bluuebunny/crossref_metadata_2025 huggingface dataset:
Appearance of superparamagnetism on heating nanosize Mn0.65Zn0.35Fe2O4
Authors: Rath Chandana , Mishra N. C. , Anand S. , Das R. P. , Sahu K. K. , Upadhyay Chandan , Verma H. C.
Published: July 2002

Nanosize particles (average size ∼12 nm) of mixed ferrite Mn0.65Zn0.35Fe2O4 were prepared by the hydrothermal precipitation route and studied using x-ray diffraction, transmission electron microscopy, differential scanning calorimetry, magnetization measurements, and Mössbauer spectroscopy. The as-prepared sample was largely ferrimagnetic and, as the sample was annealed at temperatures above 250 °C, it gradually became superparamagnetic. This unexpected behavior is explained by assuming that the cation distribution in the nanosize as-prepared sample is in a metastable state and, as the sample is heated, this distribution changes to a more stable state while the grain size remains nearly the same.
Ga Ohmic contact for n-type diamond by ion implantation
Authors: Teraji T. , Koizumi S. , Kanda H.
Published: July 2002

Electric contacts formed on an n-type diamond film by means of the Ga ion implantation were studied. The implanted Ga contacts revealed Ohmic property at room temperature. Contact resistivity for the implanted Ga contacts formed on the diamond film with donor density of 3×1018 cm−3 was 4.8×106 Ω cm2. The value is more than one order smaller than that for conventional vacuum-deposited metal contacts on the same diamond film. This contact has enabled us to measure electric properties for relatively lightly doped n-type diamond films in a temperature range between room temperature and 600 °C.
Surface relief gratings generated by a photocrosslinkable polymer with styrylpyridine side chains
Authors: Yamaki Shigeru , Nakagawa Masaru , Morino Shin’ya , Ichimura Kunihiro
Published: July 2002

The formation of holographic surface relief gratings on thin films of a liquid-crystalline polymethacrylate with styrylpyridine side chains is described. A film of the polymer was treated with hydrogen chloride to protonate the side chain moieties, followed by holographic exposure to p-polarized Ar+ laser beams at 488 nm and by post-exposure baking to give rise to surface relief gratings. The depth of the surface relief in a typical case was 15 nm which is approximately 15% of the original film thickness. On the basis of the temperature dependence of diffraction efficiencies of the grating, the mechanism of the generation of the relief gratings is discussed.
Temperature dependence of intrinsic recombination coefficients in 1.3 μm InAsP/InP quantum-well semiconductor lasers
Authors: Pikal J. M. , Menoni C. S. , Thiagarajan P. , Robinson G. Y. , Temkin H.
Published: July 2002

In this letter, we report on the temperature dependence of the intrinsic recombination coefficients in long-wavelength quantum-well lasers. Unlike previous studies, we obtain the intrinsic recombination coefficients from carrier lifetime measurements with a correction for the carrier population in the barrier and separate confinement heterostructure region. Our results show that this carrier population not only affects the value of the recombination coefficients obtained but also their temperature dependence. We measure a significant increase in the intrinsic Auger coefficient with temperature indicating that the frequently reported temperature insensitivity of this coefficient is likely due to carriers spilling out of the wells at elevated temperatures and not an intrinsic property of the Auger process.
Structural origin of coercivity enhancement and exchange-bias field in double antiferromagnet/ferromagnet bilayers
Authors: Hou C. , Fujiwara H. , Zhang K.
Published: July 2002

An experimental method to identify the structural origins of the coercivity enhancement and the exchange-bias field of polycrystalline ferro-antiferromagnetic (F/AF) coupled systems is proposed. The exchange-bias field and the coercivity of the FI layer of the samples of the structure FI/AFI are compared with those of the FII layer of the samples of the structure FII/AFII/FIII/AFIII. It is concluded that, for a given temperature, it is those AF grains which stop their growth at a critical thickness that contribute to the coercivity enhancement, and those which grow over the critical thickness that contribute to the exchange-bias field. Meanwhile the effective magnetic surface anisotropy introduced to the top surface of the AFII layer by the FIII layer can increase the exchange-bias field of the FII layer when the AFII layer is thin.
Kinetic oscillations of red photoluminescence from nanocrystalline Si/SiO2 films
Authors: Wu Xiaochun , Bittner Alexander M. , Kern Klaus , Eggs Ch. , Veprek S.
Published: July 2002

Experimental investigations on the red photoluminescence (PL) from nanocrystalline Si (nc-Si) embedded in a SiO2 matrix, fabricated by plasma-enhanced chemical vapor deposition and a subsequent post-treatment, reveal under certain conditions the occurrence of kinetic oscillations. For the red PL between 1.5 and 1.75 eV, the spectral shift and the peak intensity versus annealing times show temporal oscillations. The spectral variations are explained by the growth and decay kinetics of two oxygen thermal donors (TDs) upon annealing. The oscillatory behavior is a pure kinetic one. It reflects the regrowth of oxygen TDs by an autocatalytic process during their decay. Although the source for the autocatalysis is not clear, this PL oscillation adds more evidence that the red PL observed in our system originates from oxygen TDs-related defect states.
The direct production of CO(v=1–9) in the reaction of O(3P) with the ethyl radical
Authors: Reid Jonathan P. , Marcy Timothy P. , Kuehn Seppe , Leone Stephen R.
Published: July 2002

