 2002

The effect of pressure on the structural stability of icosahedral Zr–Al–Ni–Cu–Ag quasicrystals forming from a Zr65Al7.5Ni10Cu7.5Ag10 metallic glass with a supercooled liquid region of 44 K has been investigated by in situ high-pressure angle-dispersive x-ray powder diffraction at ambient temperature using synchrotron radiation. The icosahedral quasicrystal structure is retained up to the highest hydrostatic pressure used (approximately 28 GPa) and is reversible after decompression. The bulk modulus at zero pressure and its pressure derivative of the icosahedral Zr–Al–Ni–Cu–Ag quasicrystal are 99.10±1.26 GPa and 4.25±0.16, respectively. The compression behavior of different Bragg peaks is isotropic and the full width at half maximum of each peak remains almost unchanged during compression, indicating no anisotropic elasticity and no defects in the icosahedral Zr–Al–Ni–Cu–Ag quasicrystals induced by pressure.
Parallel-hat tempering: A Monte Carlo search scheme for the identification of low-energy structures
Authors: Zhang Yang , Skolnick Jeffrey
Published: July 2002

A new parallel-hat tempering algorithm has been developed for Monte Carlo simulations, in which a composite ensemble of noninteracting replicas of the molecule system at different temperatures is simulated, and the Markov process of each replica is driven by a hatlike weight factor with exponentially enhanced probability in both low- and high-energy regions. To test the algorithm, the methodology is applied to a homopolymeric protein chain located on a face-centered cubic lattice. We demonstrate that the ability of the current approach to search for low-energy molecule structures is significantly better than other Monte Carlo techniques found in the literature.
Dependence of the emission wavelength on the internal electric field in quantum-dot laser structures grown by metal–organic chemical-vapor deposition
Authors: Passaseo A. , Maruccio G. , De Vittorio M. , De Rinaldis S. , Todaro T. , Rinaldi R. , Cingolani R.
Published: July 2002

We show that the combination of different electric fields in In0.5Ga0.5As/GaAs quantum-dot electroluminescent devices dramatically blueshifts the emission wavelength even though the photoluminescence occurs at the expected value of 1.3 μm at room temperature. Systematic photoluminescence (PL), electroluminescence (EL), and photocurrent measurements demonstrate that the electric field associated with the built-in dipole in the dots, directed from the base of the dots to their apex, and the device junction field lead to the depletion of the ground state. As a consequence, structures grown on n-type GaAs substrates exhibit electroluminescence only from the excited states (whereas the photoluminescence comes from the ground level). Instead, by growing the same device structure on p-type GaAs substrates, i.e., by reversing the direction of the built-in electric field of the device, the effect of the permanent dipole is strongly reduced, thus allowing us to obtain EL emission at the designed wavelength of 1.3 μm at 300 K, coincident to the PL. This effect expands the possibilities for the achievement of efficient lasing in the spectral region of interest for optical transmission. The electric field associated to the dipole moment is estimated to be around 150 kV/cm.
Comparison of negative-ion and positive-ion-assisted etching of silicon
Authors: Kanakasabapathy Sivananda K. , Khater Marwan H. , Overzet Lawrence J.
Published: July 2002

Relatively electron-free, positive- and negative-ion (ion–ion) plasmas have been achieved in the afterglow of pulsed-power Cl2 discharges. The application of a pulsed dc bias phase locked to the source power modulation and exclusive to the ion–ion plasma, allows selective bombardment by positive (Cl2+) or negative (Cl−) ions onto a silicon substrate. This allows an equitable comparison of etching by equal energy ions of both polarities. We find that at 50 eV, Cl2+ etches twice as fast as Cl−.
Evidence for hole and electron trapping in plasma deposited ZrO2 thin films
Authors: Chavez J. R. , Devine R. A. B. , Koltunski L.
Published: July 2002

We have observed electron and hole trapping phenomena in thin films of ZrO2 obtained by plasma assisted deposition. Limited thickness dependent measurements suggest that the holes are trapped uniformly through the film while the electrons trap at the ZrO2/Si interface. Relaxation of the trapped holes occurs rapidly after removal of negative stress (∼90% in 15 min), while electron relaxation postpositive stress occurs more slowly (∼10% in 100 min). Cycling between states of positive trapped charge and negative trapped charge by application of the appropriate stressing voltage was observed.
Tunneling carrier escape from InAs self-assembled quantum dots
Authors: Ibáñez J. , Leon R. , Vu D. T. , Chaparro S. , Johnson S. R. , Navarro C. , Zhang Y. H.
Published: July 2002

Deep-level transient spectroscopy measurements in InAs quantum dots (QDs) grown in both n-GaAs and p-GaAs show that tunneling is an important mechanism of carrier escape from the dots. The doping level in the barrier strongly affects the tunneling emission rates, enabling or preventing the detection of a transient capacitance signal from a given QD level. The relative intensity of this signal acquired with different rate windows allows the estimation of tunneling emission energies.
Electron cyclotron resonance light source from TE011 mode microwave plasma
Authors: Chauhan Sarvesh S. , Rajyaguru Chirag C. , Ito Hiroaki , Yugami Noboru , Nishida Yasushi , Yoshida Tomoyuki
Published: July 2002

An intense ultraviolet and visible radiation source for various applications has been conceptualized, designed, and developed using a TE011 mode microwave cavity filled with plasma immersed in a magnetic field. The radiation intensity emitted from the source, as a function of the magnetic field intensity, approaches maximum at the electron cyclotron resonance. Light intensity increases with pressure inside the Hg+Ar lamp and the microwave power.
Toward an anisotropic atom–atom model for the crystalline phases of the molecular S8 compound
Authors: Pastorino C. , Gamba Z.
Published: July 2002

