bluuebunny/crossref_metadata_2025 huggingface dataset:
GaAs/Al
x
Ga
1−x
As
quantum cascade lasers
Authors: Sirtori Carlo , Kruck Peter , Barbieri Stefano , Collot Philippe , Nagle Julien , Beck Mattias , Faist Jérôme , Oesterle Ursula
Published: July 2002

A unipolar injection quantum cascade (QC) laser grown in an AlGaAs/GaAs material system by molecular beam epitaxy, is reported. The active material is a 30 period sequence of injectors/active regions made from Al0.33Ga0.67As/GaAs-coupled quantum wells. For this device a special waveguide design, which complies with a GaAs heavily doped substrate and very short Al0.90Ga0.10As cladding layers, has been optimized. At a heat-sink temperature of 77 K, the laser emission wavelength is 9.4 μm with peak optical power exceeding 70 mW and the threshold current density is 7.3 kA/cm2. The maximum operating temperature is 140 K. This work experimentally demonstrates the general validity of QC laser principles by showing laser action in a heterostructure material different from the one used until now.
Optical constants of (Al0.98Ga0.02)xOy native oxides
Authors: Knopp K. J. , Mirin R. P. , Christensen D. H. , Bertness K. A. , Roshko A. , Synowicki R. A.
Published: July 2002

We report the optical constants of oxidized crystalline and low-temperature-grown (LTG) Al0.98Ga0.02As films, as determined by variable angle spectroscopic ellipsometry. Data were acquired at three angles of incidence over 240–1700 nm and fitted to a Cauchy dispersion function. For oxidized crystalline material, we observe a variation in the real index of ±0.5% for layer thickness variations of ±6%. We show that upon oxidation, LTG material can expand by >25% while crystalline material contracts by <2%. Atomic force microscopy analysis indicates thickness-dependent variations in the oxide microstructure. Additionally, an optical scattering loss of 2.1×10−4%/pass is calculated based on surface roughness measurements for a thin layer of oxidized crystalline material.
A self-assembled microlensing rotational probe
Authors: Brody James P. , Quake Stephen R.
Published: July 2002

A technique to measure microscopic rotational motion is presented. When a small fluorescent polystyrene microsphere is attached to a larger polystyrene microsphere, the larger sphere acts as a lens for the smaller microsphere and provides an optical signal that is a strong function of the azimuthal angle. We demonstrate the technique by measuring the rotational diffusion constant of the microsphere in solutions of varying viscosity and discuss the feasibility of using this probe to measure rotational motion of biological systems.
Low-threshold high-quantum-efficiency laterally gain-coupled InGaAs/AlGaAs distributed feedback lasers
Authors: Kamp M. , Hofmann J. , Forchel A. , Schäfer F. , Reithmaier J. P.
Published: July 2002

We have developed gain-coupled lasers based on metal gratings patterned laterally to the laser ridge. For narrow ridge waveguides, the evanescent field of the laser mode couples to the grating. The fabrication requires no overgrowth steps and can be applied to all material systems. Ridge-waveguide gain-coupled lasers with threshold current densities of 600 A/cm2 were obtained from InGaAs/AlGaAs GRINSCH structures. The continuous wave threshold currents are around 9 mA for a cavity with 600 μm length and 2.5 μm width. Monomode emission up to output power levels of 64 mW and sidemode suppression ratios of over 45 dB have been obtained.
Ternary single electron tunneling phase logic element
Authors: Liu F. Y. , An F.-T. , Kiehl R. A.
Published: July 2002

An analysis of a ternary single electron tunneling phase logic element is presented. The analysis is based on Monte Carlo simulations and an analytical treatment of a resistively loaded tunneling junction at low temperatures. We show that tristable operation can be obtained over a 19% dc bias operating range by optimizing the pump frequency and amplitude. For large ac frequencies, our optimizations also show that simple linear relationships exist between the optimal parameters (the optimal dc bias and pump amplitude) and frequency. Finally, we show that the ternary phase state of a clocked element can be controlled by an input signal provided that the clock turn-on is not too abrupt. The results should be of use in the design of ternary and other multilevel tunneling phase logic families.
Generation of tunable far-infrared radiation by the interaction of a superluminous ionizing front with an electrically biased photoconductor
Authors: Hashimshony D. , Zigler A. , Papadopoulos K.
Published: July 2002

Tunable radiation in the 0.1 THz to a few THz range by the interaction of a superluminous photoconducting front with an electrostatic “frozen-wave” configuration in a semiconductor is reported. The interaction converts the energy contained in the “frozen wave” into far-infrared radiation, whose frequency depends on the energy in the laser pulse creating the superluminous front and the wavelength of the static wave.
Effects of nitrogen implantation in silicon for shallow p+-n junction formation
Authors: Kang Chang-Yong , Cho Won-Joo , Kang Dae-Gwan , Lee Young-Jong , Hwang Jeong-Mo
Published: July 2002

This letter will present the effects of nitrogen implantation on shallow p+-n junction formation in silicon. The p+-n junctions fabricated at different implantation conditions and heat budgets were characterized by secondary ion mass spectroscopy, current–voltage and capacitance–voltage measurements, and analyzed by transport of ions in matter simulation. The capacitance–voltage measurements of nitrogen implanted samples revealed the one-sided abrupt junction properties, and the current–voltage measurements indicated the shallow junction characteristics due to boron diffusion suppression by nitrogen. The activation energy is about 0.39 eV for temperatures below 80 °C, and its dominant leakage mechanism is phonon-assisted tunneling. At a reverse bias of −3 V, the leakage current density was 5.217×10−8 A/cm2 at −3 V, which is comparable to that of a conventional p+-n junction.
Direct evidence of Cd diffusion into Cu(In, Ga)Se2 thin films during chemical-bath deposition process of CdS films
Authors: Nakada T. , Kunioka A.
Published: July 2002