A new product channel that yields vibrationally excited CO(v=1–9) in the reaction of the ethyl radical with O(3P) is experimentally observed by time-resolved Fourier transform infrared emission spectroscopy. The branching ratios for the different vibrational states are estimated to be 0.21±0.06, 0.27±0.03, 0.14±0.02, 0.08±0.02, 0.07±0.02, 0.07±0.02, 0.06±0.02, 0.05±0.02, and 0.05±0.02 for v=1–9, respectively. Previously, only the CH3+H2CO, CH3CHO+H, and C2H4+OH channels were known. Kinetics tests are provided to verify that the CO is produced directly in the reaction and not from secondary chemistry. The two possible new product channels are CO+CH4+H and CO+CH3+H2. The implications of this previously unexplored reaction channel for combustion chemistry and the possible mechanisms for this reaction are discussed.
Paramagnetic defects at the interface of ultrathin oxides grown under vacuum ultraviolet photon excitation on (111) and (100) Si
Authors: Stesmans A. , Afanas’ev V. V.
Published: July 2002

Electron-spin-resonance monitoring of Si dangling-bond-type interface defects is used to study ultrathin (∼20 Å) Si-oxide films grown by vacuum ultraviolet (VUV)-enhanced oxidation of Si at 300 K. Large densities (up to ∼9×1012 cm−2) of Pb and Pb0 centers (interfacial Si3≡Si⋅) are observed in VUV-grown (111) and (100) Si/SiO2, respectively. As compared to standard thermal Si/SiO2, two major differences emerge: the VUV Si/SiO2 interface is under substantially enhanced stress, while no Pb1 defects are discerned in VUV (100) Si/SiO2. It is inferred that Pb1 generation requires a minimum amount of oxide relaxation. Microscopic understanding is provided for the known inferior electrical interface quality threatening low-thermal-budget oxide fabrication.
High frequency one-dimensional phononic crystal characterized with a picosecond transient grating photoacoustic technique
Authors: Dhar Lisa , Rogers John A.
Published: July 2002

Periodic patterns of relief defined on a substrate using conventional lithographic techniques are phononic crystals for surface localized acoustic waves. Picosecond transient grating characterization of the acoustic behavior of Rayleigh waves in structures of binary, one-dimensional patterns of relief with feature sizes of a few microns show that these crystals exhibit complete phononic band gaps at frequencies of a few tenths of a gigahertz. These types of structures could be useful for applications in surface acoustic wave devices; understanding their acoustic behavior will help to establish strategies for transient grating examination of complex microstructures.
Thermal expansion of bulk and homoepitaxial GaN
Authors: Kirchner V. , Heinke H. , Hommel D. , Domagala J. Z. , Leszczynski M.
Published: July 2002

The thermal behavior of Mg-doped and intentionally undoped bulk crystals and homoepitaxial GaN was investigated in a wide temperature range from 12 to 600 K. With high-resolution x-ray diffraction, both lattice parameters a and c were determined and the thermal expansion coefficients were calculated. Within the experimental accuracy, mean values were extracted for the temperature ranges 12–100, 100–250, and 250–600 K. These values are essential, especially, for the interpretation of measurements of other GaN properties performed at low temperatures.
Raman scattering studies on silver nanoclusters in a silica matrix formed by ion-beam mixing
Authors: Gangopadhyay P. , Kesavamoorthy R. , Nair K. G. M. , Dhandapani R.
Published: July 2002

The formation and growth of nanoscale silver clusters in silica matrices are studied by means of optical absorption and Raman scattering spectroscopy. The metallic silver layer of the Ag/SiO2 couple was irradiated by argon ions for ion-beam mixing of silver with the silica matrix. Postannealing of the embedded Ag atoms in silica matrices results in the growth of Ag nanoclusters. The quadrupolar surface acoustic vibrational mode of the Ag nanoclusters has been observed in low-frequency Raman scattering experiments. The average size of these Ag nanoclusters is estimated from the quadrupolar mode frequency. Systematic growth of Ag nanoclusters embedded in silica matrices is observed with the increase of postannealing temperature. We have estimated the activation energy for the diffusion of Ag atoms in irradiated a-SiO2 from the coarsening of these embedded Ag atoms. Development of a peak at about 3 eV in the optical absorption spectra for the samples postannealed at higher temperatures corroborates further the growth of Ag nanoclusters in the silica matrices. New Raman modes at 1368 and 1573 cm−1 are observed in Ag embedded silica samples and the possible origin of these modes is discussed.
A compact x-ray system for macromolecular crystallography
Authors: Gubarev Mikhail , Ciszak Ewa , Ponomarev Igor , Gibson Walter , Joy Marshall
Published: July 2002

We describe the design and performance of a high flux x-ray system for a macromolecular crystallography that combines a microfocus x-ray generator (40 μm full width at half maximum spot size at a power level of 46.5 W) and a collimating polycapillary optic. The Cu Kα x-ray flux produced by this optimized system through a 500 μm diam orifice is 7.0 times greater than the x-ray flux previously reported by Gubarev et al. [M. Gubarev et al., J. Appl. Crystallogr. 33, 882 (2000)]. The x-ray flux from the microfocus system is also 2.6 times higher than that produced by a rotating anode generator equipped with a graded multilayer monochromator (green optic, Osmic, Inc. CMF24-48-Cu6) and 40% less than that produced by a rotating anode generator with the newest design of graded multilayer monochromator (blue optic, Osmic, Inc. CMF12-38-Cu6). Both rotating anode generators operate at a power level of 5000 W, dissipating more than 100 times the power of our microfocus x-ray system. Diffraction data collected from small test crystals are of high quality. For example, 42 540 reflections collected at ambient temperature from a lysozyme crystal yielded Rsym=5.0% for data extending to 1.70 Å, and 4.8% for the complete set of data to 1.85 Å. The amplitudes of the observed reflections were used to calculate difference electron density maps that revealed positions of structurally important ions and water molecules in the crystal of lysozyme using the phases calculated from the protein model.
Ground state gas and solution phase conformational dynamics of polar processes: Furfural systems
Authors: Baldridge Kim K. , Jonas Volker , Bain Alex D.
Published: July 2002