We analyze two anisotropic atom–atom models used to describe the crystalline α, β, and γ phases of S8 crystals, the most stable compound of elemental sulfur in solid phases, at ambient pressure and T≲400 K. The calculations are performed via a series of classical molecular dynamics (MD) simulations, with flexible molecular models and using a constant pressure–constant temperature algorithm for the numerical simulations. All intramolecular modes that mix with lattice modes, and are therefore relevant to the onset of structural phase transitions, are taken into account. Comparisons with experimental data and previous results obtained with an isotropic atom–atom molecular model are also performed.
Molecular dynamics simulation study of the properties of doubly occupied N2 clathrate hydrates
Authors: van Klaveren E. P. , Michels J. P. J. , Schouten J. A. , Klug D. D. , Tse J. S.
Published: July 2002

We have performed molecular dynamics calculations to obtain a number of properties of a doubly occupied structure II N2 clathrate hydrate, in particular to study its behavior under higher pressures. The calculated neutron diffraction pattern is in agreement with the experimental result. The effect of the presence of the filling of the small cages and of the large cages (in either single or double occupancy) on the calculated pattern is demonstrated and discussed. The calculated Raman spectra show that the average vibrational frequency of the N2 molecules decreases in going from the singly occupied small cages to the doubly occupied large cages and then to the singly occupied large cages, respectively. The frequency distributions are explained in terms of radial distribution functions. When applying large pressures at low temperatures, a clathrate-amorphous transition occurs for a partially doubly occupied clathrate. The transition occurs at about the same pressure as for single occupations, but the densification is larger for the latter. In both cases, the transition is reversible.
High-resolution energy-dispersive x-ray spectroscopic analysis of ultrathin ion diffusion barriers using microcalorimetry
Authors: Geer R. E. , Wu D. , Wollman D. A.
Published: July 2002

The advent of microcalorimetry for x-ray detectors holds the promise for high-resolution compositional microanalysis applicable to nanometer-scale devices and structures. To demonstrate this capability, microcalorimeter-based energy dispersive x-ray spectroscopy (EDS) has been used to analyze ultrathin TaSiN films under development as ion diffusion barriers in sub-0.1 μm integrated circuit interconnect structures. Microanalysis of TaSiN films has been carried out using microcalorimetry-based EDS for comparison with similar data acquired using conventional Si(Li) EDS detectors. The elimination of elemental peak overlaps provided by the improved energy resolution of the microcalorimeter x-ray detector is demonstrated for TaSiN EDS spectra from films as thin as 3.5 nm. In addition, variation of the electron probe beam energy is examined to reduce the x-ray generation volume relative to the TaSiN film thickness. It is shown that microcalorimetery-based EDS is a potentially powerful technique for the characterization/metrology of ultrathin barrier films used for microelectronics, with x-ray energy-resolving capabilities similar to techniques such as wavelength dispersive spectroscopy.
Optical characterization of wurtzite gallium nitride nanowires
Authors: Lee M. W. , Twu H. Z. , Chen C.-C. , Chen C.-H.
Published: July 2002

The optical properties of the gallium nitride (GaN) nanowires are examined by the transmission method in the ultraviolet-visible range (1–5 eV) and by the reflection method in the infrared range (500–4000 cm−1). The absorption edge of the GaN nanowires is blueshifted by 0.2 eV from the bulk edge. The temperature dependence of the energy gap is expressed by, Eg(T)=3.724−9.97×10−4/(861+T) eV. The plasma frequency and the free-carrier density of the GaN nanowires, deduced from the infrared reflectance minima, are estimated to be ωp=1100±120 cm−1 and nf=3.73×1017 cm−3, respectively. Obtaining the free-carrier density from the infrared reflectance spectra is especially useful in research on nanostructured semiconductors.
High-frequency capacitance–voltage measurement of plasma-enhanced chemical-vapor-deposition-grown SiO2/n-GaN metal-insulator-semiconductor structures
Authors: Chen P. , Wang W. , Chua S. J. , Zheng Y. D.
Published: July 2002

This work reports on the high-frequency capacitance–voltage (C–V) measurements of metal-insulator-semiconductor structures fabricated by depositing SiO2 film on an n-type GaN epitaxial layer. The SiO2 film was grown by plasma-enhanced chemical vapor deposition at 310 °C, and the GaN layer was grown by low-pressure metalorganic chemical vapor deposition on a sapphire substrate. High-frequency C–V measurements have been carried out in darkness with different bias ranges and sweep rates. With a bias between ±20 V, the small flatband shift and the very small hysteresis indicate that the interface trap concentration in the sample is low, and the interface state density is 2.1×1011 eV−1 cm−2. However, a pronounced increase of hysteresis with an extended bias range was observed. When the bias is over ±50 V, the increase of the hysteresis is much larger, indicating the nonuniform distribution of different slow deep states in the structure. The extracted interface state density is in the range of 1012 eV−1 cm−2.
Application of microwave breakdown spectroscopy to detect halide impurities in the air
Authors: Suto Hiroshi , Matsuura Mutsumi , Iinuma Koichi , Uchida Shunsuke , Takayama Ken
Published: July 2002

