bluuebunny/crossref_metadata_2025 huggingface dataset:
On the use of the thermal wave resonator cavity sensor for monitoring hydrocarbon vapors
Authors: Lima J. A. P. , Marı́n E. , da Silva M. G. , Sthel M. S. , Cardoso S. L. , Takeuti D. F. , Gatts C. , Vargas H. , Rezende C. E. , Miranda L. C. M.
Published: July 2002

A gas sensing device based on a thermal wave resonator cavity is outlined. It is experimentally tested by monitoring the presence of several hydrocarbon vapors in air via the measurement of the thermal diffusivity. It is also shown that its time-dependent response may be used to follow the vapor diffusion. It is shown that its characteristic response time is linearly correlated to the thermal diffusivity value of the mixture. The steps toward the development of a practical sensing device are further discussed.
Kinetics and nucleation model of the C49 to C54 phase transformation in TiSi2 thin films on deep-sub-micron n+ type polycrystalline silicon lines
Authors: Kittl J. A. , Prinslow D. A. , Apte P. P. , Pas M. F.
Published: July 2002

A detailed kinetic study of the C49 to C54 phase transformation in TiSi2 thin films was performed, to obtain the full time, temperature, and linewidth dependence of the fraction transformed during rapid thermal annealing on patterned deep-sub-micron lines. A Johnson–Mehl–Avrami kinetic analysis showed Avrami exponents of 0.8±0.2 for all submicron lines and 1.9±0.2 for a 40 μm side square structure. The activation energy of 3.9 eV was independent of linewidth. Transformation times increased dramatically as linewidth decreased. A kinetic model based on the density of nucleation sites as a function of linewidth and C49 grain size is proposed and shown to fit the data.
Parallel atomic force microscopy using cantilevers with integrated piezoresistive sensors and integrated piezoelectric actuators
Authors: Minne S. C. , Manalis S. R. , Quate C. F.
Published: July 2002

We have fabricated and operated two cantilevers in parallel in a new mode for imaging with the atomic force microscope (AFM). The cantilevers contain both an integrated piezoresistive silicon sensor and an integrated piezoelectric zinc oxide (ZnO) actuator. The integration of sensor and actuator on a single cantilever allows us to simultaneously record two independent AFM images in the constant force mode. The ZnO actuator provides over 4 μm of deflection at low frequencies (dc) and over 30 μm deflection at the first resonant frequency. The piezoresistive element is used to detect the strain and provide the feedback signal for the ZnO actuator.
Electron spin resonance observations of excimer-laser-induced paramagnetic centers in tellurite glasses
Authors: Prohaska J. D. , Li J. , Wang J. S. , Bartram R. H.
Published: July 2002

The ESR observations of three distinct paramagnetic centers in TeO2 glasses are reported. One center is intrinsic to the glass and the other two are induced by KrF-excimer-laser radiation. The intrinsic center with a broad ESR spectrum is tentatively identified as an oxygen-associated hole center. One radiation-induced center fades slowly at room temperature; its proposed structure is that of an electron trapped in a diffuse orbital associated with a modifier ion. The other radiation-induced center is stable at room temperature and corresponds to the VO⋅ center observed in crystalline paratellurite.
Phonons in self-assembled (In,Ga,Al)Sb quantum dots
Authors: Bennett Brian R. , Shanabrook B. V. , Magno R.
Published: July 2002

Quantum dots of InSb, GaSb, and AlSb were grown on GaAs substrates by molecular beam epitaxy and characterized by atomic force microscopy and Raman spectroscopy. There is a clear correlation between the observation of quantum dots by atomic force microscopy and a phonon mode at an energy a few wavenumbers below the longitudinal optic phonon energy for thick (In,Ga,Al)Sb layers. In the case of nominally AlSb quantum dots, a two-mode behavior is observed and attributed to the segregation of Ga into the AlSb during growth.
Open boundary conditions for a time-dependent analysis of the resonant tunneling structure
Authors: Talebian M. A. , Pötz W.
Published: July 2002

Exact boundary conditions for open quantum devices with constant potential asymptotics are presented and implemented into a numerical treatment of the one-dimensional time-dependent Schrödinger equation. A time-dependent analysis of the resonant tunneling double-barrier (RTDB) structure demonstrates their superiority over previous approaches. Our self-consistent study reveals microscopic details of the transient current in the system under an external time-dependent bias. Two time regimes characterize the charge transport in a RTDB in response to an applied bias. The ultrafast response of the system occurs in the contact regions within approximately 100 fs. This response time is of the order of the inverse bulk plasma frequency and is essentially independent of the applied bias. The second time regime is characterized by current oscillations which sensitively depend on the applied bias and structural parameters.
A wideband coherent terahertz spectroscopy system using optical rectification and electro-optic sampling
Authors: Nahata Ajay , Weling Aniruddha S. , Heinz Tony F.
Published: July 2002

We present a scheme for exploiting the nonresonant second-order nonlinearities in electro-optic media to extend the bandwidth of coherent spectroscopy in the far-infrared using ultrafast laser pulses. Using optical rectification and electro-optic sampling in 〈110〉 ZnTe for the generation and coherent detection of freely propagating THz radiation, respectively, we have demonstrated spectral sensitivity beyond 3 THz. This was accomplished by achieving phase matching for both optical rectification and electro-optic sampling over a broad range of THz frequencies.
Coexistence of wurtzite GaN with zinc blende and rocksalt studied by x-ray power diffraction and high-resolution transmission electron microscopy
Authors: Xie Yi , Qian Yitai , Zhang Shuyuan , Wang Wenzhong , Liu Xianming , Zhang Yuheng
Published: July 2002