The conductorlike continuum solvation model, modified for ab initio in the quantum chemistry program GAMESS, implemented at the Møller–Plesset Order 2 (MP2) level of theory has been applied to a group of push–pull pyrrole systems to illustrate the effects of donor/acceptor and solvation on the stability and energetics of such systems. The most accurate theoretical gas and solution phase data to date has been presented for the parent furan-2-carbaldehyde (furfural) system, and predictions made for three additional analogues, thiophene-2-carbaldehyde, pyrrole2-carbaldehyde, and, cyclopentadiene-1-carbaldehyde. Solvent effects on internal rotational barriers in all systems were evaluated over six different values of dielectric, using the new method. Calculated electrostatic energies are shown to be highly sensitive to level of theory incorporated.
Reorientational motion of the HS− ions in hydrogensulfides of alkali metals (NaHS, KHS, RbHS): A quasielastic neutron scattering study
Authors: Haarmann F. , Jacobs H. , Asmussen B. , Nöldeke C. , Kearley G. J. , Combet J.
Published: July 2002

The isostructural hydrogensulfides of the alkali metals sodium, potassium, and rubidium were investigated by quasielastic neutron scattering in order to gain insight into the dynamics of the molecular anion HS− as a function of temperature. A phase transition between the rhombohedral middle temperature modification (MTM) and the cubic high temperature modification (HTM) of the title compounds was of special interest. A two site jump model is clearly supported by the data of the MTM and an eight site jump model is consistent with the data of the HTM. The jump distances are in reasonable agreement with distances of the hydrogen positions determined by neutron powder diffraction on the fully deuterated samples. At the MTM→HTM phase transition the correlation times of the reorientational motion decrease by about one order of magnitude. Within the investigated temperature range (T=170 K to T=450 K) they change from 10−8 s to 10−12 s. They show Arrhenius behavior. The activation enthalpies of the MTM decrease with increasing radius of the cation from 26(1) kJ mol−1, 22(1) kJ mol−1 to 18(2) kJ mol−1 for NaHS, KHS and RbHS respectively. In the HTM they are equal within the estimated error 6(1) kJ mol−1 and 7(1) kJ mol−1 for NaHS and KHS.
Micromagnetics of ferromagnetic equilateral triangular prisms
Authors: Koltsov D. K. , Cowburn R. P. , Welland M. E.
Published: July 2002

A systematic study of micromagnetic configurations in thin equilateral triangular prisms as a function of lateral size and thickness is reported. The resulting map of a micromagnetic ground state is presented in the form of a micromagnetic phase diagram with two regions corresponding to buckle and Y-state configurations. These configurations led to the formation of a configurational anisotropy field, an ab initio calculation of which is presented and the results are compared with experiment. The calculated six-fold symmetric anisotropy field agreed well with experiment but was found to depend on the applied field in phase and magnitude. The anisotropy field dependence on the triangular nanostructure edge roughness was also investigated and related to the stability of the micromagnetic phases.
Enhanced wavelength tuning of an InGaAsP-InP laser with a thermal-strain-magnifying trench
Authors: Cohen D. A. , Mason B. , Dolan J. , Burns C. , Coldren L. A.
Published: July 2002

We have used temperature-dependent strain from differential thermal expansion to increase the temperature tuning rate, dλ/dT, of the modal wavelength of an InP-based laser. The effectiveness of the strain may be further enhanced with a deep trench etch beneath the laser waveguide. We have obtained a 50% increase in the tuning rate, without degradation of the threshold current, and a maximum increase of 86%.
Basis-set extrapolation of the correlation energy
Authors: Varandas A. J. C.
Published: July 2002

A simple theoretically motivated model to extrapolate the correlation energy based on correlation-consistent polarized X-tuple basis sets is suggested. It has the form EXcor=E∞cor[1+A3 X−3(1+A4 X−1)], where EXcor is the energy for the X-tuple basis set, E∞cor and A3 are parameters to be determined from a set (Xmin−Xmax) of correlation consistent basis sets at a given level of theory, and A4 is a function of A3. Even for the simple (2,3) extrapolation scheme, the method is shown to yield energies for 33 test data sets that are more accurate than those obtained from pure correlation consistent sextuple-zeta basis sets at a much lower computational cost. Other extrapolation schemes have also been investigated, including a simple one-parameter rule EXcor=E∞cor(1–2.4X−3).
Light-emission properties in nanocrystalline BaTiO3
Authors: Yu Jian , Sun Jinglan , Chu Junhao , Tang Dingyuan
Published: July 2002

This letter reports photoluminescence properties of nanocrystalline BaTiO3. Two intense emission bands centered at 543 and 694 nm and a weak one at 574 nm have been observed at room temperature. The change of luminescence peak with different heat treatments correlates to the evolution of Ti4+ defects in the nanocrystalline BaTiO3 matrice. The light-emission mechanism is explained within the framework of self-trapped excitons by combining quantum-size confinement and dielectric confinement effects.
Strain-induced diffusion in a strained Si1−xGex/Si heterostructure
Authors: Lim Y. S. , Lee J. Y. , Kim H. S. , Moon D. W.
Published: July 2002