In order to determine ppm orders of air pollutants, e.g., halide impurities, spectroscopy of the photons emitted from the plasma generated by a focused microwave beam in the air has been applied. The microwave beam was supplied by a pulsed magnetron (microwave frequency: 9.4 GHz; peak intensity: 100 kW; pulse width: 5 μs; pulse repetition frequency: 10 Hz), focused in a vacuum chamber, where pressure and impurities had been controlled to target values, and a plasma was generated using the air there. At the same time, the energy spectrum of the photons from the plasma was measured to determine the concentrations of chlorine and fluorine in the air by a measurement system consisting of a diffraction grating, streak camera, and charge-coupled-device camera. As a result of preliminary experiments, where the concentration of CCl2F2 was changed from 0.05 to 2 vol % in decompressed air (pressure: 10 Torr), it was demonstrated that 1700 ppm of chlorine could be determined from the background spectrum caused by the first positive band of nitrogen (B3Πg–A3Σu+), its second positive band (C3Πu–B3Πg), and first negative band (B3Σu+–X2Σg+). The experimental data demonstrated that ppm orders of halide impurities can be determined by improving pulse repetition frequency of the microwave source.
Light-induced increase in the open-circuit voltage of thin-film heterogeneous silicon solar cells
Authors: Lord K. , Yan B. , Yang J. , Guha S.
Published: July 2002

We observe a significant light-induced increase in open-circuit voltage, Voc, of solar cells whose intrinsic (i) layer consists of an amorphous and microcrystalline mixed phase. The increase depends on the i-layer thickness and light-soaking intensity. An increase of as large as 150 mV or 20% of the original Voc is observed. The original Voc is restored after subsequent thermal annealing. The possible mechanism for the Voc increase is discussed.
Photoluminescence of transparent strontium–barium–niobate-doped silica nanocomposites
Authors: Lu S. G. , Mak C. L. , Wong K. H. , Cheah K. W.
Published: July 2002

Optically transparent nanocomposites of strontium–barium–niobate (SBN) and silica were fabricated via a novel sol–gel method. The nanocomposites were annealed in temperatures of 600–800 °C to yield the nanometer-sized tetragonal tungsten–bronze phase of SBN crystallites. Photoluminescence studies at temperatures from 10 K to room temperature were performed. The undoped silica matrix showed a strong emission band at 3.2 eV and a weak emission band at 2.65 eV. They were noticeably suppressed in our SBN/SiO2 nanocomposites. Extra sharp peaks on top of the 3.2 eV band were seen. An additional emission band at 2.3 eV due to transitions within the NbO6 complex of SBN crystallites was also observed. This emission band showed a large blueshift with decreasing SBN crystallite size. This demonstrates that the photoluminescence spectra of ferroelectric-doped silica nanocomposites have a strong dependence on crystallite size.
Stability of an oscillating tip in noncontact atomic force microscopy: Theoretical and numerical investigations
Authors: Couturier G. , Nony L. , Boisgard R. , Aimé J.-P.
Published: July 2002

This article is a theoretical and a numerical investigation of the stability of a tip–cantilever system used in noncontact atomic force microscopy (NC-AFM) when it oscillates close to a surface. No additional dissipative force is considered. The theoretical approach is based on a variational method exploiting a coarse grained operation that gives the temporal dependence of the nonlinear coupled equations of motion in amplitude and phase of the oscillator. Stability criterions for the resonance peak are deduced and predict a stable behavior of the oscillator in the vicinity of the resonance. The numerical approach is based on results obtained with a virtual NC-AFM developed in our group. The effect of the size of the stable domain in phase is investigated. These results are in particularly good agreement with the theoretical predictions. They also show the influence of the phase shifter in the feedback loop and the way in which it can affect the damping signal.
Effect of scattering by strain fields surrounding edge dislocations on electron transport in two-dimensional electron gases
Authors: Jena Debdeep , Mishra U. K.
Published: July 2002

We present a theory of deformation potential carrier scattering of two-dimensional electron gases from the strain fields surrounding edge dislocations. The scattering rate is evaluated in closed form without any fitting parameters. The result is directed towards understanding mobility limiting scattering mechanisms for two-dimensional electron gases at AlGaN/GaN heterointerfaces.
Photoinduced surface relief formation on azopolymer films: A driving force and formed relief profile
Authors: Sumaru Kimio , Fukuda Takashi , Kimura Tatsumi , Matsuda Hiro , Yamanaka Tadae
Published: July 2002

A driving force of photoinduced surface relief (PSR) formation is examined by considering the complex electric susceptibility of the absorbing medium. It is introduced into a fluid mechanics model established for the viscous fluid layer on a flat substrate. As a result, a theoretical model for PSR formation covering thoroughly from the origin of the driving force to the dynamic process forming the surface relief is established. A series of systematic experiments are carried out by means of one-dimensional sinusoidal and Gaussian beam irradiation and the results are reproduced well by the model calculation. The experimental results reported in former literature are also reviewed using this model and the result supports strongly our assumption: driving force derived from the interaction between the absorbing medium and optical electric field acts as a body force on the optically plasticized azopolymer. Further, from the consideration based on the model, some important features of PSR formation which have not been reported yet are suggested.
Nanometer-spaced electrodes with calibrated separation
Authors: Kervennic Y. V. , Van der Zant H. S. J. , Morpurgo A. F. , Gurevich L. , Kouwenhoven L. P.
Published: July 2002

