bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of the progressive growth of columns by excimer laser irradiation of silicon
Authors: Sánchez F. , Morenza J. L. , Trtik V.
Published: July 2002

The growth mechanism of columns produced by excimer laser irradiation of a silicon target is investigated. The micrometric columns are formed by a cumulative process with well-defined steps. The first steps lead to silicon hillocks, which evolve to columns through a little-understood mechanism. To investigate it, we irradiated a Si(100) single crystal in air with the necessary pulses of an excimer laser beam to obtain two samples, one with a partially and one with a fully developed column structure. Then we alternated scanning electron microscopy observations with irradiation pulses on the same target area. The progressive evolution of the laser-generated structures is presented. Columns grow at notably high rates, from around 0.5 μm/pulse during the early stages to 0.2 μm/pulse on average for 10–20 μm tall columns. The experimental results lead us to suggest a hydrodynamic growth mechanism.
Room-temperature blue luminescence of thermally oxidized Si1−x−yGexCy thin films on Si (100) substrates
Authors: Cheng Xuemei , Zheng Youdou , Liu Xiabing , Zang Lan , Lo Zhiyun , Zhu Shunming , Han Ping , Jiang Ruolian
Published: July 2002

We measured at room temperature the photoluminescence spectra of the thermally oxidized Si1−x−yGexCy thin films which were grown on silicon substrates by plasma-enhanced chemical vapor deposition and then wet oxidized at 1100 °C for 20 min. The photoluminescence band with a peak at ∼393 nm under the exciting radiation of λ=241 nm was observed. Possible mechanism of this photoluminescence is discussed.
Photoelectrical properties of a ferroelectric liquid crystalline photoconductor
Authors: Kogo Kyoko , Maeda Hiroki , Kato Hiroaki , Funahashi Masahiro , Hanna Jun-ichi
Published: July 2002

An organic ferroelectric photoconductor, 2-[(S)-4-(6-methylheptyloxy)phenyl]-6-decylnaphthalene, was prepared and its photoconductive and ferroelectric behaviors were characterized by steady-state and transient electrical measurements. This material exhibited fast carrier transport with an ambipolar mobility of 2.6×10−4 cm2/V s, independent of temperature and electric field. Under a polarized microscope, a fast switching with memory effect was observed visually and the spontaneous polarization was evaluated to be 0.6 nC/cm2 in chiral smectic C phase.
Nonlinear electrical conduction and broad band noise in the charge-ordered rare earth manganate Nd0.5Ca0.5MnO3
Authors: Guha Ayan , Ghosh Arindam , Raychaudhuri A. K. , Parashar S. , Raju A. R. , Rao C. N. R.
Published: July 2002

Measurements of the dc transport properties and the low-frequency conductivity noise in films of charge-ordered Nd0.5Ca0.5MnO3 grown on Si substrate reveal the existence of a threshold field in the charge-ordered regime beyond which strong nonlinear conduction sets in along with a large broad band conductivity noise. Threshold-dependent conduction disappears as T→TCO, the charge-ordering temperature. This observation suggests that the charge-ordered state gets depinned at the onset of the nonlinear conduction.
Effects of Mo-free C40 Ti(Si1−xGex)2 precursors and the thickness of an interposed Mo layer on the enhanced formation of C54 Ti(Si1−xGex)2
Authors: Luo Jian-Shing , Huang Jun-Chieh , Lin Wen-Tai , Chang C. Y. , Shih P. S.
Published: July 2002

The effects of Mo-free C40 Ti(Si1−xGex)2 precursors and the thickness of an interposed Mo layer between Ti films and Si0.76Ge0.24 substrates on the lowering of formation temperature of C54 Ti(Si1−xGex)2 were studied. Metastable C40 Ti(Si1−xGex)2 precursors were grown by pulsed KrF laser annealing. Upon rapid thermal annealing, the Mo-free C40 phase could not be directly transformed to the C54 phase without going through the C49 phase. When the thickness of the interposed Mo layer increased, up to 2.5 nm, the temperature at which the C54 phase was initially formed changed from 750 to 600 and then to 650 °C. The present result showed that with increasing Mo concentration in the reacted layer, the phase stability shifted from C54 to C40 and no C49 was observed. It seems that apart from the C40 template mechanism, the electron/atom ratio also plays an important role in the enhanced formation of the C54 phase.
Low-temperature growth of crystalline silicon on a chlorine-terminated surface
Authors: Guo Lihui , Toyoshima Yasutake , Kondo Michio , Matsuda Akihisa
Published: July 2002

We report on the role of surface termination during growth of crystalline silicon at low temperatures. Microcrystalline silicon was fabricated using plasma-enhanced chemical-vapor deposition with a hydrogen-diluted dichlorosilane (SiH2Cl2)/monosilane (SiH4) mixture to study the role of hydrogen and chlorine in crystal formation. When varying the fraction of SiH2Cl2, x=[SiH2Cl2]/([SiH4]+[SiH2Cl2]), good crystallinity was obtained for x=0 and 1, whereas the crystallinity was markedly deteriorated for intermediate x. Optical emission spectroscopy of the plasma suggests that film precursors different from SiHx fragments and atomic chlorine are generated for x≠0 and that atomic hydrogen is generated in all cases. Infrared reflection absorption spectroscopy indicates that the surface coverage is hydrogen for x=0, chlorine for x=1, and a hydrogen–chlorine mixture for intermediate x. These results imply that low-temperature growth of crystalline silicon is facilitated on a chlorine-terminated surface as well as on a hydrogen-terminated surface under the presence of atomic hydrogen. The cooperative roles of chlorine and hydrogen are proposed in the crystal growth of Si.
Generation of amplitude-squeezed light from 1550 nm distributed feedback semiconductor laser under wavelength selective optical feedback conditions
Authors: Jérémie F. , Chabran C. , Gallion P.
Published: July 2002

