bluuebunny/crossref_metadata_2025 huggingface dataset:
Conductance oscillations due to a controllable impurity in a quantum box
Authors: Joe Yong S. , Cosby Ronald M. , Dharma-Wardana M. W. C. , Ulloa Sergio E.
Published: July 2002

We present calculations of conductance in a multiply connected nanostructure with a quantum box geometry. Well-defined conductance oscillations appear which are attributed to the quantum interference effect in the presence of a controllable impurity in the quantum box. As the strength of the impurity potential is modulated, conductance oscillations arise from the constructive and destructive interference for the two electronic paths around the centrally located impurity and a third tunneling path through the impurity. We discuss the dependence of these oscillations on the size of the impurity, in terms of circulating or bound states in the quantum box formed by multiple reflections of the phase-coherent electron. The conductance oscillations are predicted to be strong for realistic structural parameters and robust against increasing temperature.
Microstructural aspects and mechanism of the C49-to-C54 polymorphic transformation in titanium disilicide
Authors: Ma Z. , Allen L. H. , Allman D. D. J.
Published: July 2002

Microstructural study of the C49–TiSi2 to C54–TiSi2 polymorphic transformation has been performed to elucidate microstructural evolution and possible mechanism of the phase transformation. It has been shown that the nucleation of the C54–TiSi2 is heterogeneous, and preferentially takes place at triple grain junctions or grain boundaries. The interphase interfaces between C49 and C54 disilicides are often ragged with incoherent characteristics. The growth of the C54 phase is found to proceed by advancing the highly mobile incoherent interfaces in all directions toward the heavily faulted C49 phase. No rigorous orientation relationships are found between the two phases. The microstructural features of the transformation bear some massive characteristics.
Impression and diffusional creep of anisotropic media
Authors: Yang Fuqian , Li J. C. M.
Published: July 2002

Some diffusion problems in the impression and diffusional creep of anisotropic materials are analyzed. It is found that at the limit of low stress, both the diffusional creep rate and impression velocity are proportional to the applied stress. For a parallelepiped crystal under simple tension or compression in the Z direction, the diffusional creep rate depends on all three principal diffusivities, DX, DY, and DZ. Two limiting cases depend on the quantity √DXDY/DZ. When this quantity is large the creep rate is proportional to √DZ but when it is small the creep rate is independent of DZ. For a cylindrical crystal under tension or compression in the axial Z direction, the creep rate depends on Dr/DZ. When the ratio is large, the creep rate is the same as the isotropic case except that the effective diffusivity is √DrDZ. When the ratio is small, the creep rate is proportional to Dr and independent of DZ. For the impression creep of a half-space the punch velocity is proportional to the geometric mean of the principal diffusivities parallel and perpendicular to the loading direction, and inversely proportional to the punch dimension. For impression creep of a thin film deposited on an impermeable substrate under the same punching stress, the impression velocity is dependent only on the diffusivity parallel to the thin film, and inversely proportional to the square of the punch dimension. Without the substrate, the impression velocity is dependent only on the diffusivity perpendicular to the thin film, inversely proportional to the thickness of the film, and independent of the punch dimension.
Polycrystalline Zn3P2 Schottky photodiode: Vacuum surface effects
Authors: Hava S.
Published: July 2002

Thin-film polycrystalline Mg/Zn3P2 Schottky photodiodes were fabricated. Their optical and electrical properties under 1 atm air and 10−5 Torr vacuum conditions were measured. It was observed that the spectral quantum efficiency improved under vacuum. These changes were a result of surface effects. Surface ionic charge at the bulk-surface interface and at the grain boundary induces a surface depletion region which is affected by vacuum operation, hence affecting the photodiode properties.
Concurrent thermal and electrical modeling of sub-micrometer silicon devices
Authors: Lai Jie , Majumdar Arun
Published: July 2002