X-ray powder diffraction and high-resolution transmission electron microscopy (HRTEM) have found coexistence of three structures of GaN: wurtzite, zinc blende, and rocksalt in the samples prepared through benzene thermal process at 280–300 °C under pressure about 5 MPa. Previously, GaN with rocksalt structure only appeared at high pressure more than 37 GPa. HRTEM showed that there was a transitional region between the two cubic phases of GaN with zinc blende and rocksalt. X-ray photoelectron spectra analysis of the sample gave the average composition of Ga0.95N.
Reduction of interlayer coupling in NiFeCo/Cu/NiFeCo/FeMn spin valve structures by lowering argon sputtering pressure
Authors: Park Chang-Min , Shin Kyung Ho
Published: July 2002

Sputtered spin valve heterostructures of NiFeCo/Cu/NiFeCo/FeMn were prepared to examine the effect of the argon sputtering pressure on the interlayer coupling between two ferromagnetic layers. The coupling strength was found to be closely related to the argon sputtering pressure. As the argon sputtering pressure is varied from 6 to 1 mTorr, the coupling field is reduced from 13.6 to 0.5 Oe. In particular, the sample deposited at 1 mTorr exhibits a coupling field of 0.5 Oe and a resistance change of 1.7% for the field step of 0.1 Oe leading to a sensitivity as large as 17% Oe. The reduced interlayer coupling is attributed to the modified topography of NiFeCo/Cu/NiFeCo interfaces. Atomic force microscope analyses support the dominant role of a topography-related effect in reducing the interlayer coupling strength.
Creation of a monoenergetic pulsed positron beam
Authors: Gilbert S. J. , Kurz C. , Greaves R. G. , Surko C. M.
Published: July 2002

We have developed a versatile, pulsed source of cold (ΔE=0.018 eV), low-energy positrons (E≈0–9 eV). Multiple pulses of 105 positrons, each 10 μs in duration, are extracted from a thermalized, room temperature positron plasma stored in a Penning trap. The frequency, duration, and amplitude of the pulses can be varied over a wide range.
Microdischarge devices fabricated in silicon
Authors: Frame J. W. , Wheeler D. J. , DeTemple T. A. , Eden J. G.
Published: July 2002

Cylindrical microdischarge cavities 200–400 μm in diameter and 0.5–5 mm in depth have been fabricated in silicon and operated at room temperature with neon or nitrogen at specific power loadings beyond 10 kW/cm3. The discharges are azimuthally uniform and stable operation at N2 and Ne pressures exceeding 1 atm and ∼600 Torr, respectively, has been realized for 400 μm diameter devices. Spectroscopic measurements on neon discharges demonstrate that the device behaves as a hollow cathode discharge for pressures >50 Torr. As evidenced by emission from Ne and Ne+ (2P,2F) states as well as N2 (C→B) fluorescence (316–492 nm), these discharge devices are intense sources of ultraviolet and visible radiation and are suitable for fabrication as arrays.
Stress overshoot in stress-strain curves of Zr65Al10Ni10Cu15 metallic glass
Authors: Kawamura Yoshihito , Shibata Tsutomu , Inoue Akihisa , Masumoto Tsuyoshi
Published: July 2002

The essential features of the stress overshoot in the stress-strain curves of Zr65Al10Ni10Cu15 (at. %) metallic glass that has a wide supercooled liquid region were revealed. The stress overshoot was dependent on temperature, strain rate, and stress relaxation. During the stretch, a change in strain rate gave rise to stress overshoot or undershoot which was sensitive to the variable quantities in the strain rate.
Dual-wavelength laser emission from a coupled semiconductor microcavity
Authors: Pellandini P. , Stanley R. P. , Houdré R. , Oesterle U. , Ilegems M. , Weisbuch C.
Published: July 2002

We report photopumped operation of a monolithic coupled semiconductor microcavity laser. The structure consists of two λ-sized GaAs vertical cavities, one on top of the other, coupled together through a common mirror. Due to a wedge induced into each cavity, the detuning between the cavities can be continuously varied when moving across the sample. Depending on the detuning, laser action is simultaneously achieved at two different wavelengths or occurs only at one wavelength. At resonance, we observe coupled dual-wavelength laser emission at two widely spaced wavelengths (13 nm) with the same threshold and same dependence on pump power.
Quenching of the 750.4 nm argon actinometry line by H2 and several hydrocarbon molecules
Authors: Francis A. , Czarnetzki U. , Döbele H. F. , Sadeghi N.
Published: July 2002

The quenching rate of the 750.4 nm actinometer line in argon by H2 and several hydrocarbons (CH4, C2H2, C2H4, C2H6) has been determined. Argon atoms at room temperature are excited by two-photon excitation at 184 nm to the 4p′[1/2]0 state, and the decay time of the fluorescence emission at 750.4 nm is measured. The quenching rates are inferred from the change of the decay time with the pressure of the quenching gas. For hydrocarbons, the quenching rates are of the order of 10−9 cm3 s−1. The radiative lifetime of the 4p′[1/2]0 is found to be 24 ns.
Mechanism of unidirectional liquid-crystal alignment on polyimides with linearly polarized ultraviolet light exposure
Authors: Nishikawa M. , Taheri B. , West J. L.
Published: July 2002

