bluuebunny/crossref_metadata_2025 huggingface dataset:
Controlled-atmosphere chamber for atomic force microscopy investigations
Authors: Sartore Marco , Pace Raffaele , Faraci Paolo , Nardelli Daniele , Adami Manuela , Ram Manoj K. , Nicolini Claudio
Published: July 2002

The present work describes a simple chamber suitable for morphological investigations by implementing the atomic force microscopy (AFM) in controlled experiments. The novelty of our application stems from proposing an open system located in between the expensive, ultra-high-vacuum instruments and those working in air conditions, both available on the market. The former are in fact designed to obtain a detailed inspection of the samples and to develop particular geometries on them, by means of nanolithography or nanomanipulation, while the latter are designed for and used in all the situations in which the environmental conditions do not cause artifacts, problems, or formation of spurious particles on the samples during imaging. We have developed an ad hoc system based on a high-vacuum chamber (up to 10−6 Torr), which allows us to work under controlled-atmosphere conditions. The system, therefore, can be used with most of the samples which suffer from higher pressures, and exploits all the benefits arising from a controlled environment. We have equipped the chamber with an AFM and a sample-holder/mover. An external X–Y–Z motion controller, completely automated, allows the easy positioning of the sample under the sensing cantilever and the consequent relative approach. Experiments with the proposed system are presented, in which the control of environmental conditions during AFM measurements has been investigated with satisfactory results.
Low-temperature growth of epitaxial LiNbO3 films on sapphire (0001) substrates using pulsed laser deposition
Authors: Lee See-Hyung , Song Tae Kwon , Noh T. W. , Lee Jai-Hyung
Published: July 2002

Epitaxial LiNbO3 films were grown on sapphire (0001) substrates using pulsed laser deposition. A single-crystal LiNbO3 was used as a target. Growth behaviors of the LiNbO3 films deposited at various deposition conditions such as temperature, oxygen pressure, and annealing condition were studied. Deposition temperature is found to be an important parameter which enables us to grow LiNbO3 films without the LiNb3O8 phase: formation of the Li deficient phase can be suppressed by depositing the films at temperature below 500 °C. Oxygen pressure during deposition influences crystallographic orientations of the films. An x-ray pole figure shows that epitaxial LiNbO3 film was grown, but with twin boundaries. The ordinary refractive index of the film was found to be 2.28, which is in good agreement with the bulk value.
High resolution characterization of structural inhomogeneities in YBa2Cu3O7−δ crystals with sharp superconducting transitions
Authors: Qadri S. B. , Osofsky M. S. , Browning V. M. , Skelton E. F.
Published: July 2002

Structural inhomogeneities in two single crystals of YBa2Cu3O7−δ are characterized using high resolution x-ray diffraction. Results of these analyses show a distribution in the c-axis lattice parameters. These are interpreted as evidence of a distribution in oxygen content. Values of 7−δ range from 6.71 to 7.00 in one crystal and from 6.80 to 6.99 in the other. Nevertheless, the measured superconducting transition widths for both crystals are less than 0.5 K, as determined by ac susceptibility. When examined with conventional x-ray diffraction techniques, both crystals also appeared to be free of gross defects, but high resolution rocking curves reveal variations in defect density. These results suggest that crystals presumed to be homogeneous based on narrow superconducting transition widths or conventional x-ray diffractometry, may not be. Therefore, crystals found to exhibit exotic physical properties should be carefully examined for the presence of structural anomalies. If such defects are present, the experimental results may be attributed to sample imperfections, rather than intrinsic phenomena.
PtIn2 ohmic contacts to n-GaAs via an In-Ga exchange mechanism
Authors: Chen D. Y. , Chang Y. A. , Swenson D.
Published: July 2002

Using sputter-deposited PtIn2 films as metallizations, it is demonstrated that the recently identified exchange mechanism may be utilized to form ohmic contacts to n-GaAs. Specific contact resistances as low as 3.0×10−6 Ω cm2 are obtained upon annealing in the temperature range of 800–850 °C. Contacts processed under optimum conditions show little degradation in electrical properties after 100 h of thermal aging at 400 or 500 °C. Auger depth profiles of as-deposited and annealed samples are consistent with the hypothesis of an exchange of In and Ga atoms at the contact interface.
Lift-off lithography using an atomic force microscope
Authors: Bouchiat V. , Esteve D.
Published: July 2002

We present a technique to fabricate nanostructures with an atomic force microscope (AFM). By taking advantage of the AFM tip sharpness, we engrave a narrow furrow in a soft polyimide layer. The furrow is then transferred using dry etching to a thin germanium layer which forms a suspended mask. Metallic layers are then evaporated through this mask. Metallic lines with a 40 nm linewidth and single-electron transistors have been fabricated. This lift-off technique can be used on any substrate and allows easy alignment with previously fabricated structures.
Thermal writing using a heated atomic force microscope tip
Authors: Mamin H. J.
Published: July 2002

Resistive heating of an atomic force microscope tip was used to perform thermally induced surface modifications. Heating was achieved by dissipating power in the legs of an electrically conducting silicon cantilever. Temperatures of up to 170 °C were obtained using 40 mW of input power. Electrical measurements used to monitor the temperature showed thermal time constants of 0.35–0.45 ms, depending on whether the tip was in contact with a substrate. The heated tip was used to demonstrate thermomechanical writing on a polycarbonate substrate, as well as thermal writing of an optical phase change material.
Velocity dependence of rotational loss in Evershed-type superconducting bearings
Authors: Hull John R. , Mulcahy Thomas M. , Labataille Joseph F.
Published: July 2002