Excess noise that degrades the squeezing of a 1550 nm distributed feedback semiconductor laser is identified using a Mach Zehnder interferometer. This excess noise is reduced using wavelength selective optical feedback. Amplitude-squeezed states then are generated at room temperature.
Intersubband relaxation time for InxGa1−xAs/AlAs quantum wells with large transition energy
Authors: Ghislotti G. , Riedo E. , Ielmini D. , Martinelli M.
Published: July 2002

Intersubband relaxation time for InxGa1−xAs/AlAs multiple quantum wells presenting a large transition energy (680 meV) is measured by means of pump and probe experiments. Differential transmission decays in about 10 ps. The possible influence of intrasubband relaxation and Γ–X coupling on intersubband decay is discussed.
High-density transition layer in oxynitride interfaces on Si(100)
Authors: Wang J. , Lee D. R. , Park C. , Jeong Y. H. , Lee K.-B. , Park Y. J. , Youn S. B. , Park J.-C. , Choi H. M. , Huh Y.-J.
Published: July 2002

Nitrided SiO2 thin films on Si wafers were studied by x-ray reflectivity measurements and their electron-density profiles were evaluated. Interfacial layers of the oxides were found to have densities higher than that of either crystalline Si substrates or strained interfacial layers of thermal oxides. The high density probably results from nitrogen incorporation near the interfaces. The present results suggest that strongly retarded boron penetration through nitrided gate oxides is due to their high-density interfacial layers.
Smoothening transition during initial epitaxial growth of Mo on sapphire
Authors: Ryan P. A. , Tsui F.
Published: July 2002

The initial epitaxial growth of Mo (111) on Al2O3 (0001) substrates has been studied systematically as a function of growth temperature and rate, using real-time reflection high energy electron diffraction (RHEED) and in situ scanning probe microscopy. The evolution of RHEED specular intensity during Mo growth exhibits a distinct peak at Mo coverage of ∼10 Å, where the Mo surface is the smoothest. The observed Mo coverage at the specular peak is independent of growth temperature and rate, indicating that it is energetically stable. However, our findings reveal that surface kinetics plays a key role in determining the observed behavior. At each growth temperature, the highest specular intensity occurs at an optimum growth rate, at which the Mo surface is the smoothest. The temperature dependence of the optimum rate has an Arrhenius form indicating that the observed behavior is an activated process.
High fluence ultrafast dynamics of semiconductor saturable absorber mirrors
Authors: Langlois P. , Joschko M. , Thoen E. R. , Koontz E. M. , Kärtner F. X. , Ippen E. P. , Kolodziejski L. A.
Published: July 2002

The ultrafast nonlinear dynamics of InGaAs/InP semiconductor saturable absorber mirrors are investigated using reflective pump–probe measurements. At high fluence, ultrafast induced absorption begins to dominate over absorption bleaching. Above the InGaAs quantum well band gap, the differential reflectivity shows a ∼1 ps transient due to nonequilibrium carrier dynamics. Below band gap, the signal is dominated by a strong two-photon absorption component followed by induced absorption that decays with a time constant of ∼5 ps; these components are attributed to nonlinear absorption and subsequent carrier diffusion in the InP layer.
Diminished photoluminescence polarization due to exciton ionization in strained Si1−xGex/Si(001) quantum wells
Authors: Miyazaki T. , Fukatsu S.
Published: July 2002

The photoluminescence (PL) of strained Si1−xGex/Si(001) quantum wells was measured in an edge-emitting configuration. The no-phonon PL was found to be polarized largely along the quantum-well plane at low temperature, whereas such in-plane polarization was diminished as the sample temperature was increased. Exciton dissociation occurring at higher temperature is responsible for the diminution of PL polarization, which is consistent with an analogous reduction of in-plane polarization due to field ionization of excitons.
Nonuniform silicon oxidation and application for the fabrication of aperture for near-field scanning optical microscopy
Authors: Minh Phan Ngoc , Ono Takahito , Esashi Masayoshi
Published: July 2002

In this letter, a technological approach for the fabrication of a miniature aperture for near-field scanning optical microscopy using silicon micromachining technology is described. The aperture with diameter sizes from 10 to 500 nm at the apex of a SiO2 tip on a Si cantilever is fabricated using a “Low temperature Oxidation & Selective Etching” technique. The SiO2 tip is formed by nonuniform Si wet oxidation at 950 °C with a thickness of about 1 μm. The aperture is created by selective etching SiO2 in a buffered-HF (50% HF:40% NH4F, 9cc:100cc) solution at 36 °C using a thin chromium (Cr) layer deposited on the oxidized sample as a mask. Using the fabricated probe, atomic force microscopy and corresponding near-field scanning optical microscopy images of 300 nm diameter latex spheres on mica substrate are obtained.
P
-type polycrystalline diamond layers by rapid thermal diffusion of boron
Authors: Krutko Oleh B. , Kosel Peter B. , Wu R. L. C. , Fries-Carr S. J. , Heidger S. , Weimer J.
Published: July 2002