Unidirectional liquid-crystal (LC) alignment by a linearly polarized ultraviolet light (UV) exposure was examined using two types of polyimide (PI). PI with a fluorene unit incorporated in a side chain showed LC alignment perpendicular to those of conventional PIs, i.e., perpendicular to the rubbing direction and parallel to the exposure polarization of UV. The results of the dichroic ratio measurement of LC cells, UV absorption spectra, birefringence, infrared absorption spectra, and PI conformation calculations using molecular mechanics suggest that the photodecomposition of PI by UV exposure produces anisotropic van der Waals forces, which align LC along its optical axis.
Intermittent contact scanning force microscopy: The role of the liquid necks
Authors: Luna M. , Colchero J. , Baró A. M.
Published: July 2002

We have studied the intermittent contact mode for small oscillation amplitudes and soft cantilevers with sharp tips. For appropriate working conditions the tip does not touch the mica surface. Instead, a high dissipative tip–sample interaction takes place which reduces the oscillation amplitude of the cantilever. Our experiments show that this dissipative interaction is related to the relative humidity and we believe that it is induced by a nanometer sized liquid neck between tip and sample. A phase contrast image on different surface materials is then due mainly to the wetting properties of the corresponding material rather than to their elastic or viscoelastic properties. The intermittent contact mode described in this work is especially advantageous since the tip is extremely gentle with the surface.
Combinatorial synthesis and evaluation of epitaxial ferroelectric device libraries
Authors: Takeuchi I. , Chang H. , Gao C. , Schultz P. G. , Xiang X.-D. , Sharma R. P. , Downes M. J. , Venkatesan T.
Published: July 2002

Combinatorial libraries of parallel-plate capacitors, consisting of Pt and La0.5Sr0.5CoO3 electrodes and a doped BaxSr1−xTiO3 dielectric layer, have been fabricated and analyzed to systematically study the effects of dopants on device performance. Epitaxial heterostructure libraries with sharp interfaces were generated from amorphous layers on LaAlO3 substrates. Two hundred and forty different host/dopant combinations were synthesized on a 1/2 in. by 1/2 in. substrate, with 23 capacitors for each combination. Addition of 1.5 mol % W was found to increase the figure of merit (ε/Ileak) 220-fold and reduce the high-frequency (MHz and GHz) loss tangent by fourfold.
Near-field optical imaging of light propagation in semiconductor waveguide structures
Authors: Bourzeix S. , Moison J. M. , Mignard F. , Barthe F. , Boccara A. C. , Licoppe C. , Mersali B. , Allovon M. , Bruno A.
Published: July 2002

We have investigated light propagation in optical devices by near-field scanning optical microscopy (NSOM) at the telecommunication wavelength of 1.55 μm. NSOM images obtained on the top of channel waveguides measure the mode profile perpendicular to the propagation direction and show a modulation of intensity along this direction. This modulation demonstrates the periodic variation of the mode size predicted for the propagation in small guides and marks the direction of propagation. We show that NSOM analysis can completely assess complex optical devices with subwavelength resolution: determination of the optical path, variation of the light intensity along this path, and measurement of local radiative losses.
Ferroelectric properties of c-oriented YMnO3 films deposited on Si substrates
Authors: Yoshimura Takeshi , Fujimura Norifumi , Ito Taichiro
Published: July 2002

We have proposed the use of RMnO3 (R: rare earth elements) films for metal–ferroelectric–semiconductor field effect transistor (MFSFET)-type ferroelectric random access memories (ferroelectric RAMs). This reports the production of YMnO3 films on Si substrates for MFSFET with confirmation of the distinct ferroelectricity by P–E hysteresis and capacitance–voltage (C–V) measurement. (0001)-oriented YMnO3 films were obtained on a (111)Si substrate using a pulsed-laser deposition method. Although the Pt/YMnO3/Si structure exhibits a very small remnant polarization of 1.2 nC/cm2, it has clear ferroelectric polarization switching type C–V characteristics with a memory window of 1.1 V. The dielectric constant and the dissipation factor were 27.8 and 0.035, respectively. The polarization switching characteristics are discussed.
Fluorescence lifetime three-dimensional microscopy with picosecond precision using a multifocal multiphoton microscope
Authors: Straub M. , Hell S. W.
Published: July 2002

The combination of pulsed-mode excitation multifocal multiphoton microscopy with a high-repetition, time-gated intensified CCD camera enables efficient three-dimensional (3D) fluorescence lifetime imaging. With a 200-ps gate opening at 76 MHz repetition rate, fluorescence decay can be traced in a sequence of images with varying delays between pulse and gate. Fluorophore lifetimes are measured with a precision of a few picoseconds. As an application we show that, upon two-photon excitation at 800 nm, certain pollen samples feature a multiexponential fluorescence relaxation. Our results indicate that efficient four-dimensional microscopy with hundreds of nanometer spatial and tens of picoseconds temporal resolution is within reach.
High-quality germanium photodiodes integrated on silicon substrates using optimized relaxed graded buffers
Authors: Samavedam S. B. , Currie M. T. , Langdo T. A. , Fitzgerald E. A.
Published: July 2002

The integration of Ge photodetectors on silicon substrates is advantageous for various Si-based optoelectronics applications. We have fabricated integrated Ge photodiodes on a graded optimized relaxed SiGe buffer on Si. The dark current in the Ge mesa diodes, Js=0.15 mA/cm2, is close to the theoretical reverse saturation current and is a record low for Ge diodes integrated on Si substrates. Capacitance measurements indicate that the detectors are capable of operating at high frequencies (2.35 GHz). The photodiodes exhibit an external quantum efficiency of η=12.6% at λ=1.3 μm laser excitation in the photodiodes. The improvement in Ge materials quality and photodiode performance is derived from an optimized relaxed buffer process that includes a chemical mechanical polishing step within the dislocated epitaxial structure.
Maskless photolithography: Embossed photoresist as its own optical element
Authors: Paul Kateri E. , Breen Tricia L. , Aizenberg Joanna , Whitesides George M.
Published: July 2002