The diffusion behavior at the Cu(In, Ga)Se2 (CIGS)/CdS interface of high efficiency CIGS thin film solar cells has been investigated using energy dispersive x-ray spectroscopy (EDX) and transmission electron microscopy. CdS layers were deposited on CIGS thin films using the chemical bath deposition (CBD) process. EDX analysis revealed that Cd was present in the CIGS layer approximately 100 Å from the interface boundary. In contrast to the diffusion of Cd, the Cu concentration decreased near the surface of the CIGS film, suggesting substitution of Cd for Cu atoms. These results are direct evidence of Cd diffusion into CIGS thin films during the CBD process.
Piezoelectric and electro-optic coefficients of the molecular nonlinear optical crystal 2-furyl methacrylic anhydride
Authors: Kerkoc P. , Maruo S. , Horinouchi S. , Sasaki K.
Published: July 2002

Large crystals of 2-furyl methacrylic anhydride were successfully grown, making it possible to measure accurately their piezoelectric and electro-optic coefficients. Low modulation frequency electro-optic coefficients at a wavelength of 632.8 nm are: r33=7.1±0.4 pm/V and r13=4.5±0.2 pm/V, whereas the inverse piezoelectric coefficients are: d33=8.2±0.4 pm/V and d13=2.2±0.1 pm/V.
The effect of bottom electrode on the performance of thin film based capacitors in the gigahertz region
Authors: Dube D. C. , Baborowski J. , Muralt P. , Setter N.
Published: July 2002

Capacitors in metal–insulator–metal structure have been fabricated with different bottom electrodes. It has been found that the observed losses are enormous at gigahertz frequencies unless due care is taken not only for the conductivity of the bottom electrode but also its thickness. Very thin bottom electrodes (thickness ∼100 nm) modify the dielectric response substantially and major loss contributions arise from the substrate, the electrode resistance, and the contacts. A simple approach is suggested to model and experimentally evaluate the electrode resistance.
Reduced operating voltage of organic electroluminescent devices by plasma treatment of the indium tin oxide anode
Authors: Steuber F. , Staudigel J. , Stössel M. , Simmerer J. , Winnacker A.
Published: July 2002

The impact of oxygen plasma treatment of indium tin oxide anodes on performance and durability of vapor-deposited organic electroluminescent devices is shown. Investigations focused on the long-term stability using driving conditions suitable for passive matrix driven displays. Reliability studies of solvent only cleaned samples indicate the presence of a predominating degradation process at the interface between indium tin oxide and the hole injection layer which results in a drastic rise of the operating voltage. This voltage increase could be reduced to 0.31 mV/h by oxygen plasma treatment. As hole injection layer copper phthalocyanine is compared with a star-shaped amine derivative.
Magnetic anisotropy obtained from demagnetization curves: Influence of particle orientation and interactions
Authors: Franco V. , Conde A.
Published: July 2002

The influence of easy axis orientation and interparticle interactions on the apparent magnetic anisotropy distribution, obtained from the demagnetization curve analysis, has been studied. For any particle orientation, the method is useful in detecting multimodal distributions as well as effective anisotropy field values. In the case of interacting particulate systems, the range of interaction strength that can be studied is wider than that of the reversible transverse susceptibility method. Analytical expressions for the distribution curves are given, allowing numerical predictions for different distributions of particle orientations and interaction strengths.
All optical mode locking of Fabry–Perot laser diode via mutual injection locking between two longitudinal modes
Authors: Kasuya Hideaki , Mori Masakazu , Goto Ryosuke , Goto Toshio , Yamane Kazuo
Published: July 2002

A method to mode lock a Fabry–Perot laser diode (F–P LD) by purely optical means is proposed. Two F–P modes are mutually injection locked with the aid of injected cw light at the center frequency of the two modes. The other F–P modes are cascadingly mode locked owing to the cavity-enhanced nondegenerate four-wave mixing in the F–P LD. The principle of the method was confirmed by observing the linewidth narrowing of the lasing longitudinal modes and the generated optical pulse train at the repetition frequency of one F–P mode spacing.
Bipolar transport and charge-carrier generation in polymethylphenylsilane thin films containing diphenoquinone
Authors: Naito Hiroyoshi , Ohsawa Yoshikazu , Mimura Shinya
Published: July 2002

Bipolar transient transport in polymethylphenylsilane (PMPS) thin films containing 3,5-dimethyl-3′,5′-di-tert-butyl-4,4′-diphenoquinone (MBDQ) has been studied with the standard time-of-flight experiment. It is found that with increasing MBDQ concentration the electron drift mobility is observable above 30 wt % MBDQ addition (5.0×10−7 cm2/V s at 40 wt % MBDQ addition) and the hole drift mobility is unchanged (∼1.0×10−4 cm2/V s) for 0–40 wt % MBDQ addition. It is also found that the photocarrier generation efficiency is increased by an order of magnitude over pristine PMPS for 30–40 wt % MBDQ addition. These results demonstrate that MBDQ/PMPS composites are an approach to improving photoelectric properties of bipolar polymeric composites.
Scaling of ferroelectric properties in thin films
Authors: Ganpule C. S. , Stanishevsky A. , Su Q. , Aggarwal S. , Melngailis J. , Williams E. , Ramesh R.
Published: July 2002

