bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoreflectance of GaSb/Al0.4Ga0.6Sb single quantum wells
Authors: Geddo M. , Ferrini R. , Patrini M. , Franchi S. , Baraldi A. , Magnanini R.
Published: July 2002

We report a photoreflectance study conducted in the 0.7–1.2 eV photon energy range and at temperatures from 80 to 300 K of GaSb/Al0.4Ga0.6Sb single quantum wells grown by molecular beam epitaxy. We observed clear and well-resolved structures, which could be attributed to the interband optical transitions originating in both the GaSb buffer and the quantum wells, and which could be fitted by standard critical-point line shapes. Our results demonstrate that even unintentionally doped GaSb-based quantum systems can be studied and characterized by photoreflectance, especially at low temperatures.
Changes in the electronic structure of CuInS2 thin films by Na incorporation
Authors: Fukuzaki Koichi , Kohiki Shigemi , Yoshikawa Hideki , Fukushima Sei , Watanabe Takayuki , Kojima Isao
Published: July 2002

For Na-free and Cu-deficient CuInS2 thin films with chalcopyrite structure, electron spectra of core levels and valence band were similar to each other in spite of a difference in Cu deficiency. Incorporation of Na with the CuInS2 films caused peak shifts by −0.5 eV for the core levels without changes in both linewidth and shape. The electron density of states, deeper by 1–5 eV than that from Cu d–S p hybridization centered at around 2.5 eV, increased by the Na incorporation, and increments in the electron density of states were larger for the films with larger the Cu deficiency. The Na incorporation yielded a surface layer expressed as (Na,Cu)InS2 on the Cu deficient CuInS2 films.
Enhancement of the interband Stark effects in strained CdxZn1−xTe/ZnTe coupled double quantum wells
Authors: Kim T. W. , Lee K. H. , Park H. L.
Published: July 2002

The effects of an electric field on the interband transitions in CdxZn1−xTe/ZnTe coupled double quantum wells have been investigated both experimentally and theoretically. Photoluminescence (PL) measurements have been performed to investigate the excitonic transitions in CdxZn1−xTe/ZnTe coupled double quantum wells. Transmission electron microscopy images show that a 35-Å Cd0.18Zn0.82Te quantum well and a 50-Å Cd0.18Zn0.82Te quantum well are separated by a 35-Å Cd0.1Zn0.9Te potential barrier. PL spectra at 300 K show the excitonic transitions. When an electric field is applied to a coupled double quantum well, the Stark shift of the interband transition energy in the CdxZn1−xTe/ZnTe coupled double quantum wells is much more sensitive to the applied electric field than that of the Cd0.1Zn0.9Te/ZnTe single quantum wells. The electronic subband energies and energy wave functions in the quantum wells are calculated by an envelope function approximation, which takes into account the strain effects. These results indicate that CdxZn1−xTe/ZnTe coupled double quantum wells hold promise for potential applications in optoelectronic devices, such as new types of modulators and tunable lasers.
Strain-dependent morphology of spontaneous lateral composition modulations in (AlAs)m(InAs)n short-period superlattices grown by molecular beam epitaxy
Authors: Norman A. G. , Ahrenkiel S. P. , Moutinho H. , Al-Jassim M. M. , Mascarenhas A. , Mirecki Millunchick J. , Lee S. R. , Twesten R. D. , Follstaedt D. M. , Reno J. L. , Jones E. D.
Published: July 2002

The nature of spontaneous lateral composition modulation and its relationship to surface morphology during the growth of (AlAs)m(InAs)n short-period superlattices by molecular beam epitaxy are investigated as a function of the global strain between the short-period superlattice and (001)InP substrate. For samples grown under tension, transmission electron and atomic force microscopy reveal composition modulations along directions close to 〈310〉 coupled to a surface cusping. For samples grown under compression, we observe composition modulations roughly along the elastically soft 〈100〉 directions coupled to a surface rippling. For high strains (⩾0.7%), with individual InAs layer thicknesses ⩽1.6 monolayers, we observe weak or no composition modulations.
Exciton transport into a single GaAs quantum wire studied by picosecond near-field optical spectroscopy
Authors: Richter A. , Süptitz M. , Heinrich D. , Lienau Ch. , Elsaesser T. , Ramsteiner M. , Nötzel R. , Ploog K. H.
Published: July 2002

We report a time-resolved near-field luminescence study of excitonic real-space transfer into single GaAs quantum wires. Excitons generated by local optical excitation in a 250 nm spot undergo diffusive transport over a length of several microns and are subsequently trapped into the quantum wire by optical phonon emission. Local energy barriers in the vicinity of the quantum wire, originating from the epitaxial growth mechanism of the nanostructure, directly influence the real-space transfer dynamics and trapping efficiency.
Multisubband hot-electron transport in GaN-based quantum wells
Authors: Zakhleniuk N. A. , Bennett C. R. , Ridley B. K. , Babiker M.
Published: July 2002

A simple rigorous analytical theory of two-dimensional (2D) nonequilibrium electrons occupying an arbitrary number of subbands in a quantum well is developed. The electric-field dependence of electron mobility and the average kinetic energy for AlN/GaN quantum wells are presented. At temperatures below 200 K the electron mobility is controlled mainly by the acoustic phonon scattering and it is a nonmonotonous function of the electric field, which has a maximum. At room and higher temperatures the interaction with both acoustic and polar optical phonons determine the hot-electron mobility and it depends very weakly on the electric field. Both the mobility and average energy of 2D electrons are smaller than that for three-dimensional (3D) electrons in the bulk semiconductor. Our theory provides a self-consistent transition from the 2D to the 3D regime of electron transport with increasing electric field accompanied by the occupation of an increasingly large number of subbands by the electrons.
Amplified resonant Raman scattering in conducting polymer thin films
Authors: Shkunov M. N. , Gellermann W. , Vardeny Z. V.
Published: July 2002