This letter demonstrates that features embossed on the surface of a layer of photoresist can direct UV light in the photoresist layer. These topographical features act as optical elements: they focus/disperse and phase shift incident light in the optical near field, inside the resist layer. A number of different surface topographies have been examined, which give 50–250 nm features after exposure and development. This method gives patterns of complex features over large areas, in a parallel process, that can then be transferred into silicon or metal. It provides a method for controlling the intensity of light inside a thin film of photoresist.
Piezoelectric doping and elastic strain relaxation in AlGaN–GaN heterostructure field effect transistors
Authors: Bykhovski A. D. , Gaska R. , Shur M. S.
Published: July 2002

We calculate the sheet electron density induced by the piezoelectric effect in AlxGa1−xN–GaN heterostructure field effect transistors. This density is limited by the elastic strain relaxation, which depends on AlGaN barrier layer thickness and on the Al molar fraction in the barrier layer. Piezoelectric doping is more important in structures with larger Al content and thinner barrier layers. These results agree with our experimental data.
A low-loss composition region identified from a thin-film composition spread of (Ba1−x−ySrxCay)TiO3
Authors: Chang H. , Takeuchi I. , Xiang X.-D.
Published: July 2002

We have generated the thin-film ternary composition spread of (Ba1−x−ySrxCay)TiO3 on an equilateral-triangle-shaped LaAlO3 substrate. Compositional variation within the triangle was achieved by a precisely controlled shutter system inside a pulsed laser deposition chamber, which allows the deposition of precursors with gradient thickness over the length of the substrate. Appropriate postannealing afforded high-quality epitaxial thin films over almost the entire composition region. Mapping of the microwave dielectric properties of the composition-spread chip was performed using a scanning evanescent microwave microscope at 1 GHz. Composition region Ba0.12–0.25Sr0.35–0.47Ca0.32–0.53TiO3 was found to have desirable properties for electronic applications.
Measurement of the electron energy distribution function in an argon radio-frequency discharge in the γ mode
Authors: Deegan C. M. , Goss J. P. , Vender D. , Hopkins M. B.
Published: July 2002

The α–γ transition in an argon radio-frequency discharge has been measured using Langmuir probes and a current–voltage monitor. At two pressures, bi-Maxwellian electron energy distribution functions were measured in the γ mode. Increasing the gas pressure results in a decrease in the high-energy component of the electron energy distribution function. A transition region in the current–voltage characteristic is also observed.
Effect of arsenic doping on {311} defect dissolution in silicon
Authors: Brindos Richard , Keys Patrick , Jones Kevin S. , Law Mark E.
Published: July 2002

Si + ions were implanted into silicon wafers with background concentrations of arsenic ranging from 1×1017 to 3×1019 cm−3 to study the interaction between arsenic atoms and excess self-interstitials. Samples were then annealed at 750 °C for a range of times between 15 and 60 min to nucleate and dissolve {311} defects. The concentration of trapped interstitials in these defects was measured using quantitative plan-view transmission electron microscopy. It is shown that, as the arsenic concentration increases, there is a reduction in the number and size of the {311} defects. This decrease in the {311} defect density with increasing arsenic well concentration is believed to be the result of interstitial trapping by the arsenic. Upon annealing, the trapped interstitial concentration in the {311} defects decreases as the defects dissolve. The time constant for the dissolution was calculated to be 33±5 min at 750 °C, and was independent of background concentration. This suggests that the arsenic traps some of the interstitials, and these traps are sufficiently stable that they do not affect the subsequent {311} dissolution at 750 °C.
Spatially distributed p-n heterojunction based on nanoporous TiO2 and CuSCN
Authors: Rost C. , Sieber I. , Siebentritt S. , Lux-Steiner M. C. , Könenkamp R.
Published: July 2002

Directed semiconductor growth in nanoporous ceramic films is reported. A p-n heterojunction with an interface that is spatially distributed across the complete thickness of the ceramic film is established. The interface area is estimated to be several 100 times larger than its geometric projection. The p-n junction shows excellent rectification and may serve as the basic building block for photovoltaic devices.
Widely tunable continuous-wave THz laser
Authors: Gousev Yu. P. , Altukhov I. V. , Korolev K. A. , Sinis V. P. , Kagan M. S. , Haller E. E. , Odnoblyudov M. A. , Yassievich I. N. , Chao K.-A.
Published: July 2002

We present experimental results on continuous-wave generation of THz radiation by strained Ge and a theoretical model for population inversion of carriers giving rise to the stimulated THz emission. Resonant acceptor states induced by strain and resonance hole scattering under applied electric field are necessary for the inversion.
Cooling of a semiconductor by luminescence up-conversion
Authors: Finkeißen E. , Potemski M. , Wyder P. , Viña L. , Weimann G.
Published: July 2002

We report the observation of phonon-mediated up-conversion of luminescence in a GaAs quantum well. This opens the possibility of light-induced lattice cooling in a semiconductor. Under appropriate conditions, pumping the sample with light at the energy of the heavy-hole exciton, we observe light-hole exciton emission, which lies ∼10 meV above the excitation energy. The use of an external magnetic field together with the resolution of excited excitonic states provides an internal thermometer to monitor the sample temperature. Temperature drops as large as 10% of the initial temperature are observed for pump densities of 4 W cm−2.
Characterization of silicon/oxide/nitride layers by x-ray photoelectron spectroscopy
Authors: Hansch Walter , Nakajima Anri , Yokoyama Shin
Published: July 2002