P -type diamond layers have been produced in polycrystalline diamond by means of rapid thermal diffusion (RTD) of boron. Using thin film of deposited boron as the source and temperatures near 1600 °C, solid-state diffusion of boron was achieved, forming conducting layers with average sheet resistances of about 356 Ω/square and diffused-layer thicknesses of about 0.6 μm. Ohmic contacts to the diffused layers were formed by depositing molybdenum films before the RTD step so that sintering of contacts occurred simultaneously with the diffusion. The advantages of this approach are that (a) low contact resistances are obtained and (b) only one high-temperature step is involved. Using this technique, molybdenum-diamond Schottky diodes were fabricated and found to have barrier potentials of 2.6 V and ideality factors of 1.6.
Oscillatory optical second-harmonic generation from Si(001) surface during thin-film epitaxy
Authors: Tok E. S. , Price R. W. , Taylor A. G. , Zhang J.
Published: July 2002

Periodic variation in optical second-harmonic generation during homoepitaxial growth of silicon on singular Si(001) surface is reported. The period of the oscillations corresponds to bilayer growth, and the oscillations are correlated with the mechanism associated with a two-dimensional layer-by-layer growth mode. This mechanism is tentatively attributed to periodic domain coverage variations analogous to the oscillatory response in linear optical technique of reflectance anisotropy. The current experiment, however, cannot distinguish this mechanism from another based on anisotropic second-harmonic generation response with respect to steps.
Efficiency enhancement of free electron Maser oscillator by mode selection with a prebunched electron beam
Authors: Abramovich A. , Kleinman H. , Eichenbaum A. , Yakover Y. M. , Gover A. , Pinhasi Y.
Published: July 2002

We present a method for enhancing the efficiency of a Free Electron Laser Maser oscillator by locking it to a preferred resonator mode. This is done by prebunching of the e beam before injection into the wiggler. In a free running oscillator, the longitudinal mode that dominates the mode competition process during the oscillation buildup period is usually the highest gain mode. However, this mode does not extract the highest energy from the e beam. Lower eigenfrequency modes would provide a higher efficiency if they could dominate the mode competition process. By prebunching the e beam at a frequency near any one of the longitudinal eigenfrequencies of the resonator (having a gain>1), we can make that mode dominant at saturation. The eigenfrequency for which the maximum efficiency is obtained is the lowest eigenfrequency of the resonator for which the net small signal gain is greater than 1. Employing an experimental setup of a prebunched beam Free Electron Maser, we demonstrated efficiency enhancement of 30% for this lowest eigenfrequency mode (as compared to the highest gain mode). Simulation results predict an efficiency enhancement of up to 50%.
Effect of high magnetic field on the two-step martensitic-phase transition in Ni2MnGa
Authors: Ma Yanwei , Awaji S. , Watanabe K. , Matsumoto M. , Kobayashi N.
Published: July 2002

Using a low-temperature x-ray diffraction apparatus in a high field, a systematic study was performed of the stoichiometric Ni2MnGa alloys in a wide temperature range from 10 K to room temperature and in fields up to 5 T. X-ray diffraction data at different fields clearly show the martensitic-phase transition is a two-step process. It is found that the martensitic-transition temperature Tm increases and the premartensitic-transition temperature Tp decreases with increasing magnetic fields up to 2 T, and then both remain unchanged in fields up to 5 T.
Subgap optical absorption and recombination center efficiency in bulk GaAs irradiated by light or heavy ions
Authors: Mangeney J. , Lopez J. , Stelmakh N. , Lourtioz J.-M. , Oudar J.-L. , Bernas H.
Published: July 2002

This letter reports the results of a comparative study on heavy- and light-ion-irradiated semiconductors for fast saturable absorbers. The linear absorption of bulk GaAs irradiated either by Au+ ions or protons was measured over a wide range of wavelengths below the gap. Good correspondence was found between the absorption measurements and the calculated elementary defect concentrations. Defect clustering is evidenced in the heavy-ion case. Pump–probe experiments were used to measure the time-resolved absorption variations for weakly irradiated GaAs samples under intense illumination. Much shorter carrier recombination times are estimated for the heavy-ion case.
Homogenization of nonuniform electric fields in mixtures of liquid dielectrics
Authors: Christen Thomas
Published: July 2002

Inhomogeneous electric fields in mixtures of liquid dielectrics are investigated theoretically in the framework of thermodynamics of binary mixtures. Due to dielectrophoretic forces, the local dielectric constant is increased in high-field regions, which in turn leads to a decrease of field enhancements. This effect could be extremely useful for field-grading applications in insulation devices. Although the effect is usually weak due to dominant entropic forces, it can be strongly enhanced close to the critical point where the effective (mutual) diffusion constant becomes small.
Effect of dopant atoms on the roughness of III–V semiconductor cleavage surfaces
Authors: Quadbeck P. , Ebert Ph. , Urban K. , Gebauer J. , Krause-Rehberg R.
Published: July 2002

We demonstrate that the presence of dopant atoms influences the roughness, morphology, and optical mirror properties of III–V semiconductor (110) cleavage surfaces. High concentrations of Te dopant atoms in GaAs lead to macroscopically curvatured (110) cleavage surfaces with high step concentrations. This “glass-like” fracture behavior is explained by the “lattice superdilation phenomenon” induced by high concentrations of Te dopant atoms in GaAs.
GaAs/AlGaAs distributed feedback quantum cascade lasers
Authors: Schrenk W. , Finger N. , Gianordoli S. , Hvozdara L. , Strasser G. , Gornik E.
Published: July 2002

We report on the realization of distributed feedback quantum cascade lasers in the GaAs/AlGaAs material system. A metallized surface relief grating is used for feedback. Both single mode and double mode emission is observed at λ≈10 μm. The coupling coefficient is measured from the mode spacing for double mode emission to be 24 cm−1. The emission wave number can be tuned with the temperature at a rate of dν/dT≈0.048 cm−1/K.
Switching dynamics between single-mode and dual-mode pulse emissions from a self-seeded laser diode
Authors: Chow K. K. , Shu C.
Published: July 2002