Results of free spin down in vacuum are reported for an Evershed-type superconducting bearing in which a permanent magnet (PM) ring is levitated over an array of high-temperature superconductors (HTSs) and under a similar PM ring in magnetic attraction. The velocity dependence of the rotational loss strongly suggests that the observed velocity-dependent losses are primarily due to eddy currents induced in the PM by inhomogeneity of the field produced by the magnetized HTS array. The results show that the Evershed-type bearing is capable of reducing these eddy-current losses to an extremely low level, so that at a maximum magnet rim velocity of 28 m/s, the fractional kinetic-energy loss per hour was 2.4×10−4. Significant levitation heights are also possible, and at a 23 mm height, we measured a low-speed coefficient of friction of 3×10−8.
Mass transfer in Stranski–Krastanow growth of InAs on GaAs
Authors: Ramachandran T. R. , Heitz R. , Chen P. , Madhukar A.
Published: July 2002

We present a quantitative study of the evolution of the material contained in two- and three-dimensional (2D and 3D) surface features during the 2D–3D morphology transition in highly strained growth, using InAs/GaAs(001) as the vehicle. The results establish a varying mass transfer between 2D and 3D surface features with increasing InAs deposition. Quasi-3D (0.6–1.2 nm high) clusters are seen to mediate the 2D–3D morphology change and to play an important role in the mass redistribution on the surface.
Probing of InAs/AlSb double barrier heterostructures by ballistic electron emission spectroscopy
Authors: Walachová J. , Zelinka J. , Vaniš J. , Chow D. H. , Schulman J. N. , Karamazov S. , Cukr M. , Zich P. , Král J. , McGill T. C.
Published: July 2002

InAs/AlSb resonant tunneling heterostructures have been studied by ballistic electron emission spectroscopy. Current thresholds attributed to quasibound states in the quantum well and emission over the AlSb barriers are observed. The observed shape of thresholds is consistent with inelastic processes in the InAs layers of the structures, where a high number of electron–hole pairs are generated. A threshold consistent with the generation of electron–hole pairs in quantum well states is observed.
Nanometer-scale linewidth fluctuations caused by polymer aggregates in resist films
Authors: Yamaguchi Toru , Namatsu Hideo , Nagase Masao , Yamazaki Kenji , Kurihara Kenji
Published: July 2002

Linewidth fluctuation in resist patterns is a serious problem in electron beam nanolithography. We have observed granular structures with a diameter of 20–30 nm in resist films, and have determined that these structures cause the linewidth fluctuations. The granules are made up of polymer aggregates. We discuss the origin of the aggregates from the result that their size depends on the polymer molecular weight. We also show that the linewidth fluctuation is reduced, though the developing rate is slow, when the pattern size is less than the aggregate size. The linewidth dependence of the fluctuation and of the developing rate can be explained by the influence of the resist polymer aggregate on the development behavior.
Etch-pit initiation by dissolved oxygen on terraces of H-Si(111)
Authors: Wade Christopher P. , Chidsey Christopher E. D.
Published: July 2002

Dissolved oxygen in 40% aqueous ammonium fluoride solution initiates the formation of etch pits in the terraces of the otherwise ideal H-Si(111) surface. The etch pits are observed byex situ scanning tunneling microscopy in an argon atmosphere following emersion from the aqueous fluoride solution. Removal of O2from the fluoride solution by sparging with argon substantially reduces the initiation of etch pits. We propose the following mechanism of etch-pit initiation. Oxygen molecules are reduced to superoxide anion radicals at the negative open-circuit potential of the silicon surface. A small fraction (less than 0.4%) of these superoxide anions abstract hydrogen atoms from the H-Si(111) terraces to form silicon radicals (dangling bonds), which are then susceptible to etching in neutral to basic aqueous solutions. Hydrogen atom abstraction by aqueous superoxide anion radical also explains the known enhancement by water of oxide growth on hydrogen-terminated silicon surfaces.
Near-room-temperature mid-infrared interband cascade laser
Authors: Olafsen L. J. , Aifer E. H. , Vurgaftman I. , Bewley W. W. , Felix C. L. , Meyer J. R. , Zhang D. , Lin C.-H. , Pei S. S.
Published: July 2002

A 25-stage interband cascade laser with a W active region and a third hole quantum well for the suppression of leakage current has exhibited lasing in pulsed mode up to 286 K. A peak output power of 160 mW/facet and a slope efficiency of 197 mW/A per facet (1.1 photons per injected electron) were measured at 196 K. Above 200 K, the characteristic temperature was higher (T0=53 K) and the threshold current densities lower than for a previously reported W interband cascade laser without the third hole quantum well.
Determination of the ordered structures of Pb(Mg1/3Nb2/3)O3 and Ba(Mg1/3Nb2/3)O3 by atomic-resolution Z-contrast imaging
Authors: Yan Y. , Pennycook S. J. , Xu Z. , Viehland D.
Published: July 2002