We have fabricated pairs of platinum electrodes with separation between 20 and 3.5 nm. Our technique combines electron beam lithography and chemical electrodeposition. We show that the measurement of the conductance between the two electrodes through the electrolyte provides an accurate and reproducible way to control their separation. We have tested the robustness of the electrodes by applying large voltages across them and by using them to measure the transport properties of Au nanoclusters. Our results show that the technique reliably produces metallic electrodes with a separation that bridges the minimum scale accessible by electron beam lithography with the atomic scale.
Laser damage studies in zinc (tris) thiourea sulfate: Nonlinear optical crystal
Authors: Gupte Sonal S. , Pradhan Ranjit D. , Marcano O. A. , Melikechi Noureddine , Desai C. F.
Published: July 2002

We report on a detailed study of laser damage in a semiorganic nonlinear optical crystal, zinc (tris) thiourea sulfate using eight nanosecond laser pulses with a repetition rate of 10 Hz at wavelengths of 532 and 1064 nm. The damage morphology shows a correlation with the mechanical hardness anisotropy. The crystal shows a damage threshold measured to be 0.6±0.1 GW/cm2 for a single 532 nm laser pulse which reduces to 0.12±0.01 GW/cm2 as the number of shots is increased to 10 000. Single shot laser damage at 1064 nm wavelength shows an onset estimated to be about 3 GW/cm2. Multiple shots at 1064 nm result in laser ablation hindering the determination of a damage threshold. We suggest a possible role of negative thermal effects resulting in the high damage threshold observed in this crystal.
Nitrogen-induced enhancement of the electron effective mass in InNxAs1−x
Authors: Hung W. K. , Cho K. S. , Chern M. Y. , Chen Y. F. , Shih D. K. , Lin H. H. , Lu C. C. , Yang T. R.
Published: July 2002

The electron effective mass in n-type InNxAs1−x (with x up to 3.0%) grown by gas-source molecular-beam epitaxy was obtained from infrared reflectivity and Hall-effect measurements. The large increase of the effective mass due to the incorporation of nitrogen is attributed mainly to the nitrogen-induced modification on the electronic states near the conduction-band edge. The well-known band anticrossing (BAC) model for the electronic structure of the III-N-V alloys cannot well describe the experimental data, especially in the region of higher electron concentration. This result provides an opportunity to examine the “universality” of the BAC model.
Development of texture in TiN films by use of in situ synchrotron x-ray scattering
Authors: Schell N. , Matz W. , Bøttiger J. , Chevallier J. , Kringhøj P.
Published: July 2002

During growth, the microstructural development of TiN films was studied—especially the change in texture with film thickness. The films were deposited by use of a magnetron sputtering source in a vacuum chamber equipped with two magnetron sources and mounted on a goniometer located at a synchrotron radiation beam line. X-ray diffraction and reflectivity measurements were carried out in situ to follow the microstructure as a function of film thickness. With the deposition parameters that were chosen, a crossover was observed: grains with a (002) plane parallel to the film surface dominated at small thicknesses, while, at larger thicknesses, (111) grains dominated. Recrystallization was identified as a mechanism that controls this texture development. The driving force for change of orientation of the individual grains arose from minimalization of the sum of the surface energy and the strain energy of the individual grains.
Dielectric loss modes of SrTiO3 thin films deposited on different substrates
Authors: Yu Zhi , Ang Chen , Guo Ruyan , Bhalla A. S. , Cross L. E.
Published: July 2002

The dielectric loss modes of SrTiO3 thin films deposited on different substrates are reported. Two dielectric loss modes around 165 and 75 K (at 10 kHz) with field-independent Tm are emphasized. The physical origin of the dielectric modes is discussed and is tentatively attributed to the motion of oxygen vacancies.
Irradiation-induced formation of nanoparticles in cadmium niobate pyrochlore
Authors: Jiang W. , Weber W. J. , Young J. S. , Boatner L. A.
Published: July 2002

Sequential irradiation with 3 MeV He+ and 10 MeV C3+ ions performed at T=150 K produces two separate amorphous buried layers in cadmium niobate pyrochlore single crystals. Further irradiation at room temperature results in the formation of nanometer-scale particles in the amorphized regions. An ion-cleaving technique was used to facilitate the observation of these nanoparticles by using scanning electron microscopy. Complete granulation with particle sizes ranging from 30 to 150 nm was observed. X-ray energy-dispersive spectrometry analysis indicates that the numerically large population of smaller particles (∼50 nm) contains a high Cd content (∼70%) and the numerically smaller population of larger particles (>100 nm) contains negligible Nb with a Cd-to-O ratio of about 1:0.54.
Ultrasound shock wave generator with one-bit time reversal in a dispersive medium, application to lithotripsy
Authors: Montaldo Gabriel , Roux Philippe , Derode Arnaud , Negreira Carlos , Fink Mathias
Published: July 2002

The building of high-power ultrasonic sources from piezoelectric ceramics is limited by the maximum voltage that the ceramics can endure. We have conceived a device that uses a small number of piezoelectric transducers fastened to a cylindrical metallic waveguide. A one-bit time- reversal operation transforms the long-lasting low-level dispersed wave forms into a sharp pulse, thus taking advantage of dispersion to generate high-power ultrasound. The pressure amplitude that is generated at the focus is found to be 15 times greater than that achieved with comparable standard techniques. Applications to lithotripsy are discussed and the destructive efficiency of the system is demonstrated on pieces of chalk.
Submicron particles of Fe/Co multilayers: Influence of interactions
Authors: Hanson M. , Kazakova O. , Blomqvist P. , Wäppling R.
Published: July 2002