Using picosecond pulsed laser excitation, we investigate the optical emission characteristics of poly(2,5-dioctyloxy-p-phenylenevinylene), (DOO-PPV), thin films at high excitation intensities (∼1–90 MW/cm2). We observe the presence of amplified resonance Raman scattering in the emission spectra of conducting polymer films. The effect results in sharp Raman lines (widths smaller than 3 Å) superimposed on a significantly broader, well known, spectrally narrowed emission band (width ∼10 nm) caused by the amplified spontaneous emission in the waveguided polymer film. At the highest used excitation intensities, Raman scattering dominates the DOO-PPV emission spectrum resulting in a highly monochromatic, single-line emission spectrum.
Formation of n–p–n structure on an insulating SrTiO3 single crystal using excimer laser irradiation
Authors: Yoon Jong-Won , Miyayama Masaru
Published: July 2002

An n–p–n semiconducting structure, similar to a p-type grain boundary between n-type grains in ceramics, was realized on an insulating SrTiO3 single crystal by using CuO diffusion and KrF excimer laser irradiation. P- and n-type semiconductivity was confirmed by Hall coefficient measurements for the laser-irradiated surface with and without CuO diffusion, respectively. Laser irradiation on the surface including a CuO-diffused line (1 μm width) gave an n–p–n structure, which showed nonlinear I–V characteristics with a nonlinear coefficient of 7. The present fabrication process is suggested to be effective for developing new oxide microelectronic devices as well as for fundamental study on grain-boundary functions of semiconducting ceramics.
Investigation of excess carrier diffusion in nitride semiconductors with near-field optical microscopy
Authors: Vertikov Andrey , Ozden Ilker , Nurmikko Arto V.
Published: July 2002

We describe a high-spatial-resolution optical technique to study transport properties in semiconductors, applicable especially to heterostructures characterized by short-carrier diffusion lengths on the 100 nm scale. The method involves spatial near-field optical imaging of photoluminescence profiles created by an interference grating within a total internal reflection configuration. We illustrate the method by applying it as a diagnostic tool to a contemporary problem, namely, to acquire insight into electron–hole pair recombination in InGaN, a blue light-emitting medium exhibiting pronounced nonrandom alloy characteristics.
High-speed >90% quantum-efficiency p–i–n photodiodes with a resonance wavelength adjustable in the 795–835 nm range
Authors: Özbay Ekmel , Kimukin Ïbrahim , Biyikli Necmi , Aytür Orhan , Gökkavas Mutlu , Ulu Gökhan , Ünlü M. Selim , Mirin Richard P. , Bertness Kris A. , Christensen David H.
Published: July 2002

We report GaAs/AlGaAs-based high-speed, high-efficiency, resonant cavity enhanced p–i–n photodiodes. The devices were fabricated by using a microwave-compatible fabrication process. By using a postprocess recess etch, we tuned the resonance wavelength from 835 to 795 nm while keeping the peak efficiencies above 90%. The maximum quantum efficiency was 92% at a resonance wavelength of 823 nm. The photodiode had an experimental setup-limited temporal response of 12 ps. When the system response is deconvolved, the 3 dB bandwidth corresponds to 50 GHz, which is in good agreement with our theoretical calculations.
Optically pumped (GaIn)As/Ga(PAs) vertical-cavity surface-emitting lasers with optimized dynamics
Authors: Ellmers C. , Hofmann M. R. , Karaiskaj D. , Leu S. , Stolz W. , Rühle W. W. , Hilpert M.
Published: July 2002

We present a vertical-cavity surface-emitting laser structure optimized for fast intrinsic emission dynamics, using the strain-compensated (GaIn)As/Ga(PAs) material system with a 2λ sin-type cavity. The high quality of the epitaxial growth is revealed by the large normal mode splitting of 10.5 meV found in reflectivity measurements. The fast dynamical response of our structure after femtosecond optical excitation at 30 K yields a pulse width of 3.2 ps and a peak delay of only 4.8 ps. A structure designed for laser emission at higher temperatures exhibits picosecond dynamics at room temperature.
Cation exchange: A scheme for synthesis of mercury-based high-temperature superconducting epitaxial thin films
Authors: Wu J. Z. , Yan S. L. , Xie Y. Y.
Published: July 2002

A cation-exchange process has been developed for growth of high-quality epitaxial thin films of highly volatile mercury-based high-temperature superconductors. By selecting epitaxial precursor matrices of TlyBa2CaCu2Ox (y=1,2) and annealing them in Hg vapor, c-axis-oriented epitaxial HgBa2CaCu2O6+δ thin films with superior quality were formed through Tl and Hg-cation exchange. At 110 K, these films can carry supercurrent density close to 1 MA/cm2 which is nearly an order of magnitude higher than the best reported previously. This cation-exchange technique also provides a general scheme for synthesis of other volatile compounds with predesigned structure and composition.
Substantial contribution of effective mass variation to electron- acoustic phonon interaction via deformation potential in semiconductor nanostructures
Authors: Pipa V. I. , Mitin V. V. , Stroscio M.
Published: July 2002