Diffusivity of a strained heterostructure was theoretically investigated, and general diffusion equations with strain potential were deduced. There was an additional diffusivity by the strain potential gradient as well as by the concentration gradient. The strain-induced diffusivity was a function of concentration, and its temperature dependence was formulated. The activation energy of the strain-induced diffusivity was measured by high-resolution transmission electron microscopy. This result can be generally applied for the investigation of the diffusion in strained heterostructures.
Electrical properties of nearly stoichiometric GaAs grown by molecular beam epitaxy at low temperature
Authors: Fukushima S. , Obata T. , Otsuka N.
Published: July 2002

Nearly stoichiometeric GaAs epilayers doped with Si or Be were grown by molecular beam epitaxy at a low temperature with closely controlled fluxes near the stoichiometric conditions. Excess As point defects in the epilayers which acted as trap sites of free carriers were studied by means of the Hall effect measurements. Under the same As/Ga flux condition, concentrations of excess As point defects which trap free carriers increase with increase of concentrations of Si or Be. With the same concentration of the dopants, on the other hand, concentrations of excess As point defects increase linearly with the As/Ga flux ratio. The dependence of carrier mobility on the trapped carrier concentrations indicates that free carriers are trapped by neutral point defects and change them into ionized ones. In the epilayers which are very close to the stoichiometric composition, Si exhibits its amphoteric nature, occupying both donor and acceptor sites and results in the self-compensation.
A Fukui function overlap method for predicting reactivity in sterically complex systems
Authors: Clark Louis A. , Ellis Donald E. , Snurr Randall Q.
Published: July 2002

A formalism is given for predicting reactivity of complex systems by combining electronic structure calculations with forcefield calculations within a transition state theory framework. The theory is employed in combination with the Fukui function to produce a simulation method capable of the ensemble sampling needed to examine sterically complex systems. An important linkage between reactivity information and energetic quantities is provided by introduction of the Fukui overlap integral. This spatial overlap integral measures the coincidence of electron donating regions on a nucleophile with electron accepting regions on the corresponding electrophilic reactant. We show that configurations with high values of this overlap integral tend to have lower density-functional theory energies. Thus, Fukui functions calculated once on single isolated reactants can be used to quickly estimate the reactivity of configurations generated using conventional forcefield-based simulations. The correlation between energies and high overlap integrals can also be used to identify initial guess configurations for transition state searches. However, in the present implementation, real transition states are not accessible because intramolecular geometry relaxation is not allowed. The proposed method is tested on electrophilic aromatic alkylation reactions. Simulation results successfully reproduce experimental substituent effects in a series of variously substituted aromatics. Especially encouraging is the ability of the simulations to predict steric effects in the reaction of toluene with a series of electrophiles of varying bulkiness. Further applications, previously inaccessible to simulation, are expected in systems where steric effects play a dominant role in determining reaction selectivity.
Atom-resolved three-dimensional mapping of boron dopants in Si(100) by scanning tunneling microscopy
Authors: Liu Lequn , Yu Jixin , Lyding J. W.
Published: July 2002

The lack of surface states within the band gap of the perfect Si(100)2×1:H surface opens the way to scanning tunneling microscopy studies of dopant atom sites in Si(100). In this letter, boron-induced features on the Si(100)2×1:H surface are studied by ultrahigh vacuum scanning tunneling microscopy. Filled state images show hillock features while empty state images show local depressions associated with dopants. Furthermore, the amplitudes of these hillock features naturally group such that at least three subsurface layers can be identified. Current image tunneling spectroscopy is performed to study the electronic structure of the boron-induced features, which are explained by a simple model based on tip-induced band bending. This technique for producing atom-resolved three-dimensional maps of electrically active dopants in silicon may be a useful metric for characterizing dopant profiles in ultrasmall electronic device structures.
In situ observation of surface processes in InAs/GaAs(001) heteroepitaxy: The role of As on the growth mode
Authors: Ohtake Akihiro , Ozeki Masashi
Published: July 2002

Surface processes of the growing thin films of InAs on GaAs(001) substrates have been studied as a function of substrate temperature and As to In flux ratio. They have been observed by reflection high-energy electron diffraction and total-reflection-angle x-ray spectroscopy in real time. At temperatures lower than ∼480 °C, InAs grows in a Stranski–Krastanov mode irrespective of the As/In flux ratio, while the growth mode of InAs strongly depends on the flux ratio above ∼500 °C. We have found that the sticking probability of In decreases as the As flux is decreased above ∼500 °C, which results in the changes in the growth mode of InAs.
Effect of plasticizer on thermodynamic properties of nylons: Model, phenomenology, and proposed experiments
Authors: Zhang M. L. , March N. H. , Peeters A. , Van Alsenoy C. , Van Doren V. E.
Published: July 2002

Motivated by experiments showing decreasing tensile modulus of nylons with increasing plasticizer content, a model involving the disruption of nylon hydrogen bonding by addition of plasticizer is proposed. This leads to proposed further experiments and suggests a complementary phenomenological analysis that involves the bulk modulus. The surface energy as a function of plasticizer concentration is then incorporated into the phenomenology. The importance of varying the temperature in the proposed thermodynamic measurements is stressed.
The decay of pair correlation functions in ionic fluids: A dressed ion theory analysis of Monte Carlo simulations
Authors: Ulander Johan , Kjellander Roland
Published: July 2002