Spectrally selected, self-seeded pulses are generated from a Fabry–Perot laser diode in an external cavity containing a two-chromatic fiber grating. Electrical switching between single-mode and dual-mode operations is demonstrated by controlling the amplitude of the ac signal applied to the laser diode. The principle is based on carrier induced frequency chirp that determines the spectral overlap of the Fabry–Perot modes with the grating reflection band during the pulse buildup time. We also investigate the switching dynamics and show that the steady states can be reached after about five to six round-trip cycles. The buildup of single-mode emissions is slightly faster than that of the dual mode.
Contribution of radicals and ions in atomic-order plasma nitridation of Si
Authors: Seino Takuya , Matsuura Takashi , Murota Junichi
Published: July 2002

Atomic-order nitridation of Si(100) by an ultraclean electron-cyclotron-resonance nitrogen plasma and contribution of radicals and ions to the nitridation have been investigated. The N atom concentration on Si(100) increases with the plasma exposure time and tends to saturate to a value corresponding to a few atomic layers. In the initial stage, the N atom concentration is normalized by the product of the relative radical density with the nitrogen plasma exposure time and the number of the incident ions is much smaller than the nitridation amount, which means the radical reaction is dominant. Assuming Langmuir-type kinetics neglecting desorption, an excellent agreement is observed by fitting the experimental data. In the saturation region, the N atom concentration is normalized by the number of incident ions and becomes higher than that corresponding to the double atomic layers. Therefore, it is suggested that nitridation of the deeper atoms below the surface is induced by the ion incidence.
Photocurrent in an electron transporting luminescent polymer
Authors: Feller F. , Monkman A. P.
Published: July 2002

Photocurrent measurements have been carried out on thin films of poly(2,5-pyridinediyl), a promising polymer for applications in light-emitting diodes. The spectral dependence of the photocurrent has been recorded using the four possible directions of the applied electric field and illumination, and clear symbatic and antibatic behavior was observed. A strong, symmetric photocurrent increase with bias has been found, while the temperature dependence is relatively weak. The analysis of the results provides evidence that electrons are the majority charge carriers and gives insight into the carrier-transport mechanism in poly(2,5-pyridinediyl).
Mapping the fluorescence yield on turbid media
Authors: Qu Jianan Y. , Huang Zhijian , Hua Jianwen
Published: July 2002

In this letter, we introduce a combined polarization and fluorescence imaging technique for the measurement of fluorescence yield on the surface of turbid media. We use the cross-polarization method to reject the specular reflection and enhance the diffusive backscattering from the turbid media. It has been found that the ratio image of fluorescence versus cross-polarized reflection is not sensitive to the geometry of fluorescence excitation and illumination, and provides a map of fluorescence yield on the surface of imaged subject. The technique reported in this letter may potentially solve the problem for imaging of early cancers which usually start from the superficial layer of tissue and have the fluorescence yield lower than surrounding normal tissue.
Strongly enhanced Tb luminescence from titania xerogel solids mesoscopically confined in porous anodic alumina
Authors: Gaponenko N. V. , Davidson J. A. , Hamilton B. , Skeldon P. , Thompson G. E. , Zhou X. , Pivin J. C.
Published: July 2002

We report strongly enhanced Tb photoluminescence (PL) from a doped titania xerogel deposited on mesoporous anodic alumina. Transmission electron microscopy has confirmed the presence of the xerogel inside the pores of alumina films of up to several microns thickness. The optical bands attributed to D35→Fi7 (i=3,4,5), D45→Fj7 (j=3,4,5,6) transitions of Tb3+, with a predominant band at 2.28 eV, are investigated between 6 and 300 K. At all temperatures, the Tb PL increases and the bands become narrower with an increase in the Tb concentration in the xerogel, the number of deposited layers employed and the thickness of the porous alumina film.
Poling of soda-lime glass for hybrid glass/polymer electro-optic modulators
Authors: Enami Y. , Poyhonen P. , Mathine D. L. , Bashar A. , Madasamy P. , Honkanen S. , Kippelen B. , Peyghambarian N. , Marder S. R. , Jen A. K-Y. , Wu J.
Published: July 2002

We report on a poling process of soda-lime glasses that reduces their surface conductivity by five orders of magnitude. We show that this process enables the in-plane poling of high Tg (180 °C) electro-optic (EO) polymers coated on ion-exchanged channel waveguides fabricated in soda-lime glasses for hybrid modulators.
Photothermal spectroscopy using multilayer cantilever for chemical detection
Authors: Li Guangming , Burggraf Larry W. , Baker William P.
Published: July 2002

Photothermal spectroscopy is demonstrated using a high-aspect-ratio multilayer cantilever to measure adsorbed dimethyl methylphosphonate, which has an optical absorbance of ∼10−5 in the near infrared range. Detection sensitivity was 160 pW for a reed of 6 mm in length, 2 mm in width, and 10 μm. Sensitivity of the cantilever is compared to a thermal diffusion model that accounts for conduction loss through the cantilever clamp and to air along the length of the cantilever surface.
Hot-electron flux observation in large-area microwave sustained plasmas
Authors: Kudela Jozef , Terebessy Tibor , Kando Masashi
Published: July 2002

Flux of hot electrons directed away from the waveguiding plasma-dielectric interface was experimentally observed in large-area microwave discharges. The energy of these electrons attains values of some 60 eV, and they are believed to be originating from the resonantly-enhanced electric field region localized near the dielectric. The phenomenon appears to play a significant role in discharge heating mechanism, which is demonstrated by plasma parameter profiles.
Depth analysis of buried iron disilicide formation by Fe ion implantation into Si
Authors: Walterfang M. , Kruijer S. , Keune W. , Dobler M. , Reuther H.
Published: July 2002