A fundamental issue in ferroic systems (ferromagnetic and ferroelectric) is the scaling of the order parameter (magnetization or polarization) with size. Specifically, in ferroelectric thin films, deviations in the polarization can occur due to: (i) competition between thermal vibrations and the correlation energy (which aligns the dipoles) and (ii) damage during fabrication. These deviations will have a profound impact on the performance of the next generation of high-density nonvolatile memories based on the spontaneous polarization. We have combined approaches, namely, focused ion-beam milling to define submicron capacitors and scanning force microscopy to examine the scaling of the fundamental ferroelectric response of these capacitors. We find that the capacitors exhibit ferroelectric properties for lateral dimensions down to at least 100 nm, suggesting that memories with densities in the range of 4–16 Gbits can be successfully fabricated.
Electric-field-induced ion migration in polymer-dispersed liquid- crystal films observed by near-field scanning optical microscopy
Authors: Mei Erwen , Higgins Daniel A.
Published: July 2002

Near-field scanning optical microscopy (NSOM) is used to study electric-field-induced ion migration in polymer-dispersed liquid-crystal (PDLC) films. An electric field is applied between the aluminum-coated NSOM probe and an optically transparent, conductive glass substrate, upon which the film is supported. Electric-field-induced reorientation of the liquid crystal beneath the probe is observed optically. PDLC films doped with tetraalkylammonium tetrafluoroborate salts show dramatically different reorientation dynamics than those prepared without ionic dopants. The behavior observed is attributed to the charging of double layers at the polymer/liquid-crystal interface that cancel the field within the “bulk” of the liquid crystal in time. Observation of subsequent liquid-crystal relaxation yields a measure of the local ion migration rate.
Reversible charging effects in SiO2 films containing Si nanocrystals
Authors: Choi Suk-Ho , Elliman R. G.
Published: July 2002

Reversible charging effects are observed in metal–insulator–semiconductor structures which have been ion implanted and annealed to produce Si nanocrystals in the insulating SiO2 layer. The shifts in current–voltage (I–V) and capacitance–voltage (C–V) curves are induced by forward constant voltage stress or UV light exposure, and can be explained by hole charging of the nanocrystals in the insulator layer. A reverse constant voltage stress is shown to recover the original I–V curve and partially recover the original C–V curve. For a sample implanted with a Si dose of 3×1016 Si cm−2, the voltage shift of the I–V curve produced by a forward voltage stress of V=−10 V for 5 s is 1.2 V, which is shown to be in reasonable agreement with simple estimates based on nanocrystal charging.
Organometallic vapor phase epitaxy growth and optical characteristics of almost 1.2 μm GaInNAs three-quantum-well laser diodes
Authors: Li N. Y. , Hains C. P. , Yang K. , Lu J. , Cheng J. , Li P. W.
Published: July 2002

We report organometallic vapor-phase epitaxy (OMVPE) growth and optical characteristics of 1.17–1.20 μm double-heterostructure laser diodes with three Ga0.7In0.3N0.003As0.997 (7 nm)/GaAs(10 nm) quantum wells (GaInNAs/GaAs QWs). Three GaInNAs/GaAs QWs were successfully grown by OMVPE using dimethylhydrazine as the N precursor. Strong room-temperature photoluminescence at the 1.17–1.19 μm regime with a full width at half maximum of 33 meV has been routinely achieved. By using three GaInNAs/GaAs QWs as the gain medium of the GaInNAs laser, room temperature operation with a threshold current density of 1.2 kA/cm2 has been successfully demonstrated.
Low-leakage-current metal–insulator–semiconductor–insulator–metal photodetector on silicon with a SiO2 barrier-enhancement layer
Authors: Seto M. , Rochefort C. , de Jager S. , Hendriks R. F. M. , ’t Hooft G. W. , van der Mark M. B.
Published: July 2002

We show that the leakage current through a metal–semiconductor–metal photodetector can be reduced by placing a thin interfacial silicon dioxide layer between the Schottky metal and the silicon substrate. We measure a factor 5.2 reduction in leakage-current density to 18 μA/cm2 at 5 V, a weaker increase in dark current with bias, and a factor 3.5 improvement in photoresponsivity to 0.39 A/W. We do not observe any noticeable reduction in device speed using this interfacial oxide.
Anisotropic patterns formed in Ag–As–S ion-conducting amorphous semiconductor films by polarized light
Authors: Tanaka Keiji , Gotoh Tamihiro , Hayakawa Hideki
Published: July 2002

Linearly polarized light can produce an anisotropic surface corrugation in amorphous chalcogenide films of Ag–As–S. The corrugation resembles a mouth whisker consisting of narrow fringes which are parallel to the electric field of light and streaks which radiate from the illuminated spot to directions nearly perpendicular to the electric field. Optical birefringence of about 0.01 appears with this pattern. A photo–electro–ionic interaction, which produces a form-birefringent structure, seems to be responsible for the formation process.
An adjustable oxide-free tunnel junction for vibrational spectroscopy of molecules
Authors: Zimmerman Darin T. , Weimer Michael B. , Agnolet Glenn
Published: July 2002

We describe an adjustable, oxide-free, tunnel junction with the stability necessary to observe the vibrational modes of molecules adsorbed on clean metal surfaces. We illustrate the capabilities of this device with inelastic tunneling data from junctions whose barriers are formed by neon/acetylene mixtures of varying concentration. From the concentration dependence of the inelastic spectra, we can distinguish between acetylene molecules chemisorbed on the metal electrodes and those that are either physisorbed or incorporated in the neon barrier.
Phase imaging: Deep or superficial?
Authors: Behrend O. P. , Odoni L. , Loubet J. L. , Burnham N. A.
Published: July 2002