The atomic structure of ordered domains in Ba(Mg1/3Nb2/3)O3 and La-doped and undoped Pb(Mg1/3Nb2/3)O3 is studied by high-resolution Z-contrast imaging. The ordered domain structure in both doped and undoped Pb(Mg1/3Nb2/3)O3 is determined to be in agreement with the charge balanced random-layer model and inconsistent with the space-charge model. It is shown that La doping in Pb(Mg1/3Nb2/3)O3 enhances not only the domain size but also the degree of ordering.
Recombination dynamics in n-AlxGa1−xAs/n-In0.5Ga0.5P type-II heterostructures
Authors: Cho Yong-Hoon , Song J. J. , Lim H. , Choe Byung-Doo , Lee J. I. , Kim D.
Published: July 2002

Recombination characteristics of n-AlxGa1−xAs/n-In0.5Ga0.5P type-II band line-up heterostructures are investigated using time-integrated and time-resolved photoluminescence (PL) measurements. It is observed that the decay time of the AlxGa1−xAs luminescence depends on whether or not the excitation photon energy,ℏωe, is larger than the In0.5Ga0.5Pband-gap energy, Eg,InGaP. If ℏωe>Eg,InGaP, photoexcited holes in the AlxGa1−xAs and In0.5Ga0.5P layers are found to be in equilibrium within about 0.4 ns. The interface-related below-band-gap (BBG) PL shows a large blueshift as the excitation intensity is increased. The extremely long decay time of the BBG PL is attributed to the somewhat smaller wave function overlap between spatially separated, two-dimensional electrons and holes due mainly to the nonabrupt interfacial nature of the employed samples. The fast transient behavior of the BBG luminescence under high excitation intensity, as well as the peak energy blueshift, are explained by the band filling effect.
Intermittent photoluminescence and thermal ionization of ZnCdSe/ZnSe quantum dots grown by molecular beam epitaxy
Authors: Zhang B. P. , Li Y. Q. , Yasuda T. , Wang W. X. , Segawa Y. , Edamatsu K. , Itoh T.
Published: July 2002

We report the intermittent photoluminescence of ZnCdSe quantum dots (QDs) embedded in a ZnSe matrix grown by molecular beam epitaxy. The luminous time of the QD is strongly dependent on temperature but not on excitation intensity. This indicates that the ionization of the QDs is determined predominantly by thermal excitation of carriers into the ZnSe matrix.
A superconducting tunnel junction x-ray detector with performance limited by statistical effects
Authors: le Grand J. B. , Mears C. A. , Hiller L. J. , Frank M. , Labov S. E. , Netel H. , Chow D. , Friedrich S. , Lindeman M. A. , Barfknecht A. T.
Published: July 2002

We have characterized a thin-film Nb/Al/AlOx/Al/Nb superconducting tunnel junction (STJ) optimized for low electronic noise as an x-ray detector in the 0.2–1 keV photon energy range. The spectra measured with this junction have high spectral purity with, to the best of our knowledge, the best energy resolution ever achieved with this type of detector in this energy band. The discrepancy between the theoretical and experimental energy resolution is only about 15%. Part of this small discrepancy may be explained by the fact that our junction has electrodes made from niobium/aluminum bilayers, while the theoretical result is for electrodes made from only one material. To the best of our knowledge, this is the first time that resolution achieved with a STJ x-ray detector is in agreement with the resolution predicted from statistical fluctuations in the creation and tunneling of quasiparticles.
Magnetoresistance due to domain walls in an epitaxial microfabricated Fe wire
Authors: Rüdiger U. , Yu J. , Kent A. D. , Parkin S. S. P.
Published: July 2002

The domain wall (DW) contribution to magnetoresistance has been investigated using an epitaxial microfabricated bcc (110) Fe wires of 2 μm linewidth. A strong in-plane uniaxial component to the magnetic anisotropy perpendicular to the wire axis causes a regular stripe domain pattern with closure domains. The stripe domain width in zero-applied magnetic field is strongly affected by the magnetic history and can be continuously varied from 0.45 to 1.8 μm. This enables a measurement of the resistivity as a function of DW density in a single wire. Clear evidence is presented that the resistivity is reduced in the presence of DWs at low temperatures.
Two distinct dielectric relaxation mechanisms in the low-frequency range in (K0.50Na0.50)2(Sr0.75Ba0.25)4Nb10O30 ceramics
Authors: Bu Sangdon , Shin Eunjoo , Park Gwangseo
Published: July 2002

The effects of space charges on the ferroelectric properties of (K0.50Na0.50)2(Sr0.75Ba0.25)4Nb10O30 ceramics were studied using the ac complex impedance technique. Using our sample, which was aged at room temperature in order to induce a motion of space charges, we observed two dielectric relaxation mechanisms, each having different physical characters in the low-frequency range at temperatures above the Curie point (Tc∼208 °C). One mechanism may be attributed to the hopping process of the space charges bound inside grains, while the other seems to be associated with the space charges at the grain boundary layers. The dielectric measurements were analyzed using equivalent circuits composed of a capacitor (1) connected in series and (2) connected in parallel with a parallel RC circuit. These results were also examined within the framework of a model based on space-charge migration, which was suggested as an essential process for aging. This leads to the conclusion that the two mechanisms are distinct.
Ferroelectric characterization of highly (0001)-oriented YMnO3 thin films grown by chemical solution deposition
Authors: Yi Woo-Chul , Choe Joon-Seon , Moon Chang-Rok , Kwun Sook-Il , Yoon Jong-Gul
Published: July 2002