Using the approach of deformed ions and the tight binding, we have demonstrated that the interaction of electrons confined in a nanostructure with acoustic phonons in a cubic crystal is described by a deformation potential tensor (DPT) whose symmetry is determined by the geometry of the nanostructure. Here in, we present additional contribution to the DPT which is caused by the deformation dependence of the electron effective mass and it increases as L−2 when the characteristic size of a nanostructure, L, decreases. For narrow GaAs-based quantum wells, this contribution is comparable with and can overcome that from the usual deformation potential coupling.
Enhancement of hard x-ray emission from a copper target by multiple shots of femtosecond laser pulses
Authors: Hironaka Yoichiro , Fujimoto Yasushi , Nakamura Kazutaka G. , Kondo Ken-ichi , Yoshida Masatake
Published: July 2002

The x-ray generation from a copper disk target by irradiation of femtosecond laser pulses (42 fs, 4T W in maximum) has been investigated. The intensity of the x rays is enhanced about 100 times by the multiple laser shots on the target. A temporal profile of the x rays consists of two components: a short pulse with a time duration of 6 ps and a pulse with exponential decay. The enhancement of the x rays corresponds to the increase of the short-pulse component. A relationship between the intensity enhancement and morphology of the irradiated target is discussed.
Polarization switching of lithium niobate with giant internal field
Authors: Peng L.-H. , Fang Y.-C. , Lin Y.-C.
Published: July 2002

We report a high-field analysis on the sidewise 180° domain motion in congruent-grown Z-cut lithium niobate (LiNbO3) crystals. Polarization switching in reverse poling is found to take place at a smaller field strength (E), exhibit a larger switching current (is), and substantiate a faster switching time (ts) compared with the forward poling case. Such an axial anisotropy is correlated to a defect-induced internal field Eint in the bulk and has a strength of 2.37 kV/mm and pointing along the crystal c axis. In the high-field regime ranging from 19 to 24 kV/mm, a linear dependence of is and 1/ts on E is resolved. These observations are ascribed to a fast nucleation process followed by a sidewise 180° domain expansion, whose velocity can be characterized according to νs=μs[E−(Ec±Eint)], where the threshold field Ec and mobility μs measure a strength of 18.74 kV/mm and 1.6±0.1 mm2/kV s, respectively.
Sensitization of Er3+ emission at 1.5 μm in SiO2 thermally grown on silicon by coimplantation of Yb
Authors: Kozanecki A. , Homewood K. , Sealy B. J.
Published: July 2002

Effects of Yb codoping on the photoluminescence of Er3+ ions at room temperature in SiO2 films thermally grown on silicon are investigated. We demonstrate that for an excitation wavelength of 488 nm Yb ions act as efficient sensitizers of the I413/2–4I15/2 emission of Er3+ ions. We have found that for the fixed dose of Yb the Er3+ intensity is directly proportional to the Er concentration. Enhancement by a factor of 15 due to Yb codoping has been observed. A model of the mechanism responsible for sensitization is discussed.
Boron pileup and clustering in silicon-on-insulator films
Authors: Vuong H.-H. , Gossmann H.-J. , Pelaz L. , Celler G. K. , Jacobson D. C. , Barr D. , Hergenrother J. , Monroe D. , Venezia V. C. , Rafferty C. S. , Hillenius S. J. , McKinley J. , Stevie F. A. , Granger C.
Published: July 2002

The dopant-defect interaction in silicon-on-insulator (SOI) material is studied for Si film thicknesses ranging from 60 to 274 nm, with regards to (1) boron pileup and (2) defect-induced boron clustering. Results are obtained on boron-implanted samples and on molecular beam epitaxy-grown deposited-boron samples. The experimental results verify simulations predicting (a) boron pileup at both upper and lower interfaces of the Si film, and (b) no reduction of the boron clustering in SOI compared with bulk silicon.
Determination of the energy-dependent conduction band mass in SiO2
Authors: Ludeke R. , Cartier E. , Schenk Andreas
Published: July 2002

The energy dependence of the conduction band mass in amorphous SiO2 was deduced from quantum interference oscillations in the ballistic electron emission microscope current, and separately from Monte Carlo simulations of the electron mean free paths obtained by internal photoemission. The results imply a strong nonparabolicity of the conduction band of SiO2.
Lithium tetra-(8-hydroxy-quinolinato) boron for blue electroluminescent applications
Authors: Tao X. T. , Suzuki H. , Wada T. , Sasabe H. , Miyata S.
Published: July 2002

We report the optical and electroluminescent properties of the organometallic complex lithium tetra-(8-hydroxy-quinolinato) boron (LiBq4). The complex was prepared by the reaction of lithium borohydride with 8-hydroxyquinoline and shows absorption cutoff wavelength of 410 nm and bright blue photoluminescence with a peak wavelength at 466 nm. The ionization potential of LiBq4 was determined to be ∼5.6 eV by a photoemission apparatus. Electroluminescence (EL) properties of three-layer light-emitting diodes (LED) of ITO/PVK:TPD/LiBq4/Alq3/Mg with LiBq4 as the active layer were characterized. The EL spectra of the three-layer device are similar to the PL spectrum of the single layer LiBq4 but with peak wavelength at 475 nm. The results indicate that boron complexes are potential blue emitters for LED applications.
A technique for evaluating optical confinement in GaN-based lasing structures
Authors: Bidnyk S. , Little B. D. , Song J. J. , Schmidt T. J.
Published: July 2002