Phase images acquired while intermittently contacting a sample surface with the tip of an atomic force microscope cantilever are not easy to relate to material properties. We have simulated dynamic force curves and compared simulated with experimental results. For some cantilever–sample combinations, the interaction remains a surface effect, whereas for others, the tip penetrates the sample significantly. Height artifacts in the “topography” images, and the role of the sample stiffness, work of adhesion, damping, and topography in the cantilever response manifest themselves to different extents depending on the indentation depth.
Scanning nonlinear dielectric microscopy with nanometer resolution
Authors: Cho Yasuo , Kazuta Satoshi , Matsuura Kaori
Published: July 2002

A very high-resolution scanning nonlinear dielectric microscope was developed for the observation of ferroelectric polarization. We demonstrate that the resolution of the microscope is of a nanometer order by measurement of the c–c domain wall of a BaTiO3 single crystal, and that this microscope is very useful not only for the domain observation of ferroelectric bulk material but also for that of thin films.
The mechanism for the high-quality single-phase growth of MnSi films on Si (111) in the presence of Sb flux
Authors: Yan Y. , Al-Jassim M. M. , Matsuda K. , Tatsuoka H. , Kuwabara H. , Pennycook S. J.
Published: July 2002

The microstructures of high-quality single-phase MnSi layers grown on Si (111) by Mn deposition and reaction with Si in the presence of Sb flux are characterized by Z-contrast imaging. It is found that there is a transition layer consisting of two Sb monolayers sandwiching a Mn layer in between the Si substrate and the single-phase MnSi film. This Sb–Mn–Sb sandwich layer effectively prevents deposited Mn atoms from direct reaction with Si atoms in the substrate to form Mn silicides. This explains why high-quality single-phase MnSi layers can be grown with remarkably smooth interface on Si (111) substrates.
High energy-barrier for defect creation in thin-film transistors based on hot-wire amorphous silicon
Authors: Stannowski B. , Schropp R. E. I. , Nascetti A.
Published: July 2002

Thin-film transistors based on amorphous silicon deposited by hot-wire chemical-vapor deposition (CVD) exhibited a high mean barrier height of 1.074 eV for defect creation after gate-voltage stress. This is 77 meV higher than for glow-discharge devices. Transistors with a SiO2 or a-SiNx:H gate dielectric showed good performance with a field-effect mobility up to 0.7 cm2/V s. Thus, good thin-film transistors with a superior stability can be deposited by hot-wire CVD at high deposition rates of 1.7 nm/s. We demonstrate that a reduced defect creation in the silicon and not the hot-wire-specific absence of interface ion bombardment is responsible for this higher stability.
Enhanced fatigue and retention properties of Pb(Ta0.05Zr0.48Ti0.47)O3 films using La0.25Sr0.75CoO3 top and bottom electrodes
Authors: Yin J. , Zhu T. , Liu Z. G. , Yu T.
Published: July 2002

Ferroelectric perovskite Pb(Ta0.05Zr0.48Ti0.47)O3 thin-film capacitors having LaxSr1−xCoO3 bottom and top electrodes have been prepared on Pt/TiO2/SiO2/Si(001) substrates by pulsed-laser deposition. It is found that La0.25Sr0.75CoO3 bottom electrodes with cubic structure strongly promote the formation of (001) texture of PTZT films and improve the fatigue and retention properties of the capacitors. The polarization of the La0.25Sr0.75CoO3/Pb(Ta0.05Zr0.48Ti0.47)O3/La0.25Sr0.75CoO3 capacitors with a Pb(Ta0.05Zr0.48Ti0.47)O3 thickness of 400 nm were subjected to no degradation after 1×1010 switching cycles at an applied voltage 5 V with a frequency of 1 MHz. The capacitor retains more than 92.6% of its polarization after a retention time up to 105 s. The possible microstructural background responsible for the excellent fatigue and retention properties was discussed.
Defect examination of diamond crystals by surface hydrogen-plasma etching
Authors: Jiang X. , Rickers C.
Published: July 2002

A method to examine the defects and defect distribution in diamond crystals has been developed. To make the crystal defects visible, a high-temperature hydrogen-plasma etching was employed. By a combination of scanning electron microscopy and atomic force microscopy, the etch pits of the (001) diamond faces parallel to the substrate were observed and analyzed. The defect distribution of chemical-vapor-deposited diamond crystallites corresponds exactly with that observed by transmission electron microscopy, and can be related to the growth mode during film deposition.
Efficient high-speed near-infrared Ge photodetectors integrated on Si substrates
Authors: Colace L. , Masini G. , Assanto G. , Luan Hsin-Chiao , Wada K. , Kimerling L. C.
Published: July 2002

We have fabricated Ge/Si heterojunction photodetectors with high responsivities of 550 mA/W at 1.32 μm and 250 mA/W at 1.55 μm and time responses shorter than 850 ps. High quality Ge was epitaxially grown on Si using ultrahigh vacuum/chemical vapor deposition followed by cyclic thermal annealing. The beneficial effect of the post-growth thermal annealing on the electrical properties of Ge epilayers, due to the reduction of threading-dislocation densities, is confirmed by the dramatic enhancement of the performance of the photodetectors.
Electrical measurements of individual semiconducting single-walled carbon nanotubes of various diameters
Authors: Zhou Chongwu , Kong Jing , Dai Hongjie
Published: July 2002