The depth distribution of the iron disilicide phases (α-FeSi2 and β-FeSi2) was investigated nondestructively by depth-selective conversion-electron Mössbauer spectroscopy after Fe+ implantation (200 keV, 3×1017 cm−2) and after subsequent rapid thermal annealing (RTA) at 900 °C for 30 s. The depth profiles of the two phases were found to be correlated with the Fe concentration profiles as determined by Auger electron sputter depth profiling. For the as-implanted state a broad distribution of a phase mixture of α- and β-FeSi2 is observed. Subsequent RTA induces a layered structure including a buried β-FeSi2 layer with a high phase content of 90% near 220 nm.
Ti:LiNbO
3
waveguide polarizer with a Zn-doped overlayer prepared by liquid-phase epitaxy
Authors: Ichikawa Junichiro , Uda Satoshi , Shimamura Kiyoshi , Fukuda Tsuguo , Nagata Hirotoshi , Minowa Junichiro
Published: July 2002

Ti:LiNbO 3 waveguide polarizers were obtained by an epitaxial growth of a Zn-doped LiNbO3 layer on the Ti:LiNbO3 waveguide. The film growth was carried out by a liquid-phase-epitaxy technique using a Li2O–V2O5 flux. Such an X-cut waveguide polarizer achieved a TE-mode (extraordinary ray) propagation with an extinction ratio over 30 dB at a light wavelength of 1.55 μm. This technique can be easily applied to Z- and Y-cut waveguide polarizers with TM- and TE-mode propagation, respectively.
Growth model of coupled-fractal networks
Authors: Ugajin R. , Ohnishi M. , Hirata S. , Ishibashi A. , Kuroki Y. , Ishimoto C.
Published: July 2002

We extended the dielectric breakdown model in order to generate networks of interacting objects, which are considered as coupled-fractal networks. In the proposed model, each object grows to be a random fractal if isolated, but connects with others if glued. The model is constructed based on a growth hypothesis in which the growth rate of each object is a product of the probability of receiving source materials from faraway and the probability of receiving adhesives from other grown objects.
Improved interface properties of p-type 6H–SiC/SiO2 system by NH3 pretreatment
Authors: Xu J. P. , Lai P. T. , Chan C. L. , Cheng Y. C.
Published: July 2002

Effects of preoxidation NH3 treatment on p-type 6H–SiC/SiO2 interface properties were investigated as compared to conventional thermally oxidized devices. It was found that NH3 treatment before oxidation can reduce the SiC/SiO2 interface states and fixed oxide charge. Furthermore, less shift of flatband voltage, and smaller increases of effective oxide charge and interface states during high-field stress were observed for the NH3 pretreated devices.
Length scales of charge transport in organic photorefractive materials
Authors: Leopold A. , Grasruck M. , Hofmann U. , Kol’chenko M. A. , Zilker S. J.
Published: July 2002

The drift length of charge carriers has a significant influence on the dynamics of the space-charge field in organic photorefractive materials. This letter introduces a relatively simple method for the determination of the drift length, which takes into account that the charge carrier mobility depends on the sample thickness. By combining results of time-of-flight and holographic time-of-flight experiments using the stochastic transport model of Scher and Montroll, the effective drift length can be determined as 2.4 μm in the investigated photorefractive glass.
Hole activation from GaAs:Zn nanoclusters for p-type conduction in ZnSe
Authors: Suemune I. , Hirose J. , Ueta A.
Published: July 2002

Growth of p-type ZnSe has been limited to nitrogen doping in molecular-beam epitaxy. As an alternative to nitrogen doping, GaAs cluster doping is proposed in this letter, where GaAs has small lattice mismatch of 0.28% to ZnSe and can potentially be heavily doped in p type. Hole activation to the valence band of the ZnSe layers could be observed by the reduction of the thickness of the GaAs layers in order to form nanoclusters. This was achieved with alternate supplies of triethylgallium and trisdimethylaminoarsenic, and the net acceptor concentration of ∼1017 cm−3 was observed.
A study of Ar+ laser-assisted Si doping of GaAs by chemical beam epitaxy
Authors: Shi B. Q. , Tu C. W.
Published: July 2002

Doping results from Ar+ laser-assisted chemical beam epitaxy with triethylgallium, tris(dimethylamino) arsenic, and silicon tetrabromide precursors are reported. Enhancements in the n-type doping concentration are observed with laser irradiation in the substrate–temperature range 390–500 °C. With a 300 W/cm2 irradiation power density, a 70-fold increase in the carrier concentration is obtained at 390 °C substrate temperature. An expression for doping concentration is derived to assess the contribution of laser-induced thermal heating to the observed doping increase. Effects of photogenerated carriers on surface decomposition of metalorganic species are discussed. We postulate that the dramatic increase in carrier concentration at low growth temperatures is due to photolysis of silicon tetrabromide.
Phonon spectrometry with a bolometer based on spin-lattice relaxation
Authors: Witowski A. M. , Moll H.-P. , Sadowski M. L. , Wyder P. , Karczewski G. , Kossut J. , Wojtowicz T.
Published: July 2002