We present a technique for evaluating optical confinement in GaN-based lasing structures by studying their spectrally resolved near-field pattern under high optical excitation. Emission spectra were found to be strongly dependent on the position of the collection optics relative to the active region when the sample was excited above the lasing threshold. The spatially resolved spectra contain a modulation signature that can be used to deduce the optical confinement characteristics. We show that the observed index-guided modes result from multiple internal reflections at angles very close to the critical angle for total internal reflection between the semiconductor layers with different refractive indices. This technique was used to evaluate the degree of optical confinement in GaN epilayers and GaN/AlGaN separate confinement heterostructures. The implications of this study on the design of GaN-based laser diodes are discussed.
Degradation of ferroelectric SrBi2Ta2O9 materials under reducing conditions and their reaction with Pt electrodes
Authors: Shimakawa Y. , Kubo Y.
Published: July 2002

The degradation of ferroelectric SrBi2Ta2O9 in an H2-containing reducing atmosphere and its recovery by subsequent annealing in oxygen have been reproduced experimentally by using bulk ceramic samples. SrBi2Ta2O9 decomposes into Bi metal and Sr–Ta oxide(s) in H2 above 300 °C, and the decomposition rate increases as the annealing temperature rises. The recovery of SrBi2Ta2O9 is achieved through oxidation of Bi metal into β-Bi2O3, followed by a reaction of β-Bi2O3 and Sr–Ta oxide(s). In the presence of Pt, which is usually used as an electrode, the decomposition of SrBi2Ta2O9 is accelerated. Bi–Pt alloy and Bi–Pt oxide are produced under the reducing and oxidizing conditions, respectively. Our findings have important implications concerning the degradation of SrBi2Ta2O9 capacitors and the reaction between SrBi2Ta2O9 materials and Pt electrodes during thin-film device fabrication.
Interdiffusion of high-Sn/high-Pb (SnPb) solders in low-temperature flip chip joints during reflow
Authors: Zuruzi A. S. , Chiu C.-h. , Chen W. T. , Lahiri S. K. , Tu K. N.
Published: July 2002

We carried out experiments and numerical simulations to investigate the transport of Sn in a composite solder joint, comprising of high-Pb and high-Sn (SnPb) alloys, in a chip-composite solder-organic substrate package during the reflow process. Both the experimental and simulation results demonstrate that surface diffusion causes the transport of Sn on the surface to be faster than that inside the solder joint. Surface diffusion also accelerates the homogenization process of the composite solder joint.
Influence of Pt heterostructure bottom electrodes on SrBi2Ta2O9 thin film properties
Authors: Kim Seung-Hyun , Kim D. J. , Maria J.-P. , Kingon A. I. , Streiffer S. K. , Im J. , Auciello O. , Krauss A. R.
Published: July 2002

The properties of SrBi2Ta2O9 (SBT) films, such as remanent polarization and leakage current density, are closely related to the film/electrode interface and surface roughness of the underlying electrode. SBT films grown on stable Pt/TiO2/SiO2/Si and Pt/ZrO2/SiO2/Si substrates exhibit high remanent polarization, low leakage current density, and low voltage saturation as compared to SBT films synthesized on Pt/Ti/SiO2/Si. It is shown that severe diffusion of Ti from the Ti interlayer onto the surface of the Pt bottom electrode and the increased surface roughness of this electrode stack play key roles in degradation of SBT properties.
Near-field spectroscopy of selectively oxidized vertical cavity surface emitting lasers
Authors: Kim J. , Boyd J. T. , Jackson Howard E. , Choquette K. D.
Published: July 2002

Selectively oxidized vertical cavity surface emitting lasers (VCSELs) have been studied by spectrally resolved near-field scanning optical microscopy. We have obtained spatially and spectrally resolved images of both subthreshold emission and lasing emission from a selectively oxidized VCSEL operating at a wavelength of 850 nm. Below threshold, highly local high gain regions, emitting local intensity maxima within the active area, were observed; these were found to serve as lasing centers just above threshold. Above threshold, the near-field spatial modal distributions of low order transverse modes were identified by spectrally analyzing the emission; these were found to be complex and somewhat different from those measured in the far field.
Characterization of InGaN thin films using high-resolution x-ray diffraction
Authors: Görgens L. , Ambacher O. , Stutzmann M. , Miskys C. , Scholz F. , Off J.
Published: July 2002

Wurtzite InGaN thin films grown by metalorganic chemical vapor deposition on sapphire substrates with and without GaN buffer layers are investigated by high-resolution x-ray diffraction measurements. The structural quality, lattice constants, strain, and indium composition of 100 nm thick films with In concentrations up to 33% are evaluated by measuring symmetric (00.2) and asymmetric (20.5) reflexes. The quality of the InGaN layers with widely different biaxial stress is measured and compared. An analytical solution for the determination of the In content of strained epitaxial layers is introduced. The results show that neglecting the strain can result in a severe miscalculation of the In concentration.
Exciton spin polarization in magnetic semiconductor quantum wires
Authors: Ray O. , Sirenko A. A. , Berry J. J. , Samarth N. , Gupta J. A. , Malajovich I. , Awschalom D. D.
Published: July 2002

Electron-beam lithography and wet etching techniques are used to laterally pattern ZnSe/(Zn,Cd,Mn)Se single quantum wells into magnetically active quantum wires with widths ranging from 20 to 80 nm. Photoluminescence spectroscopy as a function of wire width reveals a competition between elastic strain relaxation and quantum confinement. Magnetophotoluminescence measurements at low temperatures indicate a strong exciton spin polarization due to the sp–d exchange-enhanced spin splitting, ranging from 20% to 60% at 4 T.
Scanning force microscopy study of the ferroelectric phase transition in triglycine sulfate
Authors: Orlik X. K. , Likodimos V. , Pardi L. , Labardi M. , Allegrini M.
Published: July 2002