Core-level intensities for Si 2p, Si 2s, O 1s, and N 1s were measured by x-ray photoelectron spectroscopy in bulk samples of silicon, SiO2 and Si3N4. A complete and consistent set of intensity ratios is given and applied for calculations of thickness and stoichiometry in thin Si/oxide/nitride layers, which can be used for gate dielectrics in advanced metal–oxide–semiconductor field-effect transistor fabrication.
Stress measurements using ultraviolet micro-Raman spectroscopy
Authors: Dombrowski K. F. , De Wolf I. , Dietrich B.
Published: July 2002

We present measurements of mechanical stress in silicon device structures by ultraviolet (UV) micro-Raman spectroscopy. The shorter wavelength of the UV light (364 nm) is the basis for two major improvements over conventionally used blue light (458 nm): The smaller penetration depth of only 15 nm (vs 300 nm for blue light) probes the stress very close to the surface, and a smaller laser spot on the sample (0.7 μm vs 0.9 μm) results in higher spatial resolution. A comparison of stress patterns obtained in the same sample with 364 nm (UV) and 458 nm (blue) light demonstrates that areas of high stress, which are averaged out by longer wavelength light, can be detected with UV light.
The environmental stability of field emission from single-walled carbon nanotubes
Authors: Dean Kenneth A. , Chalamala Babu R.
Published: July 2002

We demonstrate long-term field emission stability from single-walled carbon nanotubes. Unballasted nanotubes operate without degradation for over 350 h at 10−9 Torr. Nanotubes are shown to be significantly less sensitive to operating environments than metallic emitters. In 10−7 Torr of H2O, we demonstrate 100 h of continuous bias field emission with no current degradation. Protrusion growth and current runaway, typical problems for unballasted metal emitters, are not observed with nanotubes. Single-walled nanotubes do show susceptibility to damage by oxidation. We suggest that the exceptional environmental stability of carbon nanotubes is due to a combination of geometry, strong carbon bonding, and the lack of protrusion growth.
Evidence of aluminum silicate formation during chemical vapor deposition of amorphous Al2O3 thin films on Si(100)
Authors: Klein T. M. , Niu D. , Epling W. S. , Li W. , Maher D. M. , Hobbs C. C. , Hegde R. I. , Baumvol I. J. R. , Parsons G. N.
Published: July 2002

Using narrow nuclear reaction resonance profiling, aluminum profiles are obtained in ∼3.5 nm Al2O3 films deposited by low temperature (<400 °C) chemical vapor deposition on Si(100). Narrow nuclear resonance and Auger depth profiles show similar Al profiles for thicker (∼18 nm) films. The Al profile obtained on the thin film is consistent with a thin aluminum silicate layer, consisting of Al–O–Si bond units, between the silicon and Al2O3 layer. Transmission electron microscopy shows evidence for a two-layer structure in Si/Al2O3/Al stacks, and x-ray photoelectron spectroscopy shows a peak in the Si 2p region near 102 eV, consistent with Al–O–Si units. The silicate layer is speculated to result from reactions between silicon and hydroxyl groups formed on the surface during oxidation of the adsorbed precursor.
Electro-optic measurements of the ferroelectric-paraelectric boundary in Ba1−xSrxTiO3 materials chips
Authors: Li Jingwei , Duewer Fred , Gao Chen , Chang Hauyee , Xiang X.-D. , Lu Yalin
Published: July 2002

The combinatorial material chip strategy is used to study the ferroelectric-paraelectric phase boundary of the Ba1−xSrxTiO3 thin film system. The electro-optic (EO) effect at different compositions is measured using a modified direct-current/alternating-current birefringence EO measurement technique. We find that Ba1−xSrxTiO3 thin films exhibit relaxor like behavior with diffused ferroelectric domains existing well past the previously defined ferroelectric-paraelectric boundary (x>0.3).
Single-mode tunable, pulsed, and continuous wave quantum-cascade distributed feedback lasers at λ≅4.6–4.7 μm
Authors: Köhler Rüdeger , Gmachl Claire , Tredicucci Alessandro , Capasso Federico , Sivco Deborah L. , Chu S. N. George , Cho Alfred Y.
Published: July 2002

Single-mode tunable quantum-cascade distributed feedback lasers emitting at 4.6–4.7 μm wavelength are reported. The lasers employ strained heterostructure material with global strain compensation to provide the large band offset needed for high-performance short wavelength operation. Pulsed, continuously tunable single-mode emission is achieved from 90 to 300 K with a tuning range of 65 nm. Peak output power levels of 100 mW at room temperature are obtained. In continuous-wave operation, current tunable single-mode emission is demonstrated around liquid-nitrogen temperature with a tuning range of 20 nm (over a current range of 450 mA). The maximum output power in continuous wave at 80 K is 150 mW.
Highly conductive diamond probes for scanning spreading resistance microscopy
Authors: Hantschel T. , Niedermann P. , Trenkler T. , Vandervorst W.
Published: July 2002