High electric fields, that are characteristic of sub-micron devices, produce highly energetic electrons, lack of equilibrium between electrons, optical phonons, and acoustic phonons, and high rates of heat generation. A simple coupled thermal and electrical model is developed for sub-micron silicon semiconductor devices consisting of the hydrodynamic equations for electron transport and energy conservation equations for different phonon modes. An electron Reynolds number is proposed and used to simplify the electron momentum equation. On a case study of the metal-oxide-semiconductor field-effect transistor with 0.24 μm gate length, the calculated transconductance of 0.175 1/Ω m agreed well with measured value of 0.180 1/Ω m at 2 V drain voltage. The maximum electron temperature is found to occur under the drain side of the gate where the electric field is the highest. Comparison with experimental data shows the predictions of optical and acoustic phonon temperature distributions to have the correct trend and the observed asymmetric behavior. Increase in substrate boundary temperature by 100 °C reduces the drain current by 17% and decreases the maximum electron temperature by 8%. The first effect increases device delay and the second effect decreases the possibility of device degradation by charge trapping in the gate oxide.
Magnetic anisotropies in thick body centered cubic Co
Authors: Liu X. , Stamps R. L. , Sooryakumar R. , Prinz G. A.
Published: July 2002

Magnetic anisotropies of a 357-Å-thick Co film in the bcc phase were examined using Brillouin light scattering. This film, one of the thickest known bcc Co structures, is found to have a fourfold magnetic anisotropy that is markedly different from those of thinner films. A large possibly strain-induced uniaxial anisotropy is also found. The film is thick enough so that the surface and n=1 bulk magnon are within a few GHz of each other and strongly hybridize. Unusual behavior of the scattering intensities suggest the possible presence of surface anisotropies and/or inhomogeneous internal fields.
Temperature dispersion of refractive indices in semiconductors
Authors: Ghosh Gorachand
Published: July 2002

The variations of refractive index with temperature, i.e., the thermo-optic coefficients (dn/dT), are analyzed critically by use of a physical model for some important semiconductors to find the refractive index at any operating temperature for any wavelength throughout the transmission region. This model is based on three physical parameters—the thermal-expansion coefficient and two optical band gaps, such as excitonic and isentropic that are lying in the UV region, instead of considering any empirical equations.
Carrier localization in porous silicon investigated by time-resolved luminescence analysis
Authors: Mihalcescu I. , Vial J. C. , Romestain R.
Published: July 2002

We analyzed the photoluminescence (PL) mechanisms of porous silicon, and in particular, the origin of the PL high quantum efficiency (QE) at room temperature. For this we used postformation treatments, anodic oxidation, and hydrofluoric acid (HF) etching (known for their strong QE enhancement effect) correlated with a PL time resolved analysis. A third parameter was the temperature which, for heating above room temperature, gave a reversible quenching of the PL. All three parameters give a similar evolution of the PL decay shape, which we consider to originate from the same evolution of the carrier dynamics. Porous silicon is described as an undulating wire. The high QE at room temperature is attributed to carrier localization inside minima of the fluctuating potential along the wire; these considerations are extended to another porous material: amorphous porous silicon. Anodic oxidation and HF dissolution diminish the wire size, giving a reduction of the localization length of the carriers and progressive suppression of the nonradiative recombination channel. A simple model permits one to link the changes of the PL decay shape to the QE evolution. The nonexponential PL decay shape is interpreted as being due to a distribution of nonradiative recombination rates, the value of the nonradiative recombination rate being limited by a tunneling effect. This highly simplified model explains the origin of the nonexponential decay shape, its modification and gives a good description of the QE evolution as a function of temperature, oxidation level, or porosity.
On the origin of electrically active defects in AlGaN alloys grown by organometallic vapor phase epitaxy
Authors: Polyakov A. Y. , Shin M. , Freitas J. A. , Skowronski M. , Greve D. W. , Wilson R. G.
Published: July 2002