Voltage-modulated scanning force microscopy is applied to study the temperature dependence of the ferroelectric domain structure of triglycine sulfate up to TC, the ferroelectric transition temperature. The polarization image contrast exhibits a power-law decrease as TC is approached, associated with the competition between Maxwell stress and the converse piezoelectric effect. Substantial differences in the static and dynamic domain patterns are detected for two samples of different aging conditions.
Low-energy grazing-angle argon-ion irradiation of silicon: A viable option for cleaning?
Authors: Zalm P. C. , van den Berg J. A. , van Berkum J. G. M. , Bailey P. , Noakes T. C. Q.
Published: July 2002

In recent publications, it has been suggested that atomically clean, flat, crystalline silicon surfaces can be obtained by low-energy (0.1–1 keV) oblique-angle (⩾45° off-normal) argon-ion bombardment at mildly elevated target temperatures (∼500 °C). Here, this procedure has been applied to a multiple boron delta-doped Si structure. It leads to a massive relocation of subsurface doping atoms because of the accompanying injection of point defects into the bulk. This greatly affects the usefulness of the proposed cleaning method and shows that it is hazardous to base claims of quality solely on results obtained with surface-sensitive (∼1 nm) analytical techniques.
Evaluation of niobium effects on the longitudinal piezoelectric coefficients of Pb(Zr, Ti)O3 thin films
Authors: Haccart T. , Cattan E. , Remiens D. , Hiboux S. , Muralt P.
Published: July 2002

Pb(Zr, Ti)O 3 (PZT) and Pb(Zr, Ti, Nb)O3 (PNZT) thin films have been deposited on platinized silicon substrates by sputtering followed by a postannealing treatment. The Nb concentration in the films varied between 1 and 7 at. % with increments of 1 at. %. The effects of Nb modification on the piezoelectric response, and particularly on the longitudinal piezoelectric coefficient d33, have been investigated. The introduction of Nb enhances the dielectric and piezoelectric properties of the PZT films. The best doping level ranges from 1 up to 2 at. %. The relative dielectric constant εr, reaches 1100, and the maximum value of d33 is equal to 115 pm/V for a 1 at. % Nb-doped PZT in comparison to 820 and 55 pm/V for an undoped PZT film. PNZT films are suitable for microsystem applications.
Image amplification and novelty filtering with a photorefractive polymer
Authors: Goonesekera Arosha , Wright Daniel , Moerner W. E.
Published: July 2002

Energy transfer between two laser beams writing a volume hologram in a photorefractive polymer composite is applied to video-rate optical processing applications. A net increase in image intensity as high as a factor of 37 can be observed within one video frame time (33 ms) using a total beam intensity of 1 W cm−2 and 77 V μm−1 applied electric field. Moving object detection (novelty filtering) is also demonstrated.
Decomposition of susceptibility spectra in a torsion-stressed Fe-based amorphous wire
Authors: Kim Cheol Gi , Yoon Seok Soo , Yu Seong-Cho
Published: July 2002

The complex susceptibility spectra are measured as functions of the alternating-current field amplitude and the torsion angle in an Fe77.5Si7.5B15 amorphous wire. The susceptibility spectra show dispersion with a relaxation frequency of 40 kHz due to irreversible motion of the inner core domain walls when the driving field is larger than the threshold field of 10 mOe. The spectra for a small driving field can be decomposed into two relaxation dispersions by using the nonlinear curve fitting, one originating from reversible wall motion of the inner core domains and with a relaxation frequency of 0.36 MHz, and the other originating from reversible magnetization rotation in the outer shell domains and with relaxation frequency of 1.82 MHz. The static susceptibilities resulting from the reversible and the irreversible magnetization processes show an asymmetric change with positive and negative torsion angles.
Electric field dependence of ferroelectric phase transition in epitaxial SrTiO3 films on SrRuO3 and La0.5Sr0.5CoO3
Authors: Park K. C. , Cho J. H.
Published: July 2002

We report on the electric field dependence of ferroelectric phase transition in epitaxial SrTiO3 (STO) thin films grown by the pulsed-laser ablation deposition method. Metallic oxides SrRuO3 (SRO) and La0.5Sr0.5CoO3 (LSCO) were used as the bottom electrodes, and the bottom electrodes influence the dielectric properties of STO films. The change in the dielectric properties originates from the different work functions and the lattice mismatches among STO film, the bottom electrode, and the substrate. However, the field-induced ferroelectric phase transition temperatures of the STO/SRO/(100) STO and the STO/LSCO/(100) LaAlO3 systems are proportional to the applied electric field, indicating a field-induced first-order ferroelectric phase transition.
Dangling-bond defect state creation in microcrystalline silicon thin-film transistors
Authors: Wehrspohn R. B. , Powell M. J. , Deane S. C. , French I. D. , Roca i Cabarrocas P.
Published: July 2002

We analyze the threshold voltage shift in microcrystalline Si thin-film transistors (TFTs), in terms of a recently developed thermalization energy concept for dangling-bond defect state creation in amorphous Si TFTs. The rate of the threshold voltage shift in microcrystalline Si TFTs is much lower than in amorphous Si TFTs, but the characteristic energy for the process, which we identify as the mean energy to break a Si–Si bond, is virtually the same. This suggests that the same basic Si–Si bond breaking process is responsible for the threshold voltage shift in both cases. The lower magnitude in microcrystalline Si TFTs is due to a much lower attempt frequency for the process. We interpret the attempt frequency in amorphous and microcrystalline silicon in terms of the localization length of the electron wave function and the effect of stabilizing H atoms being located only at grain boundaries.
Excluded volume effects on the structure of a linear polymer under shear flow
Authors: Pierleoni Carlo , Ryckaert Jean-Paul
Published: July 2002