Highly (0001)-oriented thin films of YMnO3 were grown directly on Si substrates by chemical solution deposition. The crystallinity of the films was investigated by using x-ray diffraction: θ–2θ scan, rocking curve, and pole figure. Analysis of the x-ray photoelectron spectroscopy data and Rutherford backscattering spectroscopy spectrum showed that the films had a single phase of stoichiometric YMnO3. The ferroelectric properties of YMnO3 were investigated by measuring the temperature dependence of the capacitance–voltage characteristics in the metal/ferroelectric/semiconductor structure. Screening of the ferroelectricity of YMnO3 thin film at room temperature was discussed in conjunction with the charge effects.
Excitation of intra-4f shell luminescence of Yb3+ by energy transfer from Si nanocrystals
Authors: Fujii Minoru , Hayashi Shinji , Yamamoto Keiichi
Published: July 2002

SiO 2 films containing Si nanocrystals (nc-Si) and Yb were prepared and their photoluminescence (PL) properties were studied. For the sample containing nc-Si with an average diameter of 3.1 nm, a weak peak (∼1.26 eV) attributable to the intra-4f shell transition of Yb3+ could be observed at the low-energy side of a broad peak (∼1.4 eV) of nc-Si. The intensity of the 1.26 eV peak was found to depend strongly on the size of nc-Si and increase rapidly with decreasing size. The temperature dependence of the PL spectra was studied. It was found that the degree of temperature quenching of the 1.26 eV peak depends on the size of the nc-Si and becomes small as the size decreases. These results suggest that the band-gap widening of nc-Si due to the quantum size effects is essential to efficiently excite Yb3+ by nc-Si.
Controlled doping of phthalocyanine layers by cosublimation with acceptor molecules: A systematic Seebeck and conductivity study
Authors: Pfeiffer M. , Beyer A. , Fritz T. , Leo K.
Published: July 2002

We investigate the doping of vanadyl–phthalocyanine by a fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic dye films by cosublimation of matrix and dopant. The electrical parameters of the films derived from conductivity and Seebeck measurements show that the results largely follow standard models used to describe the doping of crystalline semiconductors; e.g., a smooth shift of the Fermi level towards the valence states with increasing doping is observed. Other effects, like the superlinear increase of conductivity with the molar doping ratio, need the inclusion of additional effects like percolation.
Evidence of Be3P2 formation during growth of Be-doped phosphorus-based semiconductor compounds
Authors: de Carvalho M. M. G. , Betinni J. , Pudenzi M. A. A , Cardoso L. P. , Cotta M. A.
Published: July 2002

In this work, we present evidence that Be3P2 microcrystals are formed in Be-doped phosphorus-based semiconductor compounds grown by chemical beam epitaxy. Our results suggest that microcrystal formation occurs when high Be concentrations (>1018 cm−3) and temperatures higher than 500 °C are used for crystal growth. The main consequence of Be3P2 formation is a high phosphorus consumption close to these microcrystals that causes a large density of P vacancies in the semiconductor layer. This results in reduced electrical mobility, lattice parameter reduction, and poor crystalinity of the film in general.
Efficient p-type doping of 6H-SiC: Flash-lamp annealing after aluminum implantation
Authors: Wirth H. , Panknin D. , Skorupa W. , Niemann E.
Published: July 2002

Flash-lamp annealing was used for activation and crystal recovery of highly aluminum-implanted 6H-SiC wafers. In comparison with conventional furnace annealing, the free hole concentration can be remarkably increased at high acceptor atom concentrations (⩾5×1020 cm−3). The lowest resistivity measured at room temperature was 0.01 Ω cm. In this case, the layers are characterized by metallic conduction with weak dependence of the hole concentration on the temperature. This effect is caused by freezing-in of the enhanced solubility of aluminum in SiC at the extraordinary high temperature of about 2000 °C during the light-flash.
Fractal aggregation and optical absorption of copper nanoparticles prepared by in situ chemical reduction within a Cu2+-polymer complex
Authors: Huang C. , Yang C. Z.
Published: July 2002

A polymer-matrix nanocomposite containing copper particles has been prepared by in situ chemical reduction within a Cu2+-poly(itaconic acid-co-acrylic acid) complex solid film. The copper particle size in the order of 10 nm is controlled by the initial content of the metal ions in the complex. Their fractal pattern and the value of the fractal dimension indicate that there exists a cluster-cluster aggregation process in the present system. Optical absorption spectra of copper-polymer nanocomposites show distinct plasma absorption bands and quantum size effect in the samples. The calculated blueshift of the resonance peak based on a quantum-sphere model gives remarkable agreement with the experimental data as the size of copper particles embedded in the polymer becomes smaller.
Thermal conductivities of evaporated gold films on silicon and glass
Authors: Chen George , Hui Ping
Published: July 2002

We present four-point probe measurements of the in-plane electrical conductivities of electron beam-evaporated gold layers of thickness 20 nm to 2 μm deposited on Si(100), Si(111), and BK7 glass substrates. The values of thermal conductivities deduced from these measurements of gold films on glass samples agree well with prediction from the surface and grain-boundary scattering model, but not for gold films on silicon samples. Thermal conductivities of gold films are found to be different for samples on hydrofluoric acid etched Si(100) and Si(111) substrates, for which surface roughness and microstructures of the gold films have been examined to understand the difference in these thermal conductivity results.
Analysis of asymmetric giant magnetoimpedance in field-annealed Co-based amorphous ribbon
Authors: Kim Cheol Gi , Jang K. J. , Kim D. Y. , Yoon S. S.
Published: July 2002