We discuss the possibility of a bolometer based on the effect of spin-lattice relaxation detected by a pickup coil. The system is composed of an epitaxial layer of CdMnTe on a GaAs substrate, grown by molecular beam epitaxy (MBE). The “phonon wave,” generated by a light pulse on the back surface of the substrate, changes the spin alignment in the MBE layer, thus also changing its magnetization. This change is detected by a pickup coil. The possible use of such a system as a bolometer and “phonon spectrometer” is discussed.
Noise and conversion properties of Y–Ba–Cu–O Josephson mixers at operating temperatures above 20 K
Authors: Harnack Oliver , Darula Marian , Beuven Stephan , Kohlstedt Hermann
Published: July 2002

We have measured the noise performance and conversion efficiency of Y–Ba–Cu–O bicrystal Josephson mixers at operating temperatures between 20 and 60 K and at operating frequencies around 90 GHz. A double-sideband mixer noise temperature of about 1600 K and a conversion efficiency of −10 dB at 20 K operating temperature has been measured using the Y-factor method. The absorbed local oscillator power was in the range of 10 nW. The dependence of the mixer performance on the normalized frequency Ω and the fluctuation parameter Γ has been studied. In accordance with the resistively shunted junction model, the experimental data show the presence of excess noise. The temperature dependence of the mixer noise temperature can be explained by the variation of the linewidth of the Josephson oscillations with the operating temperature.
Carrier transport in GaAs/AlAs type-II superlattices under electric field: Switch from X–X to Γ–Γ transfer
Authors: Hosoda M. , Ohtani N. , Kuroyanagi K. , Domoto C.
Published: July 2002

A switch in carrier transport from X–X to Γ–X–Γ is found in a GaAs/AlAs type-II superlattice under an electric field. This phenomenon is caused by an X–Γ transfer, as demonstrated by the photoluminescence, photocurrent response, and current–voltage characteristics. Under a high electric field, most of the electrons flow through the Γ path even in type-II superlattices.
Polycrystalline silicon/metal stacked gate for threshold voltage control in metal–oxide–semiconductor field-effect transistors
Authors: Polishchuk Igor , Hu Chenming
Published: July 2002

A stack structure for the gate electrode of metal–oxide–semiconductor (MOS) transistors is proposed, analyzed, and simulated. The stack consists of a very thin polycrystalline silicon (polysilicon) layer and metal. By changing the thickness of the polysilicon layer, one can change the effective work function of the gate. Thus, the stacked-gate structure allows for a method to continuously adjust MOS field-effect transistor threshold voltage through gate work-function engineering while retaining the proven SiO2/polysilicon interface.
Quantitative determination of the order parameter in epitaxial layers of ZnSnP2
Authors: Francoeur S. , Seryogin G. A. , Nikishin S. A. , Temkin H.
Published: July 2002

X-ray diffraction is applied to determine the degree of order in partially ordered epitaxial layers of ZnSnP2 grown on GaAs. The Bragg–Williams order parameter, used as a scaling coefficient for the structure factor of superstructure reflections, is extracted from the comparison of measured and calculated relative intensities of a set of carefully chosen reflections. The calculated diffraction patterns are obtained from the dynamical theory of x-ray diffraction. The effect of antiphase domains on the width of superstructure reflections is discussed. Order parameters up to 30% were measured.
Dielectric and electret properties of nanoemulsion spin-on polytetrafluoroethylene films
Authors: Schwödiauer Reinhard , Neugschwandtner Gerhard S. , Bauer-Gogonea Simona , Bauer Siegfried , Rosenmayer Tom
Published: July 2002

Nanoemulsion polytetrafluoroethylene (nano-PTFE) is a promising low-dielectric-constant polymer, which can be prepared in thin-film form by spin-on techniques. Similarities and differences of the nano-PTFE material to the well-known class of PTFE polymers with respect to morphology and dielectric and electret characteristics are discussed. Nano-PTFE shows excellent dielectric properties up to 300 °C, and a charge stability that compares favorably with other members of the PTFE class. Nano-PTFE enlarges the family of PTFE materials and may become interesting for applications in microelectronics and miniaturized electret devices.
Self-trapping of negative ions due to electron detachment in the afterglow of electronegative gas plasmas
Authors: Kaganovich I. D. , Ramamurthi B. N. , Economou Demetre J.
Published: July 2002

The spatiotemporal evolution of charged species densities and wall fluxes during the afterglow of an electronegative discharge has been investigated. It was found that plasma decay crucially depends on the product of negative-ion-detachment frequency (γd) and diffusion time τd. If γdτd>2, negative ions convert to electrons during their diffusion towards the walls. The presence of detached electrons results in “self-trapping” of the negative ions, due to emerging electric fields, and the negative-ion flux to the walls is extremely small. Thus, negative ions can be extracted in the afterglow only if γdτd<2.
Random telegraph signals and low-frequency noise in n-metal–oxide–semiconductor field-effect transistors with ultranarrow channels
Authors: Bu H. M. , Shi Y. , Yuan X. L. , Wu J. , Gu S. L. , Zheng Y. D. , Majima H. , Ishikuro H. , Hiramoto T.
Published: July 2002

The characteristics of low-frequency noise in n-metal–oxide–semiconductor field-effect transistors with ultranarrow channels have been investigated through random telegraph signals and low-frequency noise spectroscopy. Random telegraph signals with very large amplitude (∼70%) are observed in weak inversion at room temperature. Low-frequency noise spectra having both 1/fn and Lorentzian type are found separately in the same channel at various gate bias voltages. The observations strongly suggest that the low-frequency noise is dominated by carrier mobility fluctuation in weak inversion and by carrier number fluctuation under high-field conditions in an ultranarrow channel.
Ultranarrow polaritons in a semiconductor microcavity
Authors: Jensen J. R. , Borri P. , Langbein W. , Hvam J. M.
Published: July 2002