The effect of excluded volume interactions on the structure of a polymer in shear flow is investigated by Brownian dynamics simulations for chains with size 30⩽N⩽300. The main results concern the structure factor S(q) of chains of N=300 Kuhn segments, observed at a reduced shear rate β=γ̇τ=3.2, where γ̇ is the bare shear rate and τ is the longest relaxation time of the chain. At low q, where anisotropic global deformation is probed, the chain form factor is shown to match the form factor of the continuous Rouse model under shear at the same reduced shear rate, computed here for the first time in a wide range of wave vectors. At high q, the chain structure factor evolves towards the isotropic equilibrium power law q−1/ν typical of self-avoiding walk statistics. The matching between the excluded volume and ideal chains at small q, and the excluded volume power law behavior at large q are observed for q orthogonal to the main elongation axis but not yet for q along the elongation direction itself, as a result of interferences with finite extensibility effects. Our simulations support the existence of anisotropic shear blobs for polymers in good solvent under shear flow for β>1 provided chains are sufficiently long.
The melting point of chain polymers
Authors: Hu Wenbing
Published: July 2002

Based upon a consideration of the similarity in the driving force of polymer crystallization to the mesophase formation from the melt state, the equilibrium melting point of chain polymers relating to the details of molecular structure and the impure environment of crystallites was studied by classical lattice statistics and dynamic Monte Carlo simulations. The compact packing energy change of local chains during crystallization can be represented as an orientation-dependent attraction in the lattice model. The results as a function of driving force, chain length, content of random comonomers, and content of the diluent like small molecules or noncrystalline chains were compared with the semiempirical expressions that have been verified for several polymers. Good agreements were obtained. As a result, the classical lattice statistics can predict a disorder–order phase transition as well as a phase separation behavior in a mixture system containing polymers, and supply the thermodynamic background to explore details of these behaviors in computer simulations.
Ballistic transport through GaAs–AlGaAs superlattices in transverse magnetic fields
Authors: Rakoczy D. , Smoliner J. , Heer R. , Strasser G.
Published: July 2002

In this work, ballistic electron transport through the lowest miniband of a biased GaAs–AlGaAs superlattice is investigated in transverse magnetic fields. As method we employ a solid-state version of ballistic electron emission microscopy/spectroscopy using a metal-insulator-metal injector structure that replaces the tip of the scanning tunneling microscope (STM). The ballistic electron current measured as a function of the collector bias shows a peak at flatband conditions indicating coherent transport through the superlattice miniband. With increasing transverse magnetic fields, this peak is quenched and evidence of sequential LO-phonon scattering inside the superlattice is found. Using an extended transfer matrix method, the observed effects are quantitatively explained; differences to previous STM based measurements are discussed.
Multidimensional reactive rate calculations in dissipative chaotic systems
Authors: Hershkovitz E. , Wiesenfeld L.
Published: July 2002

Using concepts from transient chaos and stochastic dynamics, we develop a perturbative solution for multidimensional activated rate processes. The solution is applicable to the underdamped regime where system dynamics prevails over bath fluctuations. The baseline of the method is the partition of the multidimensional reactive flux in a chaotic system to a sum of independent fluxes in one-dimensional systems. The partition is based on the underlying dynamics of the multidimensional system. The method is fast and explains the high and low temperature dependence of multidimensional reaction rates.
Formation of icosahedral phase from amorphous Zr65Al7.5Cu12.5Ni10Ag5 alloys
Authors: Lee J. K. , Choi G. , Kim D. H. , Kim W. T.
Published: July 2002

The kinetic behavior of quasicrystalline phase formation in amorphous Zr65Al7.5Cu12.5Ni10Ag5 alloys has been studied by using differential scanning calorimetry (DSC), x-ray diffractometry, and transmission electron microscopy. DSC trace obtained during continuous heating from 473 to 873 K shows two distinct exotherms with peak temperatures of 727 and 776 K. The first and second exothermic reactions, respectively, correspond to the formation of quasicrystalline phase from the amorphous matrix and the formation of tetragonal CuZr2 and hexagonal Zr6NiAl2 phases from the previously formed quasicrystalline phase. Partial replacement of Cu by Ag enhanced the formation of quasicrystalline phase from the amorphous structure. Johnson–Mehl–Avrami analysis of isothermal transformation data suggests that the formation of quasicrystalline phase is not entirely polymorphic in nature and may involve partitioning of solute at a later stage.
Temperature dependence of the electron–hole-plasma electroluminescence from metal–oxide–silicon tunneling diodes
Authors: Liu C. W. , Chen Miin-Jang , Lin I. C. , Lee M. H. , Lin Ching-Fuh
Published: July 2002

The temperature performance of metal–oxide–silicon tunneling light-emitting diodes was studied. An electron–hole-plasma model can be used to fit all the emission spectra from room temperature to 98 K. At constant voltage bias in the accumulation region, the normalized integral emission intensity slightly increases at low temperature with activation energy as low as 12 meV. From room temperature down to 98 K, the extracted band gaps are ∼80 meV lower than the value of Varshni equation, and the linewidth drops from 65 to 30 meV. The transverse optical and longitudinal optical phonons are involved in the light-emission process due to the reduction of extracted band gaps and the resemblance between electroluminescence and photoluminescence spectra at similar temperature.
Static electric properties of conjugated cyclic ketones and thioketones
Authors: Eckart Udo , Fülscher Markus P. , Serrano-Andrés Luis , Sadlej Andrzej J.
Published: July 2002