Individual semiconducting single-walled carbon nanotubes (SWNTs) of various diameters are studied by electrical measurements. Transport through a semiconducting SWNT involves thermal activation at high temperatures, and tunneling through a reverse biased metal–tube junction at low temperatures. Under high bias voltages, current–voltage (I–V) characteristics of semiconducting SWNTs exhibit pronounced asymmetry with respect to the bias polarity, as a result of local gating. SWNT transistors that mimic conventional p-metal-oxide-semiconductor field-effect transistor with similar I–V characteristics and high transconductance are enabled.
Large dielectric constant (ε/ε>6000) Ba0.4Sr0.6TiO3 thin films for high-performance microwave phase shifters
Authors: Carlson C. M. , Rivkin T. V. , Parilla P. A. , Perkins J. D. , Ginley D. S. , Kozyrev A. B. , Oshadchy V. N. , Pavlov A. S.
Published: July 2002

We deposited epitaxial Ba0.4Sr0.6TiO3 (BST) films via laser ablation on MgO and LaAlO3 (LAO) substrates for tunable microwave devices. Postdeposition anneals (∼1100 °C in O2) improved the morphology and overall dielectric properties of films on both substrates, but shifted the temperature of maximum dielectric constant (Tmax) up for BST/LAO and down for BST/MgO. These substrate-dependent Tmax shifts had opposite effects on the room-temperature dielectric properties. Overall, BST films on MgO had the larger maximum dielectric constant (ε/ε0⩾6000) and tunability (Δε/ε⩾65%), but these maxima occurred at 227 K. 30 GHz phase shifters made from similar films had figures of merit (ratio of maximum phase shift to insertion loss) of ∼45°/dB and phase shifts of ∼400° under 500 V (∼13 V/μm) bias, illustrating their utility for many frequency-agile microwave devices.
High-efficiency transparent organic light-emitting devices
Authors: Parthasarathy G. , Adachi C. , Burrows P. E. , Forrest S. R.
Published: July 2002

We demonstrate organic light-emitting devices (OLEDs) employing highly transparent cathodes comprised of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline (BCP) and an ultrathin film of Li capped with radio-frequency magnetron-sputtered indium–tin–oxide. The cathodes are incorporated onto a conventional bilayer small-molecule OLED. The operating voltages and the total device external quantum efficiencies emitted from the top and substrate surfaces (1.0±0.05)% are comparable to the best conventional undoped OLEDs employing thick metallic cathodes. The device characteristics are independent of the position of Li within the compound cathode, suggesting that Li readily diffuses through BCP to enhance electron injection. An increase of a factor ∼3.5 in the external quantum efficiency is observed compared to devices containing no Li. These results suggest that Li donates electrons to the BCP, increasing its conductivity to the point that band bending occurs to aid in the injection of charge.
Characteristic length of hot-electron transport in silicon metal–oxide–semiconductor field-effect transistors
Authors: Sakamoto T. , Kawaura H. , Baba T. , Iizuka T.
Published: July 2002

Characteristic length of hot-electron transport in an inversion layer at a Si surface is estimated by using lateral hot electron transistor, which has an upper gate and two lower gates. The inversion layer formed by biasing the upper-gate voltage is separated into three channel regions (the emitter, collector, and base) by the two lower gates. We find that the characteristic length depends on both the upper-gate voltage and the injection energy and ranges from 19 to 27 nm. These results show that hot electrons are affected by electron–electron scattering or surface–roughness scattering and that hot-electron transport plays a crucial role in Si metal–oxide–semiconductor field effect transistors with gate lengths of 20 nm or less.
Polarized blue light-emission from epitaxially oriented bis(phenyloxazolyl)benzene crystals
Authors: Yamada Yuya , Yanagi Hisao
Published: July 2002

Epitaxially oriented crystals of a π-conjugating blue light-emitting molecule, 1,4-bis[2-(5phenyloxazolyl)]benzene (POPOP), were prepared by vapor deposition onto the KCl (001) surface. Rodlike crystals with submillimeter length grew along the KCl 〈110〉 directions having the (102) plane of the monoclinic cell contact with the KCl surface. The POPOP molecules were aligned parallel on this plane taking the herringbone structure. Due to this orientation, the electronic transition dipoles between the frontier molecular orbitals were all lying, so that the fluorescence emitted from the crystals under ultraviolet excitation was highly polarized in parallel to the substrate surface. This polarized emission with the transverse-electric mode was partly confined within the crystals and self-waveguided along their rod axes.
Nanoscale InGaAs concave disks fabricated by heterogeneous droplet epitaxy
Authors: Mano Takaaki , Watanabe Katsuyuki , Tsukamoto Shiro , Koguchi Nobuyuki , Fujioka Hiroshi , Oshima Masaharu , Lee Chae-Deok , Leem Jae-Young , Lee Hwack Joo , Noh Sam Kyu
Published: July 2002

The detailed cross-sectional structure of InGaAs quantum dots fabricated by a heterogeneous droplet epitaxy method was investigated by means of cross-sectional transmission electron microscopy observation. It was confirmed that concave disks without any dislocations or wetting layer were formed at the upper part of the flat surface. This result was consistent with the change of photoluminescence intensity and peak position. The sizes of the disks were estimated to be 30 and 12 nm in lateral and vertical directions, respectively. From this estimation, the occurrence of a phase-separation effect is suggested.
Minority carrier lifetime degradation in carbon-doped base of InGaP/GaAs heterojunction bipolar transistors grown by low-pressure metalorganic chemical vapor deposition
Authors: Yang Q. , Scott D. S. , Chung T. , Stillman G. E.
Published: July 2002