The asymmetric giant magnetoimpedance (GMI) profile has been realized in weak-field-annealed Co-based amorphous ribbon at the annealing temperature of 380 °C in open air. Asymmetric GMI profiles with respect to applied field become profound as the annealing field increases over 500 mOe. The asymmetric GMI profile at the frequencies of ac current over 0.5 MHz is well ascribed for by the rotational transverse magnetization of single domain under a uniaxial anisotropy in amorphous core and a unidirectional anisotropy due to the exchange coupling with the bias field in the crystalline layer, underlying surface oxidation layer developed during the annealing in open air.
Switching of vertical giant magnetoresistance devices by current through the device
Authors: Bussmann K. , Prinz G. A. , Cheng S.-F. , Wang D.
Published: July 2002

Experiments are reported that demonstrate current-perpendicular-to-the-plane giant magnetoresistance devices can be switched repeatably between the high- and low-resistance states by passing current vertically through the structure. The lithographically patterned devices, having diameters in the range of 0.3–0.7 μm, operate at room temperature and exhibit distinctly separate switching of the soft and hard layers. Designs for magnetoelectronic random access memory can utilize this scheme for storing and reading information.
High-quality Ge epilayers on Si with low threading-dislocation densities
Authors: Luan Hsin-Chiao , Lim Desmond R. , Lee Kevin K. , Chen Kevin M. , Sandland Jessica G. , Wada Kazumi , Kimerling Lionel C.
Published: July 2002

High-quality Ge epilayers on Si with low threading-dislocation densities were achieved by a two-step ultrahigh vacuum/chemical-vapor-deposition process followed by cyclic thermal annealing. On large Si wafers, Ge on Si with threading-dislocation density of 2.3×107 cm−2 was obtained. Combining selective area growth with cyclic thermal annealing produced an average threading-dislocation density of 2.3×106 cm−2.We also demonstrated small mesas of Ge on Si with no threading dislocations. The process described in this letter for making high-quality Ge on Si is uncomplicated and can be easily integrated with standard Si processes.
Quantitative microwave evanescent microscopy
Authors: Gao Chen , Duewer Fred , Xiang X.-D.
Published: July 2002

We have developed a scanning evanescent microwave microscope with shielded tip geometry allowing quantitative characterization of the electrical impedance of insulating and conducting materials. By modeling the tip–sample capacitance, quantitative estimates of the sample (both dielectric and conducting) electrical impedance (real and imaginary) and tip–sample separation can be made. Measurements of the tip–sample capacitance versus tip–sample separation have been made and agree with estimated values. Also, the slope of the tip–sample capacitance with respect to the tip–sample distance is calculated to implement tip–sample distance regulation for dielectric materials.
Direct identification of the synergism between methyl radicals and atomic hydrogen during growth of amorphous hydrogenated carbon films
Authors: von Keudell A. , Schwarz-Selinger T. , Meier M. , Jacob W.
Published: July 2002

The simultaneous interaction of methyl radicals (CH3) and atomic hydrogen (H) with the surface of amorphous hydrogenated carbon (a-C:H) film is investigated. Two identical quantified beam sources for H and CH3 are used. The growth and/or erosion during the simultaneous interaction of the two beams with an amorphous hydrogenated carbon film is monitored by using in situ real-time ellipsometry at a substrate temperature of 320 K. Interaction with the CH3 beam alone causes slow growth, corresponding to a sticking coefficient for CH3 of ∼3×10−5. Simultaneous interaction of the atomic hydrogen beam and the CH3 radical beam yields a sticking coefficient for CH3 of 3×10−3, which is two orders of magnitude larger than for CH3 alone. From a microscopic modeling of this synergistic growth, the reaction probability for CH3 adsorbing at an adsorption site, which is created by atomic hydrogen at the surface, is derived to be 0.14.
Hole-induced transient bandgap renormalization: A mechanism for photo-induced absorption in defect-engineered semiconductors
Authors: Juodawlkis Paul W. , Ralph Stephen E.
Published: July 2002

In semiconductors, photoexcitation often results in a transient band edge-absorption bleaching due to the dominance of conduction-band filling over bandgap renormalization. In this letter, we show that the presence of electron traps can act to reverse this behavior so that photoexcitation results in an absorption increase in these same semiconductors. We associate this photo-induced absorption with bandgap renormalization caused by photoexcited holes that remain after the electrons have become trapped. We develop a dynamic model that predicts the wavelength dependence of photo-induced absorption and accurately describes the measured nonlinear-absorption recovery of low-temperature-grown InGaAs/InAlAs multiple quantum wells.
Spectral analysis of polymer microring lasers
Authors: Polson R. C. , Levina G. , Vardeny Z. V.
Published: July 2002

The emission spectra of microring lasers made of π-conjugated polymer films that coat glass optical fibers are analyzed with the help of a Fourier transform. This method allows for the assignment of photopumped emission lines to integral Bessel functions and a more precise determination of the laser threshold excitation intensity based on harmonic analysis.
Giant magnetoresistance of the La1−xAgxMnO3 polycrystalline inhomogeneous granular system
Authors: Tang T. , Cao Q. Q. , Gu K. M. , Xu H. Y. , Zhang S. Y. , Du Y. W.
Published: July 2002