The results of ab initio calculations of static electric properties of a series of cyclic conjugated ketones and thioketones are presented. Dipole moments, dipole polarizabilities, as well as the first and second hyperpolarizabilities, were evaluated at different levels of theory: SCF, CASSCF, MP2, CCSD, CCSD(T), and CASPT2 methods were used. Both MP2 and CASPT2 methods have been shown to perform well in the calculation of such properties. The dependence of the electric properties on the extent of the π-electron conjugation and on the replacement of the keto by the thioketo group is analyzed. Cyclic conjugated thioketones are shown to exhibit significant nonlinear electric properties which make them prospective building blocks for nonlinear optical materials.
Energy backtransfer and infrared photoresponse in erbium-doped silicon p–n diodes
Authors: Hamelin N. , Kik P. G. , Suyver J. F. , Kikoin K. , Polman A. , Schönecker A. , Saris F. W.
Published: July 2002

Temperature-dependent measurements of the photoluminescence (PL) intensity, PL lifetime, and infrared photocurrent, were performed on an erbium-implanted silicon p–n junction in order to investigate the energy transfer processes between the silicon electronic system and the Er 4f energy levels. The device features excellent light trapping properties due to a textured front surface and a highly reflective rear surface. The PL intensity and PL lifetime measurements show weak temperature quenching of the erbium intra-4f transition at 1.535 μm for temperatures up to 150 K, attributed to Auger energy transfer to free carriers. For higher temperatures, much stronger quenching is observed, which is attributed to an energy backtransfer process, in which Er deexcites by generation of a bound exciton at an Er-related trap. Dissociation of this exciton leads to the generation of electron–hole pairs that can be collected as a photocurrent. In addition, nonradiative recombination takes place at the trap. It is shown for the first time that all temperature-dependent data for PL intensity, PL lifetime, and photocurrent can be described using a single model. By fitting all temperature-dependent data simultaneously, we are able to extract the numerical values of the parameters that determine the (temperature-dependent) energy transfer rates in erbium-doped silicon. While the external quantum efficiency of the photocurrent generation process is small (1.8×10−6) due to the small erbium absorption cross section and the low erbium concentration, the conversion of Er excitations into free e–h pairs occurs with an efficiency of 70% at room temperature.
Experimental and theoretical study of methyl-p-aminobenzoate/ammonia complexes. II. MAB(NH3)2–4
Authors: Longarte Asier , Fernández José A. , Unamuno Iñigo , Castaño Fernando
Published: July 2002

A complementary laser spectroscopy and computational study of the MAB(NH3)2–4 complexes, hereafter referred to by its stoichiometry, i.e., 1:2, 1:3, and 1:4, prepared in a supersonic expansion, is reported. Experimental evidence shows the existence of abundant fragmentation cascades, the most notorious being the observation of the 1:4 complex spectrum in the 1:3 and to 1:2 mass channels, in fact, the observed spectra of the 1:2 and 1:3 complexes are not genuine but a consequence of fragmentation. The observed 1:4 complex resonance enhanced multiphoton ionization (REMPI) spectrum has a significant redshift of −1160 cm−1 from the bare MAB 000 transition and appears over a noisy background that decreases, although it does not disappear, in resonance enhanced two-color photo ionization (R2PI) studies. “Hole burning” spectroscopy corroborates the presence of only one 1:4 isomer. Calculations at the B3LYP/6−31+G* level conduct to a number of 1:2, 1:3, and 1:4 stable isomer structures, the most stable being the 1:4 with a four ammonia chain coordinated to the NH2 group. The good agreement between calculated and experimental vibrational frequencies confirms the ammonia ring structure and allows us to assign a number of MAB(NH3)4 inter- and intramolecular vibrational bands.
The role of micro- and nanomorphology of rough silver surfaces of different nature in surface enhanced Raman scattering effect: A combined study of scanning force microscopy and low-frequency Raman modes
Authors: Pignataro Bruno , De Bonis Angela , Compagnini Giuseppe , Sassi Paola , Cataliotti Rosario Sergio
Published: July 2002

The micro- and nanomorphology of surface enhanced Raman scattering (SERS) active rough substrates obtained by plasma oxidation-reduction cycles onto original flat silver surfaces have been investigated by means of a dual technique approach. Scanning force microscopy and low-frequency Raman spectroscopy give complementary results when applied on very rough systems. Almost spherical silver colloids have been used as well-defined systems to model, by their stacking over flat silicon wafers, the plasma roughening process inducing SERS activity. The SERS activity results are strongly related to the micromorphology of the nanoparticles assembly, rather than to the silver cluster size. In particular an electromagnetic enhancement factor of 103 for the breathing mode of the polystyrene aromatic rings was found to be related to the vertical stacking of tens of clusters about 10 nm in diameter.
Electronic properties of In2O3 surfaces
Authors: Klein Andreas
Published: July 2002

Surfaces of reactively evaporated In2O3 films were investigated in situ by synchrotron-excited photoemission. Work function, valence band maximum, and electronic states in the band gap were determined as a function of oxygen pressure. Surface and bulk electronic properties can only be explained consistently with the assumption of a surface depletion layer.
Temperature dependence of the exciton decay times of different quantum structures coexisting in a ZnSe/CdSe/ZnSe heterostructure
Authors: Zhang B. P. , Wakatsuki K. , Manh D. D. , Segawa Y.
Published: July 2002