Magnetic elongated particles (150 by 450 nm) made of Fe/Co multilayers, of thickness 23 and 35 nm, were investigated using magnetic-force microscopy and magnetization measurements. We present evidence of effects of dipolar interactions both on the domain structure of elliptical particles positioned in square blocks and on the switching behavior of particles as the gap between them is varied from 450 to 100 nm.
Thickness-dependent phase evolution of polycrystalline Pb(Zr0.35Ti0.65)O3 thin films
Authors: Kelman Maxim B. , Schloss Lawrence F. , McIntyre Paul C. , Hendrix Bryan C. , Bilodeau Steven M. , Roeder Jeffrey F.
Published: July 2002

The structural and electrical properties of metalorganic chemical vapor deposition-grown Pb(Zr0.35Ti0.65)O3 thin films ranging in thickness from 700 to 4000 Å have been investigated. Cross-sectional scanning electron microscopy showed that these films are columnar, with grains extending through the thickness of the film. High-resolution x-ray diffraction showed that while the thickest films are tetragonal, with reflections corresponding to a-type and c-type domains, films thinner than 1500 Å are not. Electron backscatter diffraction and hysteresis loop measurements showed that the thinnest films are ferroelectric and have a rhombohedral crystal structure.
Magneto-optic properties of thin EuS/Co and EuS/Cu films on Si(111) substrates
Authors: Müller C. , Lippitz H. , Paggel J. J. , Fumagalli P.
Published: July 2002

Thin EuS films of 5.5 and 11 nm have been grown at 300 K on epitaxial Co(111) and Cu(111) surfaces on Si(111) substrates. The EuS layers show volumetric reflections in the RHEED pattern indicating that the EuS is deposited in form of small crystallites on the Co or Cu surface. The magneto-optic properties are investigated by polar Kerr spectroscopy at 300 K and 10 K. At 10 K, a clear signature of the intra-Eu 4f-5d transition is observed at 1.8 eV and at 4.2 eV. Below 1 eV, the magneto-optic contribution of the Co layer is dominant. The influence of the Co or Cu layer on the magnetic ordering of the EuS layer is studied by measuring the Kerr spectra as a function of temperature. The layered structures exhibit an in-plane magnetic anisotropy. Estimates of the paramagnetic EuS contribution to the spectrum at 300 K suggest only a weak coupling between the EuS film and the Co layer.
Shape preservation of Ge/Si(001) islands during Si capping
Authors: Rastelli A. , Müller E. , von Känel H.
Published: July 2002

Coherently strained Ge/Si(001) islands were overgrown with Si at temperatures ranging from 300 to 550 °C. The induced shape changes were investigated at different stages of the capping process by scanning tunneling microscopy and high-resolution transmission electron microscopy. Islands were found to strongly flatten and intermix at temperatures above ∼450 °C. By contrast, a good shape preservation as well as the recovery of a flat Si surface above the buried islands can be achieved by low temperature capping at 300 °C followed by Si growth at 550 °C.
Electronic property variations due to an embedded potential barrier layer in modulation-doped step quantum wells
Authors: Kim T. W. , Lee D. U. , Choo D. C. , Jung M. , Yoo K. H. , Comanescu G. , McCombe B. D.
Published: July 2002

Electronic property variations of a two-dimensional electron gas (2DEG) in modulation-doped step quantum wells due to an embedded potential barrier were studied by performing Shubnikov–de Haas (SdH), Van der Pauw–Hall-effect, and cyclotron resonance measurements on two kinds of InxGa1−xAs/InyAl1−yAs step quantum wells which were one without and the other with an embedded barrier. The fast Fourier transformation results for the SdH data at 1.5 K indicated the electron occupation of two subbands in both step quantum wells. The total electron carrier density and the mobility of the 2DEG in the step quantum well with an embedded barrier were smaller than those in the quantum well without an embedded barrier. The electron effective masses were determined from the slopes of the main peak absorption energies as functions of the magnetic field, and satisfied qualitatively the nonparabolicity effects in both quantum wells. The electronic subband energies, the wave functions, and the Fermi energies were calculated by using a self-consistent method taking into account exchange-correlation effects together with strain and nonparabolicity effects. These present results indicate that the electronic parameters in modulation-doped InxGa1−xAs/InyAl1−yAs step quantum wells are significantly affected by an embedded barrier.
Field emission from isolated individual vertically aligned carbon nanocones
Authors: Baylor L. R. , Merkulov V. I. , Ellis E. D. , Guillorn M. A. , Lowndes D. H. , Melechko A. V. , Simpson M. L. , Whealton J. H.
Published: July 2002

Field emission from isolated individual vertically aligned carbon nanocones (VACNCs) has been measured using a small-diameter moveable probe. The probe was scanned parallel to the sample plane to locate the VACNCs, and perpendicular to the sample plane to measure the emission turn-on electric field of each VACNC. Individual VACNCs can be good field emitters. The emission threshold field depends on the geometric aspect ratio (height/tip radius) of the VACNC and is lowest when a sharp tip is present. VACNCs exposed to a reactive ion etch process demonstrate a lowered emission threshold field while maintaining a similar aspect ratio. Individual VACNCs can have low emission thresholds, carry high current densities, and have long emission lifetime. This makes them very promising for various field emission applications for which deterministic placement of the emitter with submicron accuracy is needed.
Transport characteristics of posthard breakdown thin silicon oxide films and consideration of physical models
Authors: Omura Yasuhisa , Komiya Kenji
Published: July 2002