Shallow and deep centers were studied by means of temperature dependent Hall effect and photoluminescence (PL) measurements in two sets of undoped n-AlGaN samples grown by organometallic vapor phase epitaxy. The samples of these two series were grown under different conditions and had, as a result, electron concentrations differing by several orders of magnitude. The composition dependence of ionization energies of dominant donors in these two sets of samples is very different indicating that different types of centers are involved, but in both cases they are most probably related to some native defects. These defects behave as hydrogen-like donors for low Al compositions and become increasingly deeper with increasing Al content. The shallow-deep transition occurs at about x=0.2 in the low conductivity AlxGa1−xN series and at about x=0.5 for the high conductivity series. Several PL bands were detected in AlGaN and it is shown that the band at 3.05 eV is due to a radiative transition between deep donors in the upper part of the band gap and holes in the valence band or on shallow acceptors. For the yellow luminescence band at 2.25 eV it is demonstrated that this band consists of two overlapping bands and that the dominant band is due to a transition between the native donors and a carbon-related deep center.
Polymer composite thermistors for temperature and current sensors
Authors: Strümpler Ralf
Published: July 2002

The electrical resistivity of polymer composites is studied as a function of temperature. The initial resistivity ρ of thermoplastic or thermoset containing a metallic filler is in the range of 1–10×10−2 Ω cm. Around the curing temperature of epoxies, the resistivity increases by eight to twelve orders of magnitude. For thermoplastic polymers, however, the transition temperature is related to the melting temperature at which a strong volume increase occurs. Hence, the choice of polymer and its processing determine the transition temperature from a conducting state to an insulating state. For a variety of polymers we have observed transitions between 80 and 200 °C. Due to a sharp and strong transition at a predetermined temperature, such materials can be used as temperature sensors. Since the resistivity of the cold state is low, they can also carry rather high currents. The balance between heating and cooling determines then a critical value for the current. Thus, the materials can also serve as current sensors.
Novel dislocation structure and surface morphology effects in relaxed Ge/Si-Ge(graded)/Si structures
Authors: Samavedam Srikanth B. , Fitzgerald E. A.
Published: July 2002

The defect structure in relaxed graded Ge/GexSi1−x/Si structures grown on (001) exact and (001) off-cut substrates using ultra-high vacuum chemical vapor deposition was characterized using transmission electron microscopy (TEM), atomic force microscopy, and electron beam induced current. The samples grown on off-cut (001) substrates showed a remarkable improvement in surface roughness and dislocation pile-up densities. By applying both a dislocation blocking criterion and surface roughness to graded Si-Ge/Si(001) structures, we can predict the formation of dislocation pile-ups in graded structures. Nonparallel misfit dislocation networks in off-cut wafer samples are not as efficient at blocking perpendicular dislocation motion, leading to a large reduction in dislocation pile-up density. The lower pile-up density on layers grown on off-cut wafers results in less stress-induced surface instability during growth, leading to surfaces with much lower roughness. TEM studies revealed that the array of 60° dislocations, that usually forms to relieve the misfit stress, transforms into a lower energy hexagonal dislocation network consisting of edge dislocations with Burgers vectors of the type 1/2〈110〉, 1/2〈1̄10〉, and 〈100〉. Such reactions were found to be more prevalent in the samples grown on off-cut substrates. Favorable intersections of {111} type planes on the off-cut substrates were found to aid such reactions.
Homoepitaxial growth of GaN under Ga-stable and N-stable conditions by plasma-assisted molecular beam epitaxy
Authors: Tarsa E. J. , Heying B. , Wu X. H. , Fini P. , DenBaars S. P. , Speck J. S.
Published: July 2002

The structure, morphology, and optical properties of homoepitaxial GaN layers grown by molecular beam epitaxy on metalorganic chemical vapor deposition (MOCVD)-grown GaN “template” layers were investigated as a function of the group III/group V flux ratio during growth. GaN layers grown with a low III/V ratio (N-stable growth) displayed a faceted surface morphology and a tilted columnar structure with a high density of stacking faults. In contrast, films grown with a high III/V ratio (Ga-stable growth) displayed comparable structure to the underlying MOCVD-grown template. The transition from N-stable to Ga-stable growth modes was found to occur over a narrow range of Ga fluxes at a growth temperature of 650 °C. Evidence of Ga accumulation and step-flow growth was observed for films grown under Ga-stable conditions, leading to the formation of spiral growth features at the surface termination of mixed edge/screw dislocations. Photoluminescence measurements indicate that the deep-level (∼550 nm) emission is increased relative to the near-band edge emission for films grown under N-stable conditions.
Speckle photography during dynamic impact of an energetic material using laser-induced fluorescence
Authors: Asay Blaine W , Laabs Gary W. , Henson Bryan F. , Funk David J.
Published: July 2002