A series of bulk polycrystalline La1−xAgxMnO3 samples with x ranging nominally from 0 to 0.5 were prepared by conventional solid-state reaction processing in air. X-ray diffraction patterns show that the samples contain a single perovskite phase when x⩽0.25, and are composed of two phases (a magnetic perovskite phase and a nonmagnetic Ag-rich phase) for x>0.25. It is found that, in this series of polycrystalline samples, maximum magnetoresistance occurs for x=0.30, i.e., for a composite of the two phases whose magnetoresistance ratio is about 25.5% at room temperature. The enhancement of the magnetoresistance effect in such an inhomogeneous granular system can be attributed to the spin-dependent scattering of electrons at the interface of the two phases.
Polycrystalline silicon produced by Ni–silicide mediated crystallization of amorphous silicon in an electric field
Authors: Jang Jin , Park Seong Jin , Kim Kyung Ho , Cho Bong Rae , Kwak Won Kyu , Yoon Soo Young
Published: July 2002

Crystallization of amorphous silicon (a-Si) by Ni–silicide mediated crystallization (SMC) has been studied in an electric field with Ni area density between 3.3×1013 and 5.7×1014 atoms/cm2 on the a-Si. The needlelike crystallites of ∼1000 Å in width and several micron length grow in the 〈111〉 directions with the 〈011〉 normal to the film surface when Ni area density was between 5.1×1013 and 2.9×1014 atoms/cm2. However, dendritic crystallites have been found in the a-Si matrix without complete crystallization of the a-Si when the Ni area density was 3.3×1013 atoms/cm2. The field-effect mobility of the thin-film transistor using the SMC poly-Si was 60–112 cm2/V s when the average Ni bulk density in the poly-Si was around 3.0×1018 atoms/cm3, and it decreases with increasing Ni density.
Anharmonic effects in photoinduced electron transfer
Authors: Evans Deborah G.
Published: July 2002

Electron transfer in condensed phase media is typically studied within the standard spin-boson model. The electron transfer is described between two coupled electronic states that are coupled linearly to a bath of displaced harmonic oscillators (bosonic degrees of freedom). The dynamics of this dissipative two level system is rich and well studied as a function of the extent of the coupling to the harmonic bath. Many recent experimental studies have focused on systems where the electron transfer is coupled to a set of high frequency intramolecular modes and lower frequency solvent modes. From semiclassical modeling and experimental studies, it is found that these modes can show anharmonic distortions. This work is a study of the effects of anharmonicities in the nuclear degrees of freedom on the electron transfer process. An approximation method, the Gaussian wave packet dynamics-path integral approach, developed by Coalson [J. Phys. Chem. 100, 7896 (1996)], is adapted to study condensed phase electron transfer in all coupling regimes. Along each path in the path integral sum, the anharmonic nuclear dynamics is calculated using the Gaussian wave packet approximation. The effects of anharmonic intramolecular modes and the effects of anharmonicities in a weakly coupled solvent bath on the donor electronic state population are found to vary greatly with system variables including exothermicity, the bath spectral density, and the nonadiabatic coupling strength.
Unexpected differential sensitivity of nuclear spin–spin-coupling constants to bond stretching in BH4−, NH4+, and SiH4
Authors: Sauer Stephan P. A. , Raynes William T.
Published: July 2002

Correlated ab initio calculations are reported, which show that stretching one of the Si–H bonds in silane from its equilibrium value increases the one-bond coupling J(Si, H) of the other three bonds by more than the increase of that of the stretched bond. This “unexpected differential sensitivity” in silane is considerably greater in terms of reduced coupling constants than the same effect discovered previously in methane. This UDS effect is also found to occur in the BH4− and NH4+ ions and comes overwhelmingly from the Fermi contact term. For the proton–proton coupling UDS due to the stretch of a single bond, also found earlier in methane, occurs in NH4+, just fails to appear in silane and is absent in BH4−. The origins of UDS and the signs of the coupling constant derivatives are discussed.
Temperature dependence of hydrophobic interactions: A mean force perspective, effects of water density, and nonadditivity of thermodynamic signatures
Authors: Shimizu Seishi , Chan Hue Sun
Published: July 2002

Temperature-dependent properties of hydrophobic interactions are investigated by simulating the potential of mean force (PMF) between two methane-like solutes in TIP4P model water. Independent results from test particle insertion and free energy perturbation are compared to ensure that zero-PMF baselines are accurate. PMFs are computed under atmospheric pressure at five temperatures from 5 to 95 °C using constant-pressure simulations. The temperature dependence we observe does not agree with previous results from constant-volume simulations, highlighting the important effects of temperature-dependent water density on PMFs. Heat capacity changes upon association of two solutes are estimated at the PMF contact minimum, desolvation barrier, and the solvent (water)-separated minimum. The magnitude of the heat capacity change upon contact formation is much smaller than that predicted by the solvent accessible surface area (SASA). More surprisingly, the heat capacity change upon bringing two methanes from infinity to the desolvation barrier is large and positive. This implies that the thermodynamic signatures of the free energy barrier to desolvation have signs opposite to desolvation itself. This feature is not predicted by either SASA or a volume-based solvent exclusion model. The implications of these and other observations on implicit-solvent model potentials are discussed. Formulations based on thermodynamic perturbation and Widom’s potential distribution theory are developed to relate PMF and hydration mean forces to the underlying structural properties of aqueous solutions. In particular, we provide a theoretical perspective to understand PMF in terms of local water density and the occurrences of configurations with highly unfavorable solute–solvent repulsive interactions.
Microstructure of the nitride layer of AISI 316 stainless steel produced by intensified plasma assisted processing
Authors: Jiang J. C. , Meletis E. I.
Published: July 2002