We have achieved a record high ratio (19) of the Rabi splitting (3.6 meV) to the polariton linewidth (190 μeV), in a semiconductor λ microcavity with a single 25 nm GaAs quantum well at the antinode. The narrow polariton lines are obtained with a special cavity design which reduces the exciton broadening due to scattering with free charges and has a very low spatial gradient of the cavity resonance energy. Since the static quantum-well disorder is very small, the polariton broadening is dominantly homogeneous. Still, the measured linewidths close to zero detuning cannot be correctly predicted using the linewidth averaging model.
Direct response of twin-slot antenna-coupled hot-electron bolometer mixers designed for 2.5 THz radiation detection
Authors: Ganzevles W. F. M. , Swart L. R. , Gao J. R. , de Korte P. A. J. , Klapwijk T. M.
Published: July 2002

We measure the direct response of a Nb diffusion-cooled hot-electron bolometer mixer in a frequency range between 0.5 and 3.5 THz. The mixer consists essentially of a twin-slot antenna, a co-planar waveguide transmission line and a Nb superconducting bridge. It is designed for use in receivers with astronomical and atmospherical applications around 2.5 THz. We calculate the impedance of the antenna, the transmission line, and the bridge separately using models which are developed for frequencies below 1 THz and predict the direct response of the mixer. We demonstrate that these models can be applied to much higher frequencies. However, the measured central frequency is 10%–15% lower than predicted.
Wavelength switching by positively detuned side-mode injection in semiconductor lasers
Authors: Hong Y. , Shore K. A. , Lawrence J. S. , Kane D. M.
Published: July 2002

The experimental observation of a side-mode resonance regime in intermodal injection of a Fabry–Pérot semiconductor laser is reported. It is shown that wavelength switching can be easily affected by the use of positively detuned side-mode injection. Nearly degenerate four-wave mixing is observed in both positive and negative detuning regimes.
Spectral responsivity of single-quantum-well photodetectors
Authors: Vitusevich S. A. , Förster A. , Reetz W. , Lüth H. , Belyaev A. E. , Danylyuk S. V.
Published: July 2002

The photoresponsivity spectra of double-barrier resonant tunneling diodes have been measured in a wide range of light wavelength as well as applied voltage. The complex behavior of measured spectra is analyzed taking into account different channels for electron injection into the quantum well (QW). It has been shown that the photoresponse in the infrared wavelength range could arise not only from the electrons excited inside the QW, but also from the two-dimensional electrons confined on the quantum level in the spacer accumulation layer.
Ultrafast optical characterization of carrier capture times in InxGa1−xN multiple quantum wells
Authors: Özgür Ü. , Bergmann M. J. , Casey H. C. , Everitt H. O. , Abare A. C. , Keller S. , DenBaars S. P.
Published: July 2002

Subpicosecond wavelength-degenerate differential transmission optical spectroscopy was used to characterize the electron capture time in a 10-period InxGa1−xN multiple-quantum-well (MQW) structure. Photoluminescence and photoluminescence excitation spectroscopies demonstrated enhanced MQW emission for injection within ±50 meV of the barrier energy. Time-resolved differential transmission measurements for excitation in this region reveal efficient electron capture in the quantum wells with a time constant between 310 and 540 fs. A slower exponential relaxation, with strongly wavelength-dependent subnanosecond decay constants, is also observed.
Crosstalk canceling for laser-assisted magnetic recording
Authors: Hamamoto M. , Kojima K. , Sato J. , Katayama H. , Murakami Y.
Published: July 2002

A promising method of crosstalk canceling is proposed for laser-assisted magnetic recording, which makes it possible to record and read with a narrow track pitch limited to nearly the size of a focused laser spot. In this method, a track pitch narrower than the laser spot size is obtained by utilizing the canceling of the leakage flux from an adjacent track. We achieved a crosstalk of −24 dB or less with a 0.7 μm track pitch in laser-assisted magnetic recording by using a laser spot size of 1.07 μm in diameter and a magnetoresistive head with a track width of 1.4 μm.
Random telegraph signals of tetrahedral-shaped recess field-effect transistor memory cell with a hole-trapping floating quantum dot gate
Authors: Shima Masashi , Sakuma Yoshiki , Awano Yuji , Yokoyama Naoki
Published: July 2002

An AlGaAs/InGaAs heterojunction field-effect transistor (FET) memory cell in a tetrahedral-shaped recess (TSR) on the (111)B GaAs substrate was fabricated and investigated. The TSR–FET memory cell has a channel on the (111)A facet surfaces of the recess and a hole-trapping quantum dot (QD) as a floating gate at the bottom. Memory operations were achieved at temperatures up to 130 K, and random telegraph signals (RTSs) with a temperature dependence were observed in the retention characteristics. After our analysis of RTSs, the activation energy of hole capture and emission processes in the TSR QD were estimated to be 260 and 190 meV, respectively.
Photoelectric properties of the 0.44 eV deep level-to-band transition in gallium nitride investigated by optical admittance spectroscopy
Authors: Krtschil A. , Witte H. , Lisker M. , Christen J. , Krost A. , Birkle U. , Einfeldt S. , Hommel D. , Scholz F. , Off J. , Stutzmann M.
Published: July 2002