Laser and white light speckle photography have been used to observe surface displacement in a number of materials and over a varied range of strain rates. However, each suffers from limitations. We have developed a novel application of speckle photography in very difficult environments by using laser-induced fluorescence to generate the speckle pattern. This permits confinement of the free surface without undue degradation of the correlation upon which speckle methods are based. We have applied this method to measure the surface displacement of a reactive material during dynamic deformation at moderate strain rates. Conventional methods were tried but were unsuccessful, necessitating a novel approach. To the best of our knowledge, neither high-speed laser nor white light speckle photography has been performed using energetic materials. These measurements are very difficult because of the low material strength (yield strength ∼8–80 MPa), and because significant out-of-plane motion and surface disruption occur during fracture, and early during the deformation process. We report results from experiments in which these major problems have been overcome.
Temperature dependence of photoluminescence spectra in InAs/GaAs quantum dot superlattices with large thicknesses
Authors: Dai Y. T. , Fan J. C. , Chen Y. F. , Lin R. M. , Lee S. C. , Lin H. H.
Published: July 2002

In this report, we investigate the thermal relaxation of the photoluminescence (PL) in InAs/GaAs quantum dot superlattices with large thicknesses that have two to more than three times the critical thickness for spontaneous island formation. It is found that the linewidth first decreases and then increases with increasing temperature. In addition to thermionic emission, we suggest that carrier repopulation among quantum dots plays an important role in the PL quenching. The temperature dependence of PL peak energy following a Varshni relation was attributed to the dilation of lattice and electron-lattice interaction. The emission intensity quenches rapidly when the temperature rises to around 60 K, indicating the existence of defect-related centers in the vicinity of InAs/GaAs interfaces. In addition, we performed the measurement of the activation energy of PL quenching at different emission energy. We found that the loss mechanism of PL quenching based on the activation of electron-hole pairs from quantum dots to the adjacent barriers was difficult to explain the behavior in quantum dots with large thicknesses. In this case, we suggest that the PL quenching is dominated by the recombination through interface defects.
The Ngai coupling model of relaxation: Generalizations, alternatives, and their use in the analysis of non-Arrhenius conductivity in glassy, fast-ionic materials
Authors: Macdonald J. Ross
Published: July 2002

The ionic conductivity of glassy, fast-ion-conducting materials can show non-Arrhenius behavior and approach saturation at sufficiently high temperatures [J. Kincs and S. W. Martin, Phys. Rev. Lett. 76, 20 (1996)]. The Ngai coupling model was soon applied to explain some of these observations [K. L. Ngai and A. K. Rizos, Phys. Rev. Lett. 76, 1296 (1996)], but detailed examination and generalization of the coupling model suggested the consideration of a related, yet different, approach, the cutoff model. Although both the coupling and cutoff models involve a shortest nonzero response time, τc, and lead to single-relaxation-time Debye response at limiting short times and high frequencies, they involve different physical interpretations of their low- and high-frequency response functions. These differences are discussed; the predictions of both models in the frequency and time domains are compared; and the utility of both models is evaluated for explaining the non-Arrhenius conductivity behavior associated with the dispersed frequency response of zAgI+(1−z)[0.525Ag2S+0.475B2S3:SiS2] glass for z=0 and 0.4. The cutoff approach, using simulation rather than direct data fitting, yielded semiquantitative agreement with the data, but similar analysis using the coupling model led to poor results. The coupling model leads to an appreciable slope discontinuity at the τc transition point between its two separate response parts, while the cutoff model shows no such discontinuity because it involves only a single response equation with a smooth transition at τc to limiting single-relaxation-time response. The greater simplicity, utility, and generality of the cutoff model suggest that it should be the favored choice for analyzing high-conductivity data exhibiting non-Arrhenius behavior.
Equilibrium constant of segregation-induced Fe gettered by heavy boron doping in Si
Authors: Tobe Satoshi , Hayamizu Yoshinori , Kitagawara Yutaka
Published: July 2002