Nitrided austenitic 316 stainless steel produced by intensified plasma assisted processing has been studied by transmission electron microscopy (TEM), high-resolution TEM, and image simulation techniques. Cross-sectional TEM studies showed that the nitride layer is composed of a single phase that was found to possess a simple cubic structure. This nitride is produced by introducing one N atom into one of the interstitial sites of the octahedra within the unit cell of the γ-austenite. The lattice constant of the nitrided simple cubic structure was determined to be a=3.78 Å, which is expanded by about 5.4% from that of austenite. Stacking faults, antiphase domains, and antiphase domain boundaries in the nitride layer were observed using dark-field and high-resolution TEM imaging. The evolution of the nitride phase seems to be preceded by lattice expansion and formation of stacking faults due to the presence of N and is consistent with the observed lattice constant reduction with depth.
A density functional theory study of vibrational coupling between ribose and base rings of nucleic acids with ribosyl guanosine as a model system
Authors: Pelmenschikov Alexander , Hovorun Dmytro M. , Shishkin Oleg V. , Leszczynski Jerzy
Published: July 2002

The vibrational coupling between the ribose and base rings of nucleic acids is modeled by the ribose–guanine vibrational interaction in ribosyl guanosine at the density functional theory (DFT) level. Two coupling patterns are revealed for the in-plane guanine vibrations depending on the strength of the kinematic interaction between ribose and guanine through the glicosidic bond. For relatively weak interactions the coupling can be described in terms of a resonance between two vibrations which are originally close in frequency (the difference in frequency between the vibrations is within ∼20 cm−1). This coupling produces two modes corresponding to the in-phase and out-of-phase combinations of the original ribose and guanine vibrations, analogous to the symmetric and antisymmetric coupled modes of the carbonyl groups in anhydrides, imides, and 1,3-diketo compounds. For strong interactions involving a significant glicosidic bond stretch, the ribose and guanine moieties can no longer be considered as quasi-independent subsystems preserving the forms of their inherent vibrations. An unambiguous identification of the original ribose and guanine vibrations contributing to these combined modes is hardly possible. Taking into account (i) the large number of intrinsic ribose and base vibrations which can potentially participate in the coupling and (ii) the resonant character of many of these interactions, these results suggest that small changes in the ribose ring conformation and glicosidic bond orientation should result in noticeable changes of the related combined modes. This explains the phenomenon of high conformational sensitivity of the corresponding conformational marker bands of nucleic acids in vibrational spectroscopy.
Exciton–erbium interactions in Si nanocrystal-doped SiO2
Authors: Kik P. G. , Polman A.
Published: July 2002

The presence of silicon nanocrystals in Er doped SiO2 can enhance the effective Er optical absorption cross section by several orders of magnitude due to a strong coupling between quantum confined excitons and Er. This article studies the fundamental processes that determine the potential of Si nanocrystals as sensitizers for use in Er doped waveguide amplifiers or lasers. Silicon nanocrystals were formed in SiO2 using Si ion implantation and thermal annealing. The nanocrystal-doped SiO2 layer was implanted with different doses of Er, resulting in Er peak concentrations in the range 0.015–1.8 at. %. All samples show a broad nanocrystal-related luminescence spectrum centered around 800 nm and a sharp Er luminescence line at 1536 nm. By varying the Er concentration and measuring the nanocrystal and Er photoluminescence intensity, the nanocrystal excitation rate, the Er excitation and decay rate, and the Er saturation with pump power, we conclude that: (a) the maximum amount of Er that can be excited via exciton recombination in Si nanocrystals is 1–2 Er ions per nanocrystal, (b) the Er concentration limit can be explained by two different mechanisms occurring at high pump power, namely Auger de-excitation and pair-induced quenching, (c) the excitable Er ions are most likely located in an SiO2-like environment, and have a luminescence efficiency <18%, and (d) at a typical nanocrystal concentration of 1019 cm−3, the maximum optical gain at 1.54 μm of an Er-doped waveguide amplifier based on Si nanocrystal-doped SiO2 is ∼0.6 dB/cm.
Effect of size and roughness on light transmission in a Si/SiO2 waveguide: Experiments and model
Authors: Lee Kevin K. , Lim Desmond R. , Luan Hsin-Chiao , Agarwal Anuradha , Foresi James , Kimerling Lionel C.
Published: July 2002