This article describes the constant-voltage-stress-induced breakdown of thin silicon oxide films, as well as the conduction mechanisms of posthard breakdown oxide films which are characterized by means of the differential resistance technique involving posthard breakdown current and spectral analyses of posthard breakdown current fluctuation. A very thin oxide film still shows a high differential resistance just after the first hard breakdown event; additional current stress pushes the film into the final state. On the other hand, a thicker oxide film enters the final state (breakdown) after the first hard breakdown event. It is also shown that there still exists an energy barrier for electrons after the hard breakdown event as already reported. However, it is also shown that the energy barrier places an asymmetric restriction on electron flow in contrast to the past symmetric assumption. Current fluctuation of posthard breakdown oxide films is characterized in detail. Short-time and long-time observations of current fluctuations suggest that there are two different major defects inside posthard breakdown oxide films; in a significant development, Fourier-transformed results are shown to yield a very short time constant and a very long time constant. These results are consistent with past prebreakdown behavior analyses that indicated a significant relationship between the soft breakdown and the hard breakdown.
Structural properties of undoped and doped cubic GaN grown on SiC(001)
Authors: Martinez-Guerrero E. , Bellet-Amalric E. , Martinet L. , Feuillet G. , Daudin B. , Mariette H. , Holliger P. , Dubois C. , Bru-Chevallier C. , Nze P. Aboughe , Chassagne T. , Ferro G. , Monteil Y.
Published: July 2002

Transmission electron microscopy and x-ray diffraction measurements reveal the presence of stacking faults (SFs) in undoped cubic GaN thin layers. We demonstrate the importance of the defects in the interfacial region of the films by showing that the SFs act as nucleation sites for precipitates of residual impurities such as C and Si present in the GaN layers grown on SiC(001) substrates. We used the imaging secondary ion mass spectroscopy technique to locate these impurities. The systematic decrease of the SF density as a function of the layer thickness is explained by an annihilation mechanism. Finally, the effects of usual dopants on the structural properties of GaN layers are discussed. It is shown that Mg has a tendency to incorporate out of the Ga site by forming Mg precipitates for a concentration higher than 1019 cm−3 in contrast with the results found for heavily Si doped layers.
Integrated platform for silicon photonic crystal devices at near-infrared wavelengths
Authors: Chen Lu , Suzuki Y. , Kohnke Glenn E.
Published: July 2002

We demonstrate an integrated fiber-photonic crystal system with a uniform band gap of around 1550 nm with insertion loss as low as 3.5 dB. One-dimensional photonic crystals fabricated on this platform exhibit a higher-order photonic band gap with over 30 dB suppression of transmission from 1400 to 1700 nm that is consistent with simulation. These results may lead to true integration of photonic crystals into optical fiber communication systems as optical switches and routers.
Growth and properties of Cu3N films and Cu3N/γ′-Fe4N bilayers
Authors: Borsa D. M. , Grachev S. , Presura C. , Boerma D. O.
Published: July 2002

Copper nitride films were grown by molecular-beam epitaxy of copper in the presence of nitrogen from a radio-frequency atomic source on (001) γ′-Fe4N/(001)MgO or directly on MgO substrates. The structural properties of the Cu3N films were found to be very dependent on the substrate and on the deposition temperature. At optimal growth conditions, the Cu3N films grow epitaxial on both substrates. The Cu3N films grown on MgO were characterized optically to be insulators with an energy gap of 1.65 eV. On γ′-Fe4N, Cu3N films with a thickness of only 6 nm, were grown as closed layers, epitaxial and rather smooth (root-mean-square roughness of 0.7 nm). This material has ideal properties to be used as a barrier in low resistance magnetic tunnel junctions.
Bandtail characteristics in InN thin films
Authors: Shen W. Z. , Jiang L. F. , Yang H. F. , Meng F. Y. , Ogawa H. , Guo Q. X.
Published: July 2002

The Urbach bandtail characteristics in InN thin films grown by radio-frequency magnetron sputtering on sapphire (0001) substrates have been investigated both theoretically and experimentally. The bandtail parameter in InN thin films has been obtained by temperature-dependent transmission spectra, with the aid of a detailed calculation of the transmission profile. A bandtail model based on the calculation of density of occupied states and the carrier–phonon interaction has been employed to analyze the temperature-dependent bandtail characteristics. The bandtail parameter is in the range of 90–120 meV in the InN thin film. It is found that the carrier–phonon interaction in InN is weak and the structural disorder contribution (∼90 meV) dominates over the interactive terms. The high structural disorder in InN thin films may relate to the high nonradiative recombination centers.
Hot electrons in group-III nitrides at moderate electric fields
Authors: Barry E. A. , Kim K. W. , Kochelap V. A.
Published: July 2002

By the use of the Monte Carlo method, we studied the distribution function and the basic characteristics of hot electrons in InN, GaN, and AlN under moderate electric fields. We found that in relatively low fields (of the order of kV/cm) the optical phonon emission dominates in the electron kinetics. This strongly inelastic process gives rise to a spindle-shaped distribution function and an extended portion of a quasisaturation of the current–voltage (I–V) characteristics (the streaming-like regime). Formation of this regime is induced by a suppression of the electron spreading over the momenta perpendicular to the electric field. We prove that this is a universal character of the hot electron behavior for all three nitrides. The effects can be detected by the measurement of the I–V characteristics, or the thermopower of hot electrons in the transverse direction.
Reduction of reflection losses in ZnGeP2 using motheye antireflection surface relief structures
Authors: Aydin C. , Zaslavsky A. , Sonek G. J. , Goldstein J.
Published: July 2002