In GaN layers grown by molecular beam epitaxy and metalorganic vapor phase epitaxy on c-axis oriented sapphire, a defect-to-band transition at a photon energy of 0.44 eV was found by optical admittance spectroscopy. This transition was investigated as a function of temperature and modulation frequency. The height of the corresponding optical admittance peak shows a thermally activated quenching with an activation energy of 0.4±0.1 eV caused by a thermal carrier emission from the same defect state to the conduction band at higher temperatures. Based on this thermal quenching, the 0.44 eV level is assigned to an electron trap located in the upper half of the gap. The spectral photoionization cross section was determined, resulting in a photoionization energy at 80 K estimated to be below 0.425 eV. The omnipresence of the 0.44 eV electron trap in GaN layers grown by various epitaxial techniques and in different reactors implicates its intrinsic nature.
Correlation of mobile and fixed charge creation in protonated field-effect transistors
Authors: Devine R. A. B. , Vanheusden K. , Herrera G. V.
Published: July 2002

Protons have been introduced into the 40 nm gate oxides of n-channel metal–oxide–semiconductor field-effect transistors, resulting in the creation of mobile and fixed positive charge. Transistor gate lengths in the range from 5 to 70 μm have been studied. Hysteresis in the threshold voltage as large as −18 V has been measured. An inverse linear relationship has been found between the quantity of mobile and fixed charge generated. The inversion channel electron mobility is found to lie in the range 310–530 cm2 V−1 s−1.
Simultaneous bimodal surface acoustic-wave velocity measurement by scanning acoustic force microscopy
Authors: Behme G. , Hesjedal T.
Published: July 2002

We present scanning acoustic force microscopy (SAFM) mixing experiments of differently polarized surface acoustic waves (SAW) with noncollinear propagation directions. The phase velocities of the SAWs are measured at a submicron lateral scale, employing a multimode SAFM that is capable of detecting the wave’s normal and in-plane oscillation components. Hereby, the down conversion of the surface oscillations into cantilever vibrations due to the nonlinearity of the tip–sample interaction is utilized. The simultaneous determination of the phase velocities within a microscopic sample area is demonstrated for the mixing of Rayleigh and Love waves on the layered system SiO2/ST-cut quartz.
“Composite particles” and the eigenstates of Calogero–Sutherland and Ruijsenaars–Schneider
Authors: Bergère M. C.
Published: July 2002

We establish a one-to-one correspondence between the “composite particles” with N particles and the Young tableaux with at most N rows. We apply this correspondence to the models of Calogero–Sutherland and Ruijsenaars–Schneider and we obtain a momentum space representation of the “composite particles” in terms of creation operators attached to the Young tableaux. Using the technique of bosonization, we obtain a position space representation of the “composite particles” in terms of products of vertex operators. In the special case where the “composite particles” are bosons and if we add one extra quasiparticle or quasihole, we construct the ground state wave functions corresponding to the Jain series ν=p/(2np±1) of the fractional quantum Hall effect.
The influence of radiative heat transfer upon the collapse of a cylindrical shock wave
Authors: Hirschler T. , Gretler W.
Published: July 2002

We present the effects of thermal radiation upon the classical self-similar solution of the collapse of cylindrical shock waves in inviscid, ideal gas with a constant ratio of specific heats. The gas is considered to be optically thick and the shock front to be opaque. The ambient gas ahead of the shock front is assumed to be uniform and at rest and its counterpressure concerning the motion of the shock front is neglected. In order to investigate the deviation from the classical adiabatic solution caused only by a small amount of radiation, the shock front in the nonadiabatic case fulfills the same conditions as the shock front in the adiabatic case, except for the fact that in the nonadiabatic case heat is released at the shock front. For different adiabatic exponents self-similar solutions have been obtained determining the eigenvalues and the profiles of the flow variables. The influence of radiation is shown through variation of the mean free path of radiation.
Population inversion in an optically pumped single quantum well
Authors: Green T. J. , Xu W.
Published: July 2002

An optically pumped intersubband laser generator is proposed in which the continuum states above an Al0.2Ga0.8As–GaAs–Al0.2Ga0.8As single quantum well with a width of L=17 nm serve as the highest level in a four-level laser system. The design allows much greater flexibility in the choice of pumping source and simplifies considerably the device fabrication. We have obtained the electronic subband structure of the proposed device and utilized a simple rate equation approach to examine the electron density in different states under optical pumping.
Gallium arsenide fast photodetectors: Selection criteria and predicted response
Authors: Foulon F. , Bergonzo P. , Brullot B. , Rubbelynck C. , Flicstein J. , Petit P.
Published: July 2002

The main characteristics (sensitivity, response time, dynamic range) of a photoconductive detector (PCD) depend on the intrinsic physical (lattice parameter, extended and local defects) and electrical properties (resistivity, mobility, carrier lifetime) of the material. Selection criteria for semi-insulating undoped gallium arsenide (GaAs) were thoroughly examined for their capacity to select the more suitable material for the fabrication of fast PCDs, used in gamma-ray pulse (E∼1.2 MeV, τFWHM=30 ns) metrology. An analytical model, which assumes that carrier recombination takes place through EL2 type recombination centers [M. O. Manasreh and B. C. Covington, Phys. Rev. B 35, 2524 (1987)], was developed. Taking into account the initial physical and electrical properties of the GaAs material, the model accurately predicts the response of the PCDs after a neutron preirradiation step (dose range: 5×1014 to 1×1016 neutrons/cm2), which was used to optimize the PCD characteristics and in particular to reduce the response time to values lower than 100 ps. This led to the identification and validation of key parameters of the initial properties of single crystal GaAs materials influencing the PCDs final characteristics. Thus, depending on the user defined specifications, GaAs material initial properties and neutron preirradiation dose can be fixed to fabricate GaAs PCDs exhibiting the desired response time, sensitivity, and dynamic range. The study established that the PCDs characteristics are controlled only by the initial GaAs material properties when neutron preirradiation doses remain below 1×1015 neutrons/cm2, whereas they are mainly controlled by neutron induced crystal defects above this dose.