In this letter, we experimentally evaluate the effect of miniaturization and surface roughness on transmission losses within a Si/SiO2 waveguide system, and explain the results using a theoretical model. Micrometer/nanometer-sized waveguides are imperative for its potential use in dense integrated optics and optical interconnection for silicon integrated circuits. A theoretical model was employed to predict the relationship between the transmission losses of the dielectric silicon waveguide and its width. This model accurately predicts that loss increases as waveguide width decreases. Furthermore, we show that a major source of loss comes from sidewall roughness. We have constructed a complete contour map showing the interdependence of sidewall roughness and transmission loss, to assist users in their design of an optimal waveguide fabrication process that minimizes loss. Additionally, users can find an effective path to reduce the scattering loss from sidewall roughness. Using this map, we confirm that nanometer-size silicon waveguides with 0.1 dB/cm transmission loss are possible with the currently available technology.
Reorganization of adsorbed films by coadsorbing species
Authors: Roke S. , Coquel J. M. , Kleyn A. W.
Published: July 2002

The coadsorption of CO and butane on a Pt(533) stepped surface has been investigated using reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). The adsorption of butane on Pt(533) with CO preadsorbed on step-atop sites reveals that butane can force CO to tilt with a minimum angle of 42° away from the surface normal and displace CO from step-atop to step-bridge sites. The energy required for this tilting should be less than 20.5 kJ/mol. The coverage at which the compressed butane phase occurred was found to be the same at which this phase occurs on bare Pt(533). Together with the observed tilting and displacement of CO, this suggests that at low coverages butane adsorbs on the terraces, rotated 60° away from the step edge. The second monolayer phase then consists of tilted butane molecules having two hydrogen atoms in direct contact with the surface, situated near the step edge. The presence of butane also results in a downward shift of the CO stretch frequency, caused by electron donation in the 2π* antibonding CO orbital. When butane is preadsorbed at a submonolayer coverage exposure to CO leads to displacement of butane into a compressed phase and even into a multilayer phase. This effect becomes smaller as the initial butane coverage is increased to the multilayer regime.
Investigating the growth of localized defects in thin films using gold nanospheres
Authors: Mirkarimi P. B. , Stearns D. G.
Published: July 2002

The response of a thin film to a localized perturbation at the substrate has been studied using a technique for depositing high densities of monodisperse Au nanospheres of known sizes on silicon substrates. We find that the morphology of a defect nucleated by a gold sphere several tens of nm in diameter can vary strongly with the growth conditions, even when the morphology of the unperturbed film does not. In particular we observed that the angle of incidence of the deposition flux plays a strong role in defect evolution during film growth, and that defect smoothing is enhanced when the deposition flux is directed nearly normal to the substrate surface. This work opens the door for a greater understanding of film growth on substrate defects and significantly impacts emerging technologies such as extreme ultraviolet lithography.
Hole equilibria in Vlasov–Poisson systems: A challenge to wave theories of ideal plasmas
Authors: Schamel H.
Published: July 2002

A unified description of weak hole equilibria in collisionless plasmas is given. Two approaches, relying on the potential method rather than on the Bernstein, Greene, Kruskal method and associated with electron and ion holes, respectively, are shown to be equivalent. A traveling wave solution is thereby uniquely characterized by the nonlinear dispersion relation and the “classical” potential V(φ), which determine the phase velocity and the spectral decomposition of the wave structure, respectively. A new energy expression for a hole carrying plasma is found. It is dominated by a trapped particle contribution occurring one order earlier in the expansion scheme than the leading term in conventional schemes based on a truncation of Vlasov’s equation. Linear wave theory— reconsidered by taking the infinitesimal amplitude limit—is found to be deficient, as well. Neither Landau nor van Kampen modes and their general superpositions can adequately describe these trapped particle modes due to an incorrect treatment of resonant particles for phase velocities in the thermal range. It is therefore concluded that wave theories in their present form, dictated by linearity, are not yet properly shaped to describe the dynamics of ideal plasmas (and fluids) correctly.
(Ga,Mn)As as a digital ferromagnetic heterostructure
Authors: Kawakami R. K. , Johnston-Halperin E. , Chen L. F. , Hanson M. , Guébels N. , Speck J. S. , Gossard A. C. , Awschalom D. D.
Published: July 2002

(Ga,Mn)As digital ferromagnetic heterostructures are grown by incorporating submonolayer planes of MnAs into GaAs using molecular beam epitaxy. Structural and magnetic measurements indicate single-crystalline superlattice structure and ferromagnetic order with Curie temperatures (TC) up to 50 K. By varying the spacing between neighboring Mn layers, we observe that TC initially decreases with increasing spacer thickness, followed by a regime with weak dependence on the spacer thickness. The persistence of ferromagnetism for interlayer spacings of at least 200 ML (∼560 Å) suggests that the individual Mn layers are ferromagnetic.
High nonlinearity of Ba0.6Sr0.4TiO3 films heteroepitaxially grown on MgO substrates
Authors: Park B. H. , Gim Y. , Fan Y. , Jia Q. X. , Lu P.
Published: July 2002

We have heteroepitaxially deposited Ba0.6Sr0.4TiO3 (BST) thin films on (001)-oriented MgO substrates using pulsed-laser deposition. By optimizing the deposition temperature and adjusting the film thickness, we have successfully increased the dielectric nonlinearity and decreased the dielectric loss of BST films. BST thin films grown at 750 °C with a thickness of 1.1 μm showed a dielectric constant tunability of greater than 65% and a tunability/loss of 43 at a surface electric field of 80 kV/cm at room temperature. X-ray diffraction and transmission electron microscopy analyses indicated that the tunability and dielectric loss were closely related to the crystallinity of the BST films.