We report the reduction of surface reflection losses in zinc germanium phosphide (ZnGeP2, or ZGP) crystals by fabricating an antireflection (AR) structure in the substrate itself using subwavelength motheye surface patterns. The motheye AR patterning works by creating a region of gradually varying effective refractive index between air and the ternary nonlinear crystal. Motheye structures were created using interference lithography and reactive-ion etching in a SiCl4 plasma. The ZGP crystal with motheye patterning on the output surface reached a transmittance of ∼67% at a cutoff wavelength of 3.8 μm (close to the theoretical maximum of 73%), with negligible surface contamination from the motheye etching process. The motheye patterning technique could be applied to other nonlinear crystals where surface reflection losses are a concern.
Effects of coercive voltage and charge injection on memory windows of metal-ferroelectric-semiconductor and metal-ferroelectric-insulator-semiconductor gate structures
Authors: Lee Sung Kyun , Kim Yong Tae , Kim Seong-Il , Lee Cheol Eui
Published: July 2002

We have prepared Pt/SrBi2Ta2O9 (SBT)/Si metal-ferroelectric-semiconductor (MFS) and Pt/SBT/Y2O3/Si metal-ferroelectric-insulator-semiconductor (MFIS) ferroelectric gate structures and investigated the changes in memory window with different thickness of SBT and Y2O3 in the MFS and MFIS. As a result, it is found that the memory window increases with increasing thickness of SBT and decreasing thickness of Y2O3. The experimental and theoretical analysis reveals that the memory window equals to the difference between the effective coercive voltage (2Vc) applied to the ferroelectric film and the flat band voltage shift due to charge injection (Vci). Increasing the thickness of SBT, the 2Vc seems to be saturated at higher voltage, whereas the Vci starts to increase exponentially at the higher gate voltage. In contrast, the Vci decreases with decreasing thickness of Y2O3, resulting in the enhancement of the memory window due to the reduction of charge injection.
Reaction dynamics of F+HD→HF+D at low energies: Resonant tunneling mechanism
Authors: Lee Shih-Huang , Dong Feng , Liu Kopin
Published: July 2002

The complete state-resolved differential cross section σ(v′,j′,θ;Ec), investigated in a crossed-beam scattering study, is presented for the title reaction at six initial collision energies (Ec) which are below or near the barrier energy. At low energies, all reactive flux is gated through a trapped resonance state via a tunneling process. Hence, it serves as a benchmark system for better understanding the reactive resonance phenomenon. In addition to highlighting various resonance fingerprints of experimental observable, the concept of resonant tunneling reaction mechanism is elucidated. Particular emphasis is placed on its distinction from the more conventional transition-state reaction mechanism.
Growth mechanism of cavities in MeV helium implanted silicon
Authors: Grisolia J. , Claverie A. , Assayag G. Ben , Godey S. , Ntsoenzok E. , Labhom F. , Van Veen A.
Published: July 2002

A study of silicon implanted with 1.55 MeV helium 3 and thermally annealed to generate a subsurface cavity region was performed using neutron depth profiling and transmission electron microscopy (TEM). Results show that about 30% of the initial implanted helium is still present in cavities even after a 900 °C-1 h anneal. In addition, TEM measurement of cavity size on anneal temperature yields an activation energy of 1.65 eV for the growth of cavities. This value is very close to the activation energy (1.7 eV) reported for helium diffusion in silicon. Cavity growth hence results essentially from exchange of helium atoms between cavities.
Chemical, electrical, and structural properties of Ni/Au contacts on chemical vapor cleaned p-type GaN
Authors: Hartlieb P. J. , Roskowski A. , Davis R. F. , Nemanich R. J.
Published: July 2002

Chemical vapor cleaned, Mg-doped, p-type GaN(0001) surfaces and Ni/Au contacts deposited on these surfaces have been studied using several characterization techniques. Stoichiometric surfaces without detectable carbon and an 87% reduction in the surface oxygen to 2±1 at. % were achieved. The binding energies of the Ga 3d and N 1s core level photoelectron peaks were reduced by 0.5±0.1 eV following the chemical vapor clean. The band bending at the clean surface was measured to be 0.8±0.1 eV. As-deposited Ni/Au contacts on chemical vapor cleaned surfaces exhibited significantly less rectification in the low voltage region (<2 V) compared to identical contact structures on conventional HCl treated surfaces. The specific contact resistance of these contacts deposited on chemical vapor cleaned surfaces and subsequently annealed at 450 °C for 30 seconds was 3±2 Ω cm2. Improved ohmic behavior and a specific contact resistance of 4±2 Ω cm2 was obtained for contacts deposited on HCl treated surfaces and annealed using the same schedule. The formation of Au:Ga and Au:Ni solid solutions was observed for contacts on HCl treated surfaces following the 450 °C anneal. There were significantly less interfacial reactions for annealed contacts on chemical vapor cleaned surfaces. The values of specific contact resistance, sheet resistance, and transfer length of the annealed contacts deposited on both chemical vapor cleaned and HCl treated surfaces and measured from room temperature to 140 °C did not change during three successive thermal cycles within this range.
Enhanced thermoelectric cooling at cold junction interfaces
Authors: Ghoshal U. , Ghoshal S. , McDowell C. , Shi L. , Cordes S. , Farinelli M.
Published: July 2002