Study by time-resolved photoluminescence shows that two classes of quantum structures coexist in a ZnSe/CdSe/ZnSe heterostructure in which the CdSe coverage is less than the critical thickness. Excitons from class-A structures dominate the emission spectrum and exhibit temperature-independent decay times, demonstrating quantum-dot-like properties. On the other hand, excitonic transitions from class-B structures are characterized by decay times that depend linearly on temperature, indicating two-dimensional features. There is a sharp transition from class-A to class-B excitons as the emission energy goes from higher to lower energies across the emission band.
Weak-to-strong penetration transition of macromolecules into a slit in theta solvent
Authors: Cifra Peter , Bleha Tomáš , Wang Yongmei , Teraoka Iwao
Published: July 2002

Partitioning of polymer chains in the theta solution with a confined space of a slit was studied in a wide range of concentrations by using lattice Monte Carlo simulations. The slit width was equal to or greater than the radius of gyration of the theta chains. In the low concentration limit, the partitioning of the theta chains was indistinguishable from that of athermal chains compared at the same chain dimension. At higher concentrations but below the overlap concentration in the solution surrounding the slit, the partition coefficient was nearly independent of the concentration. With a further increase in the concentration, the partition coefficient increased toward unity, as the motional unit in the solution changed from a single chain to the concentration blob of the semidilute theta solution and the blob size decreased with an increasing concentration. The increase in the partition coefficient occurred, however, at concentrations much higher than those for the athermal solutions that showed a similar weak-to-strong penetration transition. We ascribe the delay to the absence of the positive second virial coefficient and the presence of chain clusters. The latter was verified in the density correlation functions of the bulk solutions as well as in the snapshots of the solutions.
Stability of droplet-target laser-plasma soft x-ray sources
Authors: Hemberg O. , Hansson B. A. M. , Berglund M. , Hertz H. M.
Published: July 2002

The spatial stability of microscopic target droplets used for laser-plasma soft x-ray generation in vacuum is investigated. A long-term drift in drop position is characterized with an ultrafast laser-diode imaging system. The drift is experimentally and theoretically shown to be due to a temperature-induced increase in target-liquid viscosity as a result of evaporation. Finally, the drift is compensated for and stable, long-term unattended operation of the source is demonstrated with an automatic phase-delay drop-to-laser synchronizing system. This is important for future compact lithography and microscopy systems.
A theoretical and experimental study of the ethyl-p-aminobenzoate (H2O)n (n=1–4) complexes
Authors: Fernández José A. , Longarte Asier , Unamuno Iñigo , Castaño Fernando
Published: July 2002

Weakly bound hydrogen bonded ethyl-p-aminobenzoate/water complexes, referred to henceforth both as EAB/(H2O)n (n=1–4) or by their stoichiometry 1:n, have been investigated with a combined approach of mass and light detector laser spectroscopic techniques and ab initio calculations. The experimental studies follow explorations with laser induced fluorescence (LIF), and include one-color resonant enhanced multiphoton ionization (REMPI), two-color REMPI (R2PI), pressure dependent R2PI and hole burning (HB) spectroscopies. Calculations were conducted at the B3LYP/6-31+G* level and for the 1:1 complex led to the existence of six stable isomers, identified as the experimental origin bands at +4, +6, +13, +89, +96, and +108 cm−1 above the bare EAB 000 transition. It has been shown that three of these bands originate in the EAB trans conformer, while the other three derive from the EAB gauche conformer. None of the experimental methods used lead us to observe the EAB(H2O)2 complex spectrum and the inspection of the EAB(H2O)3 REMPI and R2PI spectra has been shown to be a fragmentation from the EAB(H2O)4 complex. The structures and identification of the set of isomers are reported and a comparison with the results on the family complexes methyl-p-aminobenzoate/water is discussed.
Quantum model simulation of complete S→S1 population transfer by means of intense laser pulses with opposite chirp
Authors: Manz J. , Naundorf H. , Yamashita K. , Zhao Y.
Published: July 2002

The effects of short (0.5–2 ps) chirped laser pulses on the vibrational population transfer from the electronic ground state S0 to the excited state S1 are investigated via numerical simulations of the wave packet dynamics. It is demonstrated for a model system that both positively and negatively chirped laser pulses with high intensities can achieve almost complete population transfer to the vibrational ground or low excited states in the electronically excited state. The underlying mechanisms of the transfer are, however, different, i.e., suppression and compensation of intrapulse pump–dump processes for the negatively and positively chirped pulses, respectively. The process induced by the negatively chirped laser pulse is applied to design complete S0→S1 population transfer to the lowest vibrational states of the first electronically excited state of 9-(carbazolyl)-anthracene (C9A) which could not be observed by means of traditional, i.e., continuous wave or ns spectroscopy.
Optically resolving dynamic processes in commercial liquid crystal cells
Authors: Smith N. J. , Sambles J. R.
Published: July 2002

The recently developed fully leaky guided mode technique is a powerful tool for accurately resolving the director profile in standard, commercially fabricated liquid crystal cells. By employing a convergent beam to simultaneously excite many fully leaky guided modes, angular dependent reflectivity and transmissivity data can be captured in a fraction of a millisecond with a charge coupled device array, thus enabling the switching dynamics of the liquid crystal cell to be resolved. A preliminary study of the director profile (optical tensor distribution) during the relaxation of a homogeneously aligned cell (antiparallel rubbed) filled with the nematic material ZLI-2293 is presented here. By resolving the director distribution as a function of time, the time constant for the relaxation process is obtained from which the effective rotational viscosity coefficient γ* is then evaluated.