The effect of intermediate temperature annealing on the carbon-doped base region of InGaP/GaAs heterojunction bipolar transistors (HBTs) was studied. This work shows that the minority carrier lifetime in the samples doped at 5.5×1019 cm−3 decreases upon annealing at only 600 °C. Magnetotransport measurements were performed to obtain the minority carrier mobility, with which the minority carrier lifetime was extracted. The decrease in the direct current (dc) current gain upon annealing is attributed to the increase in the base bulk recombination. The correlation between the dc current gain and the magnetotransport measurements indicates that the annealing increases the carbon-related defects in the GaAs base, decreases the minority carrier lifetime in the carbon-doped base, and degrades the dc current gain of the InGaP/GaAs HBTs. These results are very important to the growth and postgrowth processing of InGaP/GaAs HBTs.
Strain relaxation and segregation effects during self-assembled InAs quantum dots formation on GaAs(001)
Authors: Garcı́a J. M. , Silveira J. P. , Briones F.
Published: July 2002

In segregation effects during InAs growth on GaAs(001) and critical thickness for InAs self-assembled quantum dots are studied using a real time, in situ technique capable of measuring accumulated stress during growth. Due to a large (∼50%) surface In segregation of floating In, self-assembled dot formation takes place when less than one monolayer of InAs is pseudomorphically grown on GaAs. A picture of the growth process is discussed on the basis of the equilibrium between InAs and floating In dominated by the stress energy.
Manakov solitons and polarization mode dispersion
Authors: Chen Y. , Haus H. A.
Published: July 2002

The effect of polarization mode dispersion (PMD) on Manakov solitons and dispersion managed solitons is treated analytically and by numerical simulation. In the analytic approach the internal motion of the Manakov soliton is represented as a damped harmonic oscillator. The PMD functions as a white noise source driving the oscillations. It is shown that the solitons can withstand PMD up to a certain instability threshold for which an analytic expression is obtained. This threshold is also evaluated for dispersion managed solitons.
Evolution of a shock wave in a granular gas
Authors: Kamenetsky V. , Goldshtein A. , Shapiro M. , Degani D.
Published: July 2002

Shock wave evolution arising during a steady one-dimensional motion of a piston in a granular gas, composed of inelastically colliding particles is treated by a computational fluid dynamics (CFD) method. It is shown that the flow reaches an asymptotic stationary (final) stage after large evolution time. At this stage particle kinetic energy dissipation leads to formation of two regions within the upstream flow: a fluidized region adjacent to the shock front and a “solid” region adjacent to the piston. In the latter the density is close to the maximum packing density and the kinetic energy of chaotic granular motion is almost zero. The shock wave velocity, mass, and the granular kinetic energy in the fluidized region are found to be constant values, whereas the mass of the solid region grows linearly with time. All properties calculated for the final evolution stage are in excellent agreement with the predictions of the asymptotic solution of the problem obtained earlier. We extend the existing hydrodynamic model of granular gas consisting of hard spheres to provide a plausible description of the static pressure and the speed of sound in the solid block. This is achieved by introducing a cut density ρcut such that dissipation is allowed only if the hydrodynamic density ρ is smaller than ρcut. The computational model reveals that all the above quantities tend to their limiting values in a nonmonotonic manner. In particular, the fluidized mass reaches its maximum when the density at the piston reaches its maximal value. The kinetic energy reaches its maximum earlier than does the fluidized mass. The maximal values of the fluidized mass and the kinetic energy are shown to exceed the comparable long-time limiting values with the difference amounting to about 1.5-fold factor. The calculated results are discussed in relation to several granular flows, characterized by formation of fluidized and compacted regions.
Light-induced nucleation and optical absorption in cesium vapor
Authors: Uchtmann H. , Kazitsyna S. Yu. , Baranovskii S. D. , Hensel F. , Rudek M. M.
Published: July 2002

The first experimental study of the spectral dependence of the nucleation of cesium vapor caused by light absorption is carried out in a diffusion cloud chamber. The spectral dependence of the nucleation rate is compared with the absorption and ionization spectra of cesium vapor. The results evidence that the observed structure in the nucleation spectrum at photon energies above the ionization threshold is correlated to the light-induced ionization of cesium dimers Cs2+. At lower photon energies light-induced nucleation can be either due to generation of ions by direct ionization of cesium clusters, to various two-step ionization processes, or to optical excitation of cesium atoms. The spectral dependence of the nucleation rate provides a new and powerful tool to study ionization spectra of metal vapors. In addition to results obtained by conventional methods a more consolidated insight into energy levels of atoms and clusters under equilibrium conditions is possible.
Role of potassium comodification on domain evolution and electrically induced strains in La modified lead zirconate titanate ferroelectric ceramics
Authors: Tan Qi , Li Jie-Fang , Viehland Dwight
Published: July 2002