We describe a thermoelectric device structure that confines the thermal gradients and electric fields at the boundaries of the cold end, and exploits the reduction of thermal conductivity at the interfaces and the poor electron-phonon coupling at the junctions. The measured temperature–current and voltage–current characteristics of a prototype cold point-contact thermoelectric cooler based on a p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.9Se0.1 material system indicate an enhanced thermoelectric figure-of-merit ZT in the range of 1.4–1.7 at room temperature.
Perpendicular nonlinear waves in an electron–positron–ion plasma
Authors: Hasegawa Hiroki , Irie Shinsuke , Usami Shunsuke , Ohsawa Yukiharu
Published: July 2002

Waves propagating perpendicular to a magnetic field in a plasma consisting of electrons, positrons, and ions are studied theoretically and numerically. In a three component plasma, there appears a frequency domain in which the magnetosonic waves cannot propagate; thus, we have two separate modes below the electron cyclotron frequency. Their dispersion relations are discussed. Then, Korteweg–de Vries equations are derived for these modes. A solitary wave of the low-frequency mode has a soliton width 1–103 times as large as the electron skin depth and has an electric potential 1–102 times as large as that in an electron–ion plasma; both of them increase with decreasing ion density. A solitary wave of the high-frequency mode has a soliton width of the order of the electron skin depth and has negligibly small electric potential. Three-fluid simulations show that the low-frequency mode solitary pulse can emit high-frequency mode solitons, if the amplitude of the original pulse is large and the ion density is low.
Effect of Ga implantation on the magnetic properties of permalloy thin films
Authors: Ozkaya L D. , Langford R. M. , Chan W. L. , Petford-Long A. K.
Published: July 2002

Several regions of a 30 nm thick Ni80Fe20 (permalloy) thin film have been implanted using a focused ion-beam system with varying doses of Ga ions. Changes to their magnetization reversal have been investigated by Lorentz microscopy. Implantation with a dose of 1015 Ga+ cm−2 and 1016 Ga+ cm−2 has increased coercivity of the implanted region while no difference has been observed with a dose of 1014 Ga+ cm−2. Domain wall pinning was shown to be a major mechanism for the increase in coercivity. Also changes to the lattice parameter, composition, thickness and grain size in relation to magnetic properties were investigated using various transmission electron microscopy techniques. The lattice parameter change within the implanted region indicates that the stress can be a major contributor to the increase in coercivity. Relative contributions of changes in other factors such as Ni/Fe ratio, thickness, and grain size within the implanted area have also been discussed.
Time-resolved study of SrTiO3 homoepitaxial pulsed-laser deposition using surface x-ray diffraction
Authors: Eres G. , Tischler J. Z. , Yoon M. , Larson B. C. , Rouleau C. M. , Lowndes D. H. , Zschack P.
Published: July 2002

Homoepitaxy of SrTiO3 by pulsed-laser deposition has been studied using in situ time-resolved surface x-ray diffraction in the temperature range of 310 °C to 780 °C. Using a two-detector configuration, surface x-ray diffraction intensities were monitored simultaneously at the (0 0 12) specular and the (0 1 12) off-specular truncation rod positions. Abrupt intensity changes in both the specular and off-specular rods after laser pulses indicated prompt crystallization into SrTiO3 layers followed by slower intra- and interlayer surface rearrangements on time scales of seconds. Specular rod intensity oscillations indicated layer-by-layer growth, while off-specular rod intensity measurements suggested the presence of transient in-plane lattice distortions for depositions above 600 °C.
Large low-field magnetoresistance and TC in polycrystalline (Ba0.8Sr0.2)2−xLaxFeMoO6 double perovskites
Authors: Serrate D. , De Teresa J. M. , Blasco J. , Ibarra M. R. , Morellón L. , Ritter C.
Published: July 2002

Large low-field magnetoresistance (LFMR) together with high Curie temperatures (TC) are requirements for some applications in magnetoelectronics. In order to optimize both parameters, we have investigated double perovskites (Ba0.8Sr0.2)2−xLaxFeMoO6 (0⩽x⩽0.4). High-temperature neutron diffraction measurements indicate a strong increase in TC with La doping (from ≈345 K for x=0 to ≈405 K for x=0.4). The LFMR is very large for x=0 (at 10 KOe≈27% at 10 K and ≈7% at 290 K) and decreases with La doping. This decrease cannot be attributed to a substantial enhancement of Fe/Mo antisite disorder, which is small as tracked by means of x-ray and high-resolution neutron diffraction, but to grain boundaries modifications.
Electrical and physical characteristics of ultrathin hafnium silicate films with polycrystalline silicon and TaN gates
Authors: Gopalan S. , Onishi K. , Nieh R. , Kang C. S. , Choi R. , Cho H.-J. , Krishnan S. , Lee J. C.
Published: July 2002

Metal-oxide-semiconductor transistors of ultrathin hafnium silicate films (equivalent oxide thickness (EOT) of 12.5–14 Å) with polycrystalline silicon and metal (TaN) gates have been demonstrated. Well-behaved transistor characteristics and EOT stability of Hf silicate with n+ polysilicon indicates good compatibility with polysilicon gate process without use of barrier layer. Transmission electron microscopy analysis indicates that the films have no top interfacial layer with both TaN and polysilicon gates. The films also remain amorphous and show no indication of phase separation even after a 950 °C dopant activation anneal. Hf silicate films also show excellent transistor characteristics with TaN gate. NH3 pretreatment results in degraded transistor characteristics for TaN and poly gate samples. Good capacitance–voltage characteristics and negligible hysteresis (<10 mV) was observed in the capacitors after a 1000 °C activation indicating good electrical stability at high temperatures and minimal charge trapping.