Scanning spreading resistance microscopy (SSRM) is a powerful method for the characterization of Si semiconductor devices based on atomic force microscopy (AFM). It requires conductive probe tips made of doped diamond. Although various solid diamond probes have been fabricated, they could not satisfy the requirements for SSRM. Therefore, we have developed a SSRM probe composed of a pyramidal diamond tip attached to a Si cantilever. This letter describes the probe fabrication process briefly and presents excellent SSRM measurements obtained on Si calibration samples. Solid diamond tips integrated in Si cantilevers were used for SSRM showing a significantly higher dynamic range than the conductive probes known to date.
Temperature dependence of the band gap in InAsyP1−y
Authors: Wada M. , Araki S. , Kudou T. , Umezawa T. , Nakajima S. , Ueda T.
Published: July 2002

The temperature dependence of the band gap in InAsyP1−y (y=0–0.67) has been determined by photoluminescence, x-ray diffraction, and absorption spectra measurements. We found that the measured data within the temperature range of 77–300 K can be expressed by the equation proposed by O’Donnell and Chen. The band gap at 77 K is given by Eg=1.407−1.073y+0.089y2, while the compositional dependence of the band gap observed at 300 K, agrees with the values previously reported. We confirmed that changes in temperature caused a slight change in the bowing parameters, and hence found that the band gap temperature dependence of InAsyP1−y (y=0–1) varies very little with changes in composition (2.5–3.5×10−4 eV/K).
Trapping of quasiparticles of a nonequilibrium superconductor
Authors: Pekola J. P. , Anghel D. V. , Suppula T. I. , Suoknuuti J. K. , Manninen A. J. , Manninen M.
Published: July 2002

We have performed experiments where hot electrons are extracted from a normal metal into a superconductor through a tunnel junction. We have measured the cooling performance of such NIS junctions, especially in the cases where another normal metal electrode, a quasiparticle trap, is attached to the superconductor at different distances from the junction in direct metal–to–metal contact or through an oxide barrier. The direct contact at a submicron distance allows superior thermalization of the superconductor. We have analyzed theoretically the heat transport in this system. From both experiment and theory, it appears that NIS junctions can be used as refrigerators at low temperatures only with quasiparticle traps attached.
Various bonding configurations of transition-metal atoms on carbon nanotubes: Their effect on contact resistance
Authors: Andriotis Antonis N. , Menon Madhu , Froudakis George E.
Published: July 2002

Our investigations reveal that the bonding of the transition-metal atoms on a single-wall carbon nanotube (SWCN) depends on the detailed contact conditions. On the basis of our results, we suggest that the early 3-d elements (Sc, Ti, and V) can be expected to be good candidates for making metal–SWCN contacts of low resistance, while contacts employing the late 3-d elements (Fe, Co, and Ni) and Cu are expected to exhibit large contact resistance.
Microstructure and its effect on field electron emission of grain-size-controlled nanocrystalline diamond films
Authors: Wu Kehui , Wang E. G. , Cao Z. X. , Wang Z. L. , Jiang X.
Published: July 2002

Nanocrystalline diamond films were grown by microwave plasma assisted chemical vapor deposition using N2 and CH4 as precursors. The microstructure of the films such as the diamond grain size, graphite content, and N incorporation, was controlled by introducing a small amount of hydrogen gas (0–10 sccm) in the growth. Effects of the growth parameters on the film microstructure were investigated using transmission electron microscopy, x-ray diffraction, Raman spectroscopy, and secondary ion mass spectroscopy. A surface stabilizing model is suggested to explain the formation mechanism of the uniformly grain size-controlled nanocrystalline diamond. A systematic investigation on the film microstructure and their field electron emission (FEE) property is presented for various films of different diamond grain sizes and graphite contents. It was found that the FEE property highly depended on the diamond/graphite mixed phase structure. Novel field emission properties (1 V/mum emission threshold and 10 mA/cm2 emission current) are obtained by optimizing the growth parameters. A transport-tunneling mechanism is applied to explain the experimental observations. Our results showed that nanocrystalline diamond film can be a very promising cold cathode material for field emission applications.
Microstructure of sputter-deposited Co/Si multilayer thin films
Authors: Fallon J. M. , Faunce C. A. , Grundy P. J.
Published: July 2002

This article presents the results of an investigation by transmission electron microscopy, electron diffraction, x-ray diffraction, and x-ray reflectivity of sputter-deposited Co/Si multilayers. Structures with individual layer thicknesses above about 5 nm retain elemental regions and also some interfacial mixing, but below this thickness very significant intermixing occurs with alloy formation through interdiffusion. Changes in composition in these interfacial regions reveal themselves as a change in atomic arrangement from crystalline to amorphous phases. This can be induced either by increasing the Si or reducing the Co layer thickness, resulting in alloy formation over a limited distance with a variable average composition. A simple model is proposed to explain the observed microstructural changes.
Cold neutron microprobe for materials analysis using tapered capillary optics
Authors: Sharov V. A. , Xiao Q.-F. , Ponomarev I. Yu. , Mildner D. F. R. , Chen-Mayer H. H.
Published: July 2002

A prototype monolithic capillary lens for focusing neutrons produced by thermally drawing straight multicapillary bundles has been characterized with cold neutrons, and gives an intensity gain of a factor of 25 at a focal distance of 8 mm, over the focal spot area of width 87 μm. This is over an order of magnitude smaller in area than for the multifiber capillary lens. The spatial resolution available with the lens has been tested with prompt gamma measurements on slivers of dysprosium. Background problems that can affect the spatial resolution of measurements taken at the focal position of the lens are addressed. The boron glass of the tapered monolithic lens provides good shielding from unfocused neutrons in the vicinity of the lens focus.
Renner–Teller induced photodissociation of HCO in the first absorption band: Determination of linewidths for the Ã 2A″ K=0,1 states by filter-diagonalization
Authors: Weiß Jan , Schinke Reinhard , Mandelshtam Vladimir A.
Published: July 2002