We analyze the decay of structural correlation functions for 1:1, 1:2, and 2:2 electrolyte solutions obtained from Monte Carlo simulations. It is found that by the use of dressed ion theory and a simple Picard iteration scheme one can extract the leading decay parameters with high accuracy, even from simulations with a rather limited number of ions in the simulation cell. The extraction scheme consists of replacing in a self-consistent manner the tails of the simulated pair distribution functions by analytical expressions evaluated by residue analysis of short-ranged parts of the correlation functions. Numerical results in this work are restricted to primitive model electrolytes where the solvent only enters as a dielectric continuum. The leading decay parameters of the simulated correlation functions are compared to results obtained from the hypernetted chain (HNC) approximation. For 1:1 and 1:2 electrolytes in aqueous solution the simulation results confirm predictions from the HNC approximation. For 2:2 electrolytes the HNC results agree qualitatively with the simulations but deviate quantitatively. To investigate artifacts induced by boundary conditions used in the simulations we analyze correlation functions obtained from simulations in a spherical cell as well as with cubic periodic boundary conditions. The results and method of analysis presented are restricted to electrolyte concentrations at which the leading decay terms of the pair distribution functions exhibit monotonic exponential decay.
Parametric study of the vacuum ultraviolet emission and electrical characteristics of a He–Xe microdischarge
Authors: Postel Olivier B. , Cappelli Mark A.
Published: July 2002

Vacuum ultraviolet emission and electrical characteristics of a simple discharge configuration consisting of two planar cylindrical electrodes operated with a dc voltage have been measured over a wide range of He/Xe mixtures and discharge pressures. Breakdown characteristics are consistent with those found in the literature, however current–voltage characteristics and the inferred discharge resistivity suggest the presence of a complex process controlling electron emission at the cathode. Ultraviolet vacuum emission maps of atomic and molecular xenon at 147, 150, and 173 nm, respectively, have been measured as a function of pressure, from 60 to 500 Torr, and gas mixture, from pure Xe to 5% Xe in He. The calibrated ratios of each emission map help to visualize the zones of strongest ultraviolet emission over a wide range of operating conditions. One-dimensional simulations of the breakdown voltage and current discharge have been performed using the commercially available discharge-modeling package SIGLO. Good agreement with experimental results is found in the case of pure helium and xenon, however, in the case of pure xenon, the gas temperature was adjusted (elevated) in order to reproduce the measured current–voltage characteristics. Modeling of the electron number density distribution indicates that the discharge is principally composed of a thick ion sheath near the cathode.
Annealing thin colloidal crystals with optical gradient forces
Authors: Korda Pamela T. , Grier David G.
Published: July 2002

We describe methods for annealing colloidal crystals using scanned optical tweezers. Tweezer-induced excitations drive a well-localized region of the colloidal lattice out of thermal equilibrium with its supporting fluid. Subsequent plastic and elastic relaxation of defects yields large defect-free domains.
Analysis of the power-law response in the fractal dielectric model by thermally stimulated currents and frequency spectroscopy
Authors: Bisquert Juan , Garcia-Belmonte Germà
Published: July 2002

Dielectric properties of a structurally disordered insulator were investigated in the framework of a theoretical model. The model consists of a self-similar structure composed of Debye relaxators which describes a system with a wide distribution of relaxation times that exhibits power-law relaxation at high frequencies in constant temperature measurements. The analysis of the correspondent behavior in a thermally stimulated depolarization experiment shows that the current in the low temperature side of the depolarization peak displays a power-law dependence on heating rate which is a consequence of the self-similar property of the system. The fractionary power-law is related to the fractal dimension by a=2(1−D/3). Therefore the analysis establishes the correlation between isothermal and nonisothermal measurements in dielectric materials that exhibit quasiuniversal nonexponential relaxation.
Ta-based interface ohmic contacts to AlGaN/GaN heterostructures
Authors: Qiao D. , Jia L. , Yu L. S. , Asbeck P. M. , Lau S. S. , Lim S.-H. , Liliental-Weber Z. , Haynes T. E. , Barner J. B.
Published: July 2002

Al/Ti based metallization is commonly used for ohmic contacts to n-GaN and related compounds. We have previously reported an ohmic contact scheme specially designed for AlGaN/GaN heterostructure field-effect transistors (HFETs) [D. Qiao et al., Appl. Phys. Lett. 74, 2652 (1999)]. This scheme, referred to as the “advancing interface” contact, takes advantage of the interfacial reactions between the metal layers and the AlGaN barrier layer in the HFET structure. These reactions consume a portion of the barrier, thus facilitating carrier tunneling from the source/drain regions to the channel region. The advancing interface approach has led to consistently low contact resistance on Al0.25Ga0.75N/GaN HFETs. There are two drawbacks of the Al/Ti based advancing interface scheme, (i) it requires a capping layer for the ohmic formation annealing since Ti is too reactive and is easily oxidized when annealing is performed in pure N2 or even in forming gas, and (ii) the atomic number of Al and that of Ti are too low to yield efficient backscattered electron emission for e-beam lithographic alignment purposes. In this work, we investigated a Ta based advancing interface contact scheme for the HFET structures. We found that the presence of Ta in this ohmic scheme leads to (1) a specific contact resistivity as low as 5×10−7 Ω cm2, (2) efficient electron emission for e-beam lithographic alignment, and (3) elimination of the capping layer for the ohmic annealing.
Light-induced multiple electronic-state coupling of O2+ in intense laser fields
Authors: Hishikawa Akiyoshi , Liu Shilin , Iwasaki Atsushi , Yamanouchi Kaoru
Published: July 2002