The influence of potassium comodification on the structure-property relationships of base (Pb1−3/2xLax)(ZryTi1−y)O3 [(PLZT) 100x/100y/100(1−y)] ceramics has been investigated. Studies were performed by dielectric response, electrically induced polarization and strain methods, and transmission electron microscopy. For 4/65/35, potassium modification resulted in a dramatic decrease (∼10 times) in the magnitude of the induced strain, the development of a pinned polarization, increase in dielectric loss, slight decrease in Tmax, and a dramatic reduction in domain size and change in domain morphology. For 7/65/35 with increasing potassium concentration, only minor changes in the strain-electrical field response was observed and a crossover between nanodomains and normal micron-sized domains occurred. For 12/65/35, significant increases in the magnitude of the induced strain were found, and a significant increase in the nanodomain size occurred. The influence of electrical field on the domain and strain responses of potassium modified PLZT were also performed. These results are discussed in terms of the influence of impurity/defect pinning sites and their distributions on domain stability and polarization switching.
On crossed product of algebras
Authors: Borowiec Andrzej , Marcinek Władysław
Published: July 2002

The concept of a crossed tensor product of algebras is studied from a few points of view. Some related constructions are considered. Crossed enveloping algebras and their representations are discussed. Applications to the noncommutative geometry and particle systems with generalized statistics are indicated.
InGaN/GaN quantum wells grown by molecular-beam epitaxy emitting from blue to red at 300 K
Authors: Damilano B. , Grandjean N. , Massies J. , Siozade L. , Leymarie J.
Published: July 2002

InGaN/GaN quantum wells (QWs) were grown by molecular-beam epitaxy on c-plane sapphire substrates. The growth of InGaN is carried out at 550 °C with a large V/III ratio to counteract the low efficiency of NH3 at that temperature and to promote the two-dimensional mode of growth. An In composition of 16%±2% was determined by high-resolution x-ray diffraction experiments. Room-temperature photoluminescence of InGaN/GaN single QWs can be obtained over the whole visible spectrum (from 0.4 to 0.66 μm) by varying the well thickness from 1 to 5 nm. These heterostructures exhibit very large Stokes shifts between the emission and the absorption edge energies.
Degradation of direct-tunneling gate oxide under hot-hole injection
Authors: Deguchi Kazuaki , Ishida Akihiro , Uno Shigeyasu , Kamakura Yoshinari , Taniguchi Kenji
Published: July 2002

The degradation of silicon-dioxide thin films induced by the hole direct-tunneling process is investigated using a substrate hot-hole (SHH) injection technique. Hot holes from the substrate, as well as cold holes in the inversion layer, are injected into the gate oxide in p-channel metal–oxide–semiconductor field-effect transistor with a p+ polycrystalline-silicon gate. The gate bias used is kept low enough to avoid simultaneous electron injection from the gate. During the SHH stress, in contrast to the case of thicker oxide films, a strong correlation is observed between the oxide film degradation and the injected hole energy, while no degradation occurs due to the direct tunneling of holes from the inversion layer. These experimental findings indicate the existence of threshold energy for the trap creation process, which has been predicted by the theoretical study of the hole-injection-induced structural transformation of oxygen vacancies in silicon dioxide.
Intense and stable blue-violet emission from porous silicon modified with alkyls
Authors: Li Hongliang , Xu Dongsheng , Guo Guolin , Gui Linlin , Tang Youqi , Ai Xicheng , Sun Zhaoyong , Zhang Xingkang , Qin G. G.
Published: July 2002

An intense blue-violet band, centered at 3.0 eV, has been observed besides the red-orange band of the photoluminescence (PL) from the chemically modified porous silicon (PS) samples. The PS samples were formed on 30–50 Ω cm p-type (100) Si wafers by anodic etching at large current densities and then treated with 5-cyano-1-pentynes in toluene solution at 110–120 °C. The intensities of the blue-violet lights increase with the increasing of the etching current densities, while the peak energies are hardly changed. The Fourier transform infrared spectra of the modified samples show that the alkyls have been bonded to the surfaces of the nanometer crystallite silicon particles. The decays of the blue-violet emission with a lifetime about 0.5 μs are different from the nanosecond lifetimes of the blue emissions in the oxidized PS samples reported before, and the 20 μs lifetimes of the concomitant orange PL bands. A carbon-compounds-related luminescence center has been proposed as the origin of this blue-violet PL band.
Useful and low cost instrument for capacitance measurements
Authors: Karadeniz Serdar , Serin Necmi
Published: July 2002

In this study we aimed to develop a practical, useful, and low cost instrument for measuring the capacitance of metal/semiconductor/metal structures. The device operates in the continuous frequency range of 2 Hz–20 kHz. The unknown capacitances were measured by both our instrument and a commercial impedance analyzer. In summary, this instrument, with power consumption of 1.2 W, will serve as practical, useful, and inexpensive equipment in the laboratory.
Exploring the ab initio/classical free energy perturbation method: The hydration free energy of water
Authors: Sakane Shinichi , Yezdimer Eric M. , Liu Wenbin , Barriocanal Jose A. , Doren Douglas J. , Wood Robert H.
Published: July 2002