We present new calculations on the Renner–Teller induced decay of the vibrational states of HCO(Ã 2A″) using accurate ab initio potential energy surfaces. The dynamics calculations are performed by employing filter diagonalization and an absorbing optical potential in the exit channel. The objective of this investigation is twofold: the completion of earlier time-dependent wave packet calculations by determining resonance widths for all vibrational states for projection quantum number K=1—up to 2.75 eV above the H+CO(re) dissociation threshold—and the determination of the widths for the long-lived K=0 states. In the latter case, a clear-cut J2(J+1)2 dependence, where J is the total angular momentum, is observed indicating that the rate determining step is K-resonance interaction between K=0 and 2 states. The experimentally observed J-independent contribution (0.22–0.5 cm−1), which dominates the linewidth for small values of J, is not accounted for by our calculations. Arguments are put forward, that it is caused by spin–orbit interaction, which is not included in our treatment.
Dielectric properties of lead zirconate titanate thin films deposited on metal foils
Authors: Zou Q. , Ruda H. E. , Yacobi B. G. , Saegusa K. , Farrell M.
Published: July 2002

Ferroelectric lead zirconate titanate (PZT) thin films deposited on metal foil substrates are suitable for developing a wide range of applications. In this letter, we report on PZT films deposited on a variety of foils, including titanium, stainless steel, brass, and nickel alloys, using sol–gel processing. The dielectric properties were shown to depend strongly on the foil material. Excellent properties for capacitors, including high dielectric constant (ε=400), low dielectric loss (loss tangent of <5%), and low leakage current (below 5×10−8 A at 5 V) were obtained on titanium and good results were also obtained on other foils, such as stainless steel and brass foils. Also, excellent high-frequency properties were observed for capacitors on titanium, stainless steel, and brass foils.
The initial implementation and applications of a general active space coupled cluster method
Authors: Olsen Jeppe
Published: July 2002

A general coupled cluster method that allows arbitrary excitations from a single reference-determinant is proposed and tested. The method is based on a generalization of the formalism of spin-strings and provides a unified method for the storage and manipulation of coupled cluster operators. An initial implementation of the method is discussed and used to study the convergence of the coupled cluster hierarchy for H2O and CH2 at equilibrium geometry, employing up to eightfold excitations. The energy and wave function contributions of the various excitation levels are examined. The dissociation curve of HF is also studied. Using single and double excitations from a minimal active space, the coupled cluster dissociation curve for HF shows a largest deviation from full configuration interaction curve of 1.3 mEh, which decreases by an order of magnitude up on the addition of triple excitations out of the active space.
Ab initio determination of the melting point of aluminum by thermodynamic integration
Authors: Jesson Ben J. , Madden Paul A.
Published: July 2002

The melting point of aluminum has been obtained in an ab initio molecular dynamics calculation by determination of the free energies of the solid and liquid phases as a function of temperature along the zero pressure isobar. The focus of the article is to demonstrate the problems which can arise in obtaining adequately sampled free energies. The time scale on which “adiabatic switching” may be performed to calculate the free energy of the ab initio system relative to a classical reference state is discussed. To provide a consistency check, two reference states for this thermodynamic integration are used, the one component plasma and the Lennard-Jones (LJ) system. These illustrate particular difficulties which can arise. In the LJ case, for example, the intermediate fluid states which arise in integrating from the LJ fluid to the full ab initio description of Al are found to freeze. Ultimately, consistent results are obtained.
Analysis and modeling of AlxGa1−xN-based Schottky barrier photodiodes
Authors: Monroy E. , Calle F. , Pau J. L. , Sánchez F. J. , Muñoz E. , Omnès F. , Beaumont B. , Gibart P.
Published: July 2002

Schottky barrier photovoltaic detectors have been fabricated on n-AlxGa1−xN(0⩽x⩽0.35) and p-GaN epitaxial layers grown on sapphire. Their characteristics have been analyzed and modeled, in order to determine the physical mechanisms that limit their performance. The influence of material properties on device parameters is discussed. Our analysis considers front and back illumination and distinguishes between devices fabricated on ideal high-quality material and state-of-the-art heteroepitaxial AlxGa1−xN. In the former case, low doping levels are advisable to achieve high responsivity and a sharp spectral cutoff. The epitaxial layer should be thin (<0.5 μm) to optimize the ultraviolet/visible contrast. In present devices fabricated on heteroepitaxial AlxGa1−xN, the responsivity is limited by the diffusion length. In this case, thick AlxGa1−xN layers are advisable, because the reduction in the dislocation density results in lower leakage currents, larger diffusion length, and higher responsivity. In order to improve bandwidth and responsivity, and to achieve good ohmic contacts, a moderate n-type doping level (∼1018 cm−3) is recommended.
Electron beam-induced increase of electron diffusion length in p-type GaN and AlGaN/GaN superlattices
Authors: Chernyak Leonid , Osinsky Andrei , Fuflyigin Vladimir , Schubert E. F.
Published: July 2002