The dissociation processes, O2+→O++O, in intense laser fields (100 fs, ∼3×1014 W/cm2) are studied at the laser wavelength of 795 and 398 nm on the basis of the momentum imaging maps of the O+ fragment ions. The covariance map measurements are performed with high momentum resolution to assign securely the dissociation pathways. From the anisotropic momentum distribution of O+ with respect to the laser polarization direction, the electronic states of O2+ are found to be coupled exclusively through parallel transitions in the course of the dissociation process. The released kinetic energy of O+ for both 795 and 398 nm are interpreted consistently by a sequential coupling model of light-dressed potential curves of O2+ in the quartet Π state manifold.
Doping-induced losses in AlAs/GaAs distributed Bragg reflectors
Authors: Asplund C. , Mogg S. , Plaine G. , Salomonsson F. , Chitica N. , Hammar M.
Published: July 2002

We have studied n- and p-type doping-induced performance degradation of AlAs/GaAs distributed Bragg reflectors (DBRs) for applications in vertical cavity lasers (VCLs). Based on high-accuracy optical reflectance and triple-axis x-ray diffraction measurements on a variety of differently doped DBR structures grown by metalorganic vapor-phase epitaxy, a fitting procedure was employed to extract the doping-dependent optical loss. A striking observation is that the reflectance of these DBRs is much more sensitive to n- than p-type doping incorporation. While in the latter case the loss can be well accounted for by intervalence-band and free-carrier absorption, additional loss mechanisms must be considered for n-type DBRs. We relate the losses to doping-enhanced interdiffusion effects resulting in increased interface scattering. These findings should have important consequences for the design of VCLs, demonstrating the importance of reduced n-type doping concentrations and/or growth temperatures, or the application of alternative device concepts, e.g., employing intracavity contacts.
Ablation plasma ion implantation experiments: Measurement of Fe implantation into Si
Authors: Qi B. , Gilgenbach R. M. , Lau Y. Y. , Johnston M. D. , Lian J. , Wang L. M. , Doll G. L. , Lazarides A.
Published: July 2002

Experiments have been performed demonstrating the feasibility of direct implantation of laser-ablated metal ions into a substrate. Initial experiments implanted iron ions into silicon substrates at pulsed, bias voltages up to negative 10 kV. Implantation of Fe ions into Si was confirmed by cross-sectional transmission electron microscopy and x-ray photoelectron spectroscopy. The 7.6 nm depth of damage layers below the Si surface is slightly less than predicted by code calculations for a maximum, effective ion energy of about 8 keV. The ion depth of penetration is limited by the overlying Fe film as well as the slow rise and fall of the voltage.
Atomic structures at a Si–nitride/Si(001) interface
Authors: Ikarashi Nobuyuki , Watanabe Koji , Miyamoto Yoshiyuki
Published: July 2002

We used high-resolution transmission electron microscopy to show that the atomic structures at a Si3N4/Si interface are clearly different from those at a SiO2/Si interface. Using first-principles calculations, we also found that, in one of the observed N-induced interfacial geometries, a dangling bond was produced on a Si atom adjacent to a N atom. We thus argue that such N-induced interfacial dangling bonds can cause degradation in the performance of metal–oxide–semiconductor transistors with Si–oxynitride (SiON) gate dielectrics when the N concentration is increased at the SiON/Si interfaces. We also argue that the difference in flatness between Si3N4 and SiON/Si interfaces and SiO2/Si interfaces is the result of the difference between their atomic structures.
Nondestructive probe of microstructures covered with a micrometer-thick insulating layer
Authors: Shin Hyun-Joon , Lee Min-Kyu
Published: July 2002

We present an imaging technique which uses a scanning photoelectron microscope to image microstructures covered with a micrometer-thick insulating layer at submicrometer depth and lateral-space resolution. Line shifting in the photoelectron spectrum caused by local charging in the insulating surface layer provides depth information. This technique was used to nondestructively image micropatterns in a semiconductor device down to 3 μm beneath a surface insulating layer. The depth resolution was 0.2 μm, and the lateral-space resolution was 0.5 μm.
Coulomb logarithm of a nonideal plasma
Authors: Ramazanov T. S. , Kodanova S. K.
Published: July 2002

The Coulomb logarithm is a fundamental plasma parameter in kinetic theory. The traditional formula for the Coulomb logarithm does not correctly account for collisional processes in systems because it is obtained by using an unscreened Coulomb potential. In the present work, the Coulomb logarithm is derived on the basis of a strongly screened pair effective potential. The effective potential takes into consideration long-range many-particle screening effects. An analytical interpolation expression for the Coulomb logarithm is obtained. This formula could be used for further calculations of kinetic properties of classical nonideal plasma.
Molecular dynamics studies of the effects of chain branching on the properties of confined alkanes
Authors: Wang Jee-Ching , Fichthorn Kristen A.
Published: July 2002