The ab initio/classical free energy perturbation (ABC-FEP) method proposed previously by Wood et al. [J. Chem. Phys. 110, 1329 (1999)] uses classical simulations to calculate solvation free energies within an empirical potential model, then applies free energy perturbation theory to determine the effect of changing the empirical solute–solvent interactions to corresponding interactions calculated from ab initio methods. This approach allows accurate calculation of solvation free energies using an atomistic description of the solvent and solute, with interactions calculated from first principles. Results can be obtained at a feasible computational cost without making use of approximations such as a continuum solvent or an empirical cavity formation energy. As such, the method can be used far from ambient conditions, where the empirical parameters needed for approximate theories of solvation may not be available. The sources of error in the ABC-FEP method are the approximations in the ab initio method, the finite sample of configurations, and the classical solvent model. This article explores the accuracy of various approximations used in the ABC-FEP method by comparing to the experimentally well-known free energy of hydration of water at two state points (ambient conditions, and 973.15 K and 600 kg/m3). The TIP4P-FQ model [J. Chem. Phys. 101, 6141 (1994)] is found to be a reliable solvent model for use with this method, even at supercritical conditions. Results depend strongly on the ab initio method used: a gradient-corrected density functional theory is not adequate, but a localized MP2 method yields excellent agreement with experiment. Computational costs are reduced by using a cluster approximation, in which ab initio pair interaction energies are calculated between the solute and up to 60 solvent molecules, while multi-body interactions are calculated with only a small cluster (5 to 12 solvent molecules). Sampling errors for the ab initio contribution to solvation free energies are ±2 kJ/mol or less when 50–200 configurations are used. Using the largest clusters and most accurate ab initio methods, ABC-FEP predicts hydration free energies of water at both state points that agree with equations of state, within the sampling error. These results are the first calculation of a free energy of solvation at extreme conditions from a fully atomistic model with ab initio methods.
An electron energy loss spectroscopy study of resonance population in ethylene chemisorbed on Pd(110)
Authors: Okuyama Hiroshi , Kato Hiroyuki , Kawai Maki , Yoshinobu Jun
Published: July 2002

We investigated energy and angle dependencies of vibrational losses of ethylene chemisorbed on Pd(110) by means of high-resolution electron energy loss spectroscopy (HREELS). A resonant enhancement of the C–H stretching modes is observed at the electron primary energy of ∼1.3 eV. It is shown that the resonance cross section is significantly dependent on the electron incidence direction. The resonance is forbidden when the electron incidence direction is aligned along the C–C axis, which is qualitatively explained in terms of the overlap integral between the incident plane wave and the unoccupied orbital. Based on the previous finding that ethylene is anisotropically adsorbed with the C–C axis preferentially aligned along the [11̄0] direction, we assign the resonance state as 2b2u. The results are compared to those obtained by near-edge x-ray absorption fine structure (NEXAFS).
Comparison of penetration depth between two-photon excitation and single-photon excitation in imaging through turbid tissue media
Authors: Gu Min , Gan Xiaosong , Kisteman Aernout , Xu Ming Gun
Published: July 2002

We show, both theoretically and experimentally, that for a turbid tissue medium where Mie scattering is dominant, multiple scattering not only reduces the illumination power in the forward direction but also exhibits an anisotropic distribution of scattered photons. Thus, a signal level under two-photon excitation drops much faster than that under single-photon excitation although image resolution is much higher in the former case. As a result, the penetration depth under two-photon excitation is limited by the strength of two-photon fluorescence and is not necessarily larger than that under single-photon excitation.
Diagnostics and modeling of a macroscopic plasma display panel cell
Authors: Callegari Th. , Ganter R. , Boeuf J. P.
Published: July 2002

A macroscopic plasma display discharge cell has been designed in order to more easily study the plasma evolution in dielectric barrier discharges occurring in the much smaller commercial ac plasma display panels (PDPs). The electrodes in the macrocell can be arranged in matrix or coplanar configurations. The dimensions of the cell are 100 times larger than those of typical PDP cells and the gas pressure is 100 times smaller. Although some of the properties of the discharge pulse obviously do not follow the classical similarity laws, we find that the macrocell is a very useful tool for improving our understanding of the discharge in a PDP cell. The large dimensions of the cell and the longer time scale because of the smaller pressure make the plasma diagnostics easier than in a real PDP cell. The results are presented here for discharges in pure neon at 5 Torr. Measurements of the time evolution of the current and imaging of the plasma with an intensified charge coupled device (ICCD) camera are presented in matrix electrode configurations and are compared with previously developed models. The experiment confirms the large power deposition in electron impact excitation of the gas atoms while the plasma spreads over the dielectric surface above the anode. The images obtained with the ICCD camera also show the existence of striations of the plasma near the dielectric surface which were not predicted by the models. Measured and calculated duration and shape of the current pulse are in reasonable agreement.
Differential electronic gating: A method to measure the shape of short THz pulses with a poorly defined trigger signal
Authors: Hovenier J. N. , van Es R. W. , Klaassen T. O. , Wenckebach W. Th. , Krätschmer M. , Klappenberger F. , Schomburg E. , Winnerl S. , Knippels G. M. H. , van der Meer A. F. G.
Published: July 2002

A simple experimental method has been developed to determine the shape of repetitive picosecond THz pulses in the presence of a large jitter in the trigger signal. This method, a modification of the recently reported differential optical gating method, is based on the femtosecond electronic gating of a high-frequency sequential oscilloscope. As a test, the shape of THz pulses from the free-electron laser has been measured.
Evolving shock-wave profiles measured in a silicon crystal by picosecond time-resolved x-ray diffraction
Authors: Hironaka Yoichiro , Yazaki Akio , Saito Fumikazu , Nakamura Kazutaka G. , Kondo Ken-ichi , Takenaka Hisataka , Yoshida Masatake
Published: July 2002

Picosecond time-resolved x-ray diffraction is used to probe single-crystal silicon under pulsed-laser irradiation (300 ps pulse at 1.4 J/cm2) at an interval of 60 ps. The observed rocking curves show shock compression of the silicon lattice by the laser irradiation. Uniaxial strain profiles perpendicular to the Si(111) plane are estimated using dynamical x-ray diffraction theory. The temporal and spatial evolution of the profiles indicates a propagating shock wave with the velocity of 9.4 km/s inside the silicon crystal. The observed maximum compression is 1.05%, which corresponds to a pressure of 2.18 GPa.