The diffusion length, L, of electrons in Mg-doped p-GaN grown by metal-organic chemical vapor deposition was found to increase linearly from 0.55 to 2.0 μm during 1500 s of electron beam irradiation. Similar trends were observed for p-type Mg-doped GaN and AlGaN/GaN superlattices grown by molecular-beam epitaxy. While the electron diffusion length in p-(Al)GaN depends on irradiation time, the diffusion length of holes in n-GaN remains unchanged, with L∼0.35 μm. We attribute the observed diffusion length change in p-(Al)GaN to an increase in the minority carrier lifetime. This increase is likely due to electron beam-induced charging of the deep metastable centers associated with Mg doping. The concentration of these centers was estimated to be ∼1018 cm−3. The minority carrier diffusion length increase in p-(Al)GaN, which occurs during electron injection, may lead to self-improvement of the bipolar transistor characteristics.
Algebraic force-field Hamiltonian expansion approach to linear polyatomic molecules
Authors: Sako Tokuei , Aoki Daisuke , Yamanouchi Kaoru , Iachello Francesco
Published: July 2002

The algebraic force-field Hamiltonian expansion recently proposed for vibrations of bent triatomic molecules [T. Sako, K. Yamanouchi, and F. Iachello, Chem. Phys. Lett. 299, 35 (1999)] is extended to linear polyatomic molecules by introducing operators constructed from the unitary algebra U(3). A comparison with the conventional force-field method is made by fitting the experimental vibrational-level energies of CO2. The results show that the algebraic Hamiltonian in local coordinates reproduces the level-energies with a smaller root-mean-square deviation and much faster convergence.
Reliability of normal-state current–voltage characteristics as an indicator of tunnel-junction barrier quality
Authors: Jönsson-Åkerman B. J. , Escudero R. , Leighton C. , Kim S. , Schuller Ivan K. , Rabson D. A.
Published: July 2002

We demonstrate that one of the most commonly used criteria to ascertain that tunneling is the dominant conduction mechanism in magnetic tunnel junctions—fits of current–voltage (I–V) data—is far from reliable. Using a superconducting electrode and measuring the differential conductance below Tc, we divide samples into junctions with an integral barrier and junctions having metallic shorts through the barrier. Despite the clear difference in barrier quality, equally reasonable fits to the I–V data are obtained above Tc. Our results further suggest that the temperature dependence of the zero-bias resistance is a more solid criterion, which could therefore be used to rule out possible pinholes in the barrier.
Wave-packet dynamics in a cyanine dye molecule excited with femtosecond chirped pulses
Authors: Misawa Kazuhiko , Kobayashi Takayoshi
Published: July 2002

Femtosecond vibrational wave-packet dynamics in a cyanine molecule is observed to be strongly dependent on the chirp direction of the excitation pulse. The slow-decay component associated with an oscillatory structure, which corresponds to the excited-state lifetime and a vibrational mode of 160±10 cm−1, respectively, is measured by the femtosecond time-resolved transmission spectroscopy. The excited-state population is substantially decreased and enhanced in the cases of negatively chirped (NC) and positively chirped (PC) excitations, respectively. A quantum mechanical calculation by means of the split operator scheme is performed to reproduce the wave-packet propagation after the chirped pulse excitation. The calculation shows that the spatial distribution of the wave packet for the NC case is narrower than that for the PC case during the excitation, and that the overlap integral between the excited- and ground-state wave packets determines the efficiency of the population dumping.
Fast magnetic fluctuation diagnostics for Alfvén eigenmode and magnetohydrodynamics studies at the Joint European Torus
Authors: Heeter R. F. , Fasoli A. F. , Ali-Arshad S. , Moret J. M.
Published: July 2002

Alfvén eigenmodes and other magnetohydrodynamic phenomena have been studied in tokamak plasmas at the Joint European Torus (JET) using a new eight-channel, 4 s, 1 MHz, 12-bit data acquisition system (KC1F) in conjunction with the JET fast Mirnov magnetic fluctuation pickup coils. To this end, the JET magnetic pickup coils were calibrated in the range 30–460 kHz using a new remote calibration technique which accounts for the presence of the first few LRC circuit resonances. Signal processing software has been developed to implement the calibration via digital filtering. A data analysis program has been written which produces spectrograms of fluctuation amplitude and toroidal mode number versus frequency and time, both interactively and for automatic overnight analyses. Modes with amplitudes δB/B⩾10−8 and toroidal mode numbers |n|<32 are now routinely detected. Since KC1F data are now available for over 4000 JET discharges, a pulse-characterization database has been developed to help select pulses of interest for detailed analysis.
Visible photoluminescence of the deposited germanium–oxide prepared from clusters in the gas phase
Authors: Negishi Yuichi , Nagao Satoshi , Nakamura Yoshiaki , Nakajima Atsushi , Kamei Shinichi , Kaya Koji
Published: July 2002

Optical properties of the deposited germanium–oxide (Ge–O) prepared from the laser ablation of GeO2 were investigated. When the GenOm clusters, having the compositions mainly at n=m, were deposited onto a substrate as a precursor, strong visible photoluminescence peaked at 500 nm was observed under the excitation of 325 nm laser light. X-ray photoelectron spectroscopy for the deposited Ge–O on the substrate consistently shows the substantial component of the oxidation state of Ge2+, which shows the contributions from 1:1 composition of Ge2+-O2−. Correspondingly, the electronic structures of the germanium–oxide cluster were studied by using photoelectron spectroscopy in gas phase. Our developed method of the halogen atom doping enables us to determine the highest occupied molecular orbital–the lowest unoccupied molecular orbital gap of the corresponding neutral GenOn clusters experimentally, and it was found that the gap of the GenOn (n=2–5) clusters reasonably corresponds to the energy of the visible light. These results imply that the GenOn clusters can be ascribed to the origin of the visible photoluminescence.