Using a recently developed approach, we perform molecular-dynamics simulations to probe the influence of chain branching on the properties of fluids confined between two solid surfaces. Two decane isomers are investigated: n-decane and 2,2-dimethyloctane. Under confinement, n-decane forms a layered structure consistent with structures observed for n-alkanes in previous simulation studies. In contrast, 2,2-dimethyloctane forms a “pillared-layered” structure consisting of a mix of molecules oriented parallel and perpendicular to the confining surfaces. Although both molecules exhibit solvation forces that oscillate between attractive and repulsive as a function of surface separation, pronounced differences are observed between the two isomers. The number of confined n-decane molecules changes in a stepwise manner as the surface separation is varied, while the number of 2,2-dimethyloctane molecules varies in a smooth fashion due to perpendicular “pillar” molecules gradually switching between parallel and perpendicular orientations. In addition, chain branching reduces the densities and structural changes in the adjacent layers, causing solvation forces and force oscillations to be less pronounced than those of linear chains. It also weakens the influence of pressure on the structure and properties. At separations corresponding to well-ordered films, the translational diffusivity of n-decane is the lowest, while at separations corresponding to disordered films, we recover the bulk diffusivity for n-decane. The diffusivity of 2,2-dimethyloctane is lower and its variation with surface separation is less than that seen for n-decane.
MgB
2
tunnel junctions and 19 K low-noise dc superconducting quantum interference devices
Authors: Zhang Y. , Kinion D. , Chen J. , Clarke John , Hinks D. G. , Crabtree G. W.
Published: July 2002

Point contact junctions made from two pieces of MgB2 can be adjusted to exhibit either superconductor-insulator-superconductor (SIS) or superconductor-normal metal–superconductor (SNS) current–voltage characteristics. The SIS characteristics are in good agreement with the standard tunneling model for s-wave superconductors, and yield an energy gap of (2.02±0.08) meV. The SNS characteristics are in good agreement with the predictions of the resistively-shunted junction model. DC superconducting quantum interference devices made from two SNS junctions yield magnetic flux and field noise as low as 4 μΦ0 Hz−1/2 and 35 fT Hz−1/2 at 19 K; Φ0 is the flux quantum.
Photoreflectance study of ion beam synthesized β-FeSi2
Authors: Birdwell A. G. , Collins S. , Glosser R. , Leong D. N. , Homewood K. P.
Published: July 2002

The photoreflectance spectra of ion beam synthesized β-FeSi2 reveals a direct gap at 0.815 eV and are shown to agree with the band gap value obtained by photoluminescence once the adjustments for the temperature difference and trap related recombination effects are made. This provides very convincing evidence for intrinsic light emission from ion beam synthesized β-FeSi2. Furthermore, we propose a simple model that helps to clarify the variety of inconsistent results obtained by optical absorption measurements. When the results of photoluminescence and photoreflectance are inserted into this model, we obtain good agreement with our measured optical absorption results.
Rapid thermal oxidation of radio frequency sputtered polycrystalline silicon germanium films
Authors: Choi W. K. , Natarajan A. , Bera L. K. , Wee A. T. S. , Liu Y. J.
Published: July 2002

The oxide growth of rf sputtered polycrystalline Si1−xGex films was found not sensitive to the Ge concentration in the films. The infrared results showed that the oxide grown on Si0.61Ge0.39 film mainly contained GeO2 and the oxide contained Ge–O–Ge and Si–O–Ge bonds when grown on Si0.73Ge0.27 film. X-ray photoelectron spectroscopy results showed the absence of Ge in the bulk of the oxides for Si1−xGex films with x<0.27 and no pile-up of Ge at the SiO2/Si1−xGex interface. The electrical breakdown fields of oxides grown on Si1−xGex films were lower than the oxide breakdown field of polysilicon. The Dit and Qf values of the SiO2/Si1−xGex system were found to be rather high at ∼2.1–2.6×1012 eV−1 cm−2 and ∼1.1–2.7×1012 cm−2, respectively.
Use of a permanent magnet in the synthetic antiferromagnet of a spin-valve
Authors: Seigler Michael A. , Anderson Paul E. , Shukh Alexander M.
Published: July 2002

A spinvalve using a permanent magnet as the pinned layer in a synthetic antiferromagnet of a spinvalve is described here. Permanent magnet materials can have the following advantages over antiferromagnetic materials: superior corrosion resistance, a large switching field even at relatively high temperatures, and not needing a high temperature anneal to set its magnetic orientation. The giant magnetoresistance, high field switching, and switching at high temperatures was investigated for various spinvalves using the following film stacks as the pinned layer: CoCrPt (CCP); CCP\CoFe, CCP\Ru\CoFe; and CCP\CoFe\Ru\CoFe. It was found that a thin CoFe layer between the CCP and Cu and between the CCP and Ru enhances giant magnetoresistance and antiferromagnetic coupling, respectively. When the CCP is used in a synthetic antiferromagnet, the CCP coercivity increases compared to a single layer of CCP. The switching field of CCP at 425 °C is >1000 Oe, at the same temperature the switching field for an antiferromagnetic material is ∼0 Oe. If used in a magnetic readback head, the improved thermal stability may allow for larger bias currents and larger readback voltages.
Force spectroscopy with a large dynamic range using small cantilevers and an array detector
Authors: Schäffer Tilman E.
Published: July 2002

The important characteristics of a detector for force spectroscopy measurements are sensitivity, linearity and dynamic range. The commonly used two-segment detector that measures the position of a light beam reflected from the force-sensing cantilever in an atomic force microscope becomes nonlinear when the beam shifts significantly onto one of the segments. For a detection setup optimized for high sensitivity, such as needed for the use with small cantilevers, it is shown both experimentally and theoretically that the dynamic range extends to an upper detection limit of only about 115 nm in cantilever deflection if <10% nonlinearity is required. A detector is presented that circumvents that limitation. This detector is based on a linear arrangement of multiple photodiode segments that are read out individually. With such an array detector, the irradiance distribution of the reflected beam is measured. The reflected beam not only shifts in position but also deforms when the cantilever deflects because the bent cantilever acts as a curved mirror. The mean of the distribution, however, is a linear function of cantilever deflection in both theory and experiment. An array detector is consequently well suited for force measurements for which both high sensitivity and a large dynamic range are required.
Effect of residual stresses and surface roughness on coercive force in amorphous alloys
Authors: Tejedor M. , Garcı́a J. A. , Carrizo J. , Elbaile L. , Santos J. D.
Published: July 2002

The residual stresses in Fe40Ni40(Si+B)19Mo1 and Fe40Ni38B18Mo4 amorphous ribbons have been studied and the thermal treatments necessary to remove them have been performed. Based on this study the dependence of coercivity Hc on the applied stress σ in samples in the as-quenched state and after the thermal treatments have been compared. The results show that the compressive residual stresses are related with the minimum in the curves Hc(σ) for the samples in the as-quenched state and when the residual stresses are removed the minimum nearly disappears. Finally, in samples without surface roughness and without residual stresses the increase of Hc when the applied stress increases is explained by a nucleation term.
Infrared spectroscopy of ethanol clusters in ethanol–hexane binary solutions
Authors: Murdoch Keith M. , Ferris Thomas D. , Wright John C. , Farrar Thomas C.
Published: July 2002

Temperature and concentration dependent infrared spectra were recorded for binary solutions of ethanol–hexane, ethanol–carbon tetrachloride, and ethanol–cyclopentane. The temperature range covered was 198 K to 298 K; concentrations ranged from 0.45 mole percent to 4.0 mole percent ethanol. Changes in the OH stretch frequency are consistent with the formation of ethanol clusters (EtOH)n, where n ranges from 2 to 6. The geometry, OH stretch frequency and proton chemical shift for 14 different ethanol clusters ranging from monomer to hexamer in cyclic and linear arrangements were investigated using density functional methods (B3LYP/6-31+G*). These clusters include both gauche and anticonformers of the monomer unit. The OH stretch frequency calculations were compared to experimental Fourier transform infrared measurements made as a function of concentration and temperature for dilute ethanol in hexane binary mixtures. Analysis of the O–H stretch frequency data (3100 to 3700 cm−1) indicate the presence of small clusters of ethanol. The data indicate that at the lower temperatures the clusters are primarily cyclic tetramers, pentamers, and hexamers. This infrared data is in excellent agreement with recent concentration and temperature dependent nuclear magnetic resonance chemical shift investigations that indicate a dramatic change in cluster size with concentration and temperature.
Modification of optical properties by strain-induced piezoelectric effects in ultrahigh-quality V-groove AlGaAs/GaAs single quantum wire
Authors: Liu Xing-Quan , Wang Xue-Lun , Ogura Mutsuo , Guillet Thierry , Voliotis Valia , Grousson Roger
Published: July 2002

We report tiny strain-induced piezoelectric effects in an ultrahigh-quality AlGaAs/GaAs V-groove quantum wire structure. Zero photoluminescence excitation (PLE) absorption intensities are observed at low temperatures. Excitation power density-dependent optical properties provide evidence that the zero PLE absorption intensities are due to an internal electric field created by tiny strain-induced piezoelectric polarization along the wires, which causes complete spatial separation of the electron and hole wave functions along the wires. Absorption is enhanced by shining a He–Ne laser as a background in order to screen the electric field, which confirms the existence of piezoelectric field effects.
Photoreflectance study in the E and E+Δ transition regions of GaP
Authors: Mishima Takehito , Miura Masa-aki , Ozaki Shunji , Adachi Sadao
Published: July 2002

Photoreflectance (PR) measurements have been carried out to determine the E0 and E0+Δ0 critical-point (CP) parameters in GaP at temperatures between 12 and 300 K using a He–Cd laser as modulation light source. The measured PR spectra provide distinct structures at ∼2.75–2.85 eV (E0) and ∼2.85–2.95 eV (E0+Δ0). These structures are successfully explained by a three-dimensional (3D) one-electron line shape plus an excitonic shape. The temperature dependence of the 3D CP and excitonic parameters (energy, amplitude, and broadening parameter) have been determined and analyzed using the Varshni equation and an empirical expression of Bose–Einstein type. The rapid decrease in the PR signal amplitudes is observed at T<100 K and can be explained by the weakened surface electric fields due to carrier freezing at such low temperatures. The 3D exciton binding energy at the E0/(E0+Δ0) edges of GaP has also been determined to be 13 meV.
Dynamics of a single-planar domain wall in ferroelectric–ferroelastic Gd2(MoO4)3
Authors: Lupascu Doru C. , Shur Vladimir Ya. , Shur Alevtina G.
Published: July 2002

The field-induced jump-like dynamics of a single planar domain wall are investigated in single crystal uniaxial ferroelectric–ferroelastic Gd2(MoO4)3 containing extended bulk defects. The kind and location of the defects interfering with the free motion of the domain wall determines which type of elastic vibration is excited by the discontinuous jump of the domain wall. Time coincidence between optically observed jumps, Barkhausen pulses, and acoustic emissions each occur for particular jumps only. The time coincidence conditions correlate with the vibration spectra and differ for the initial release of the wall, large domain wall jumps, and clamped motions.