The equilibrium constant of segregation-induced Fe gettering by heavy boron (B) doping in Si is obtained by experimental determination of the activation energy and the site density parameter of the gettering reaction. The activation energy is determined to be 0.68±0.03 eV by analyzing the temperature dependence of the equilibrium constant of the reaction. This result indicates that the Fe gettering is strongly related to formation of the Fe–B complex whose binding energy is very close to the value determined for the activation energy. Furthermore, the gettering site density is found to be proportional to the doped B concentration in the substrate crystal. It is understood from the activation energy value that the Fe gettering ability of each reaction site for heavy B doping is less than that for thin polycrystalline Si film on the back surface of a substrate. The well known high overall gettering capability of the heavily B-doped substrate is not simply due to the activation energy effect it is predominantly due to a large number of gettering sites in the overall region of the substrates and the closeness of the impurity Fe to the gettering sites.
In
1−x
Ga
x
As
1−y
P
y
/
InP
multiple quantum well solar cell structures
Authors: Raisky O. Y. , Wang W. B. , Alfano R. R. , Reynolds C. L. , Stampone D. V. , Focht M. W.
Published: July 2002

In 1−x Ga x As 1−y P y / InP -based multiple quantum well (MQW) solar cell structures with built-in sequential resonant tunneling were experimentally investigated. The MQW region extended the lower edge of the solar cell absorption compared to the control non-MQW InP sample. An average increase of 11% in photovoltaic efficiency of the MQW sample over the control sample was observed. The measured open circuit voltage (Voc) was found to be in good agreement with the predicted value. The bias and excitation wavelength dependences of photoluminescence and photocurrent at room temperature were used to investigate carrier escape and recombination dynamics in the MQW and control samples.
Analysis of memory retention characteristics of ferroelectric field effect transistors using a simple metal–ferroelectric–metal–insulator–semiconductor structure
Authors: Ashikaga Kinya , Ito Toshio
Published: July 2002

Memory retention characteristics of metal–ferroelectric–metal–insulator–semiconductor (MFMIS) field effect transistors (FETs) were investigated in detail using a simple structure, referred to as quasi-MFMIS, in which one electrode of the metal–ferroelectric–metal (MFM) capacitor is connected to a gate electrode of a conventional metal–oxide–semiconductors (MOS) FET using an external interconnection cable. It was found that the memory window of the MFMIS FET was quickly lost (after about 1000 s) and from a comparison with simulations, this was attributed mainly to a decrease in ferroelectric polarization due to a depolarization field inevitably remaining in the ferroelectric film during memory retention.
Two-step kinetics of As/P exchange reaction
Authors: Suh K. Y. , Lee Hong H. , Yoon E.
Published: July 2002

A simple two-step mechanism is used to derive the kinetics of the As/P exchange reaction which takes place on an epitaxially grown InP surface exposed to As flux. The first step involves surface exchange of arsenic with phosphorus, which is then followed by the second step, bulk exchange of arsenic (arsenic incorporation). Two possible choices are investigated for bulk exchange: the same exchange rate constant in the bulk and the same ratio of exchange rate constants in the bulk. Transient and steady-state profiles of As composition and the maximum depth of the As/P exchange reaction are derived analytically.
Effects of the piezoelectric field on quantum-confined Stark effect in (111)B InGaAs quantum-well structure
Authors: Bruno John D. , Tober Richard L.
Published: July 2002

The quantum-confined Stark effect is studied in a single In.15Ga.85As quantum well embedded within the intrinsic region of a (111)B GaAs p-i-n diode structure. An expression for the density of quasibound quantum well states is derived and used to determine the optical transition energies in the structure. Satisfactory agreement between experimental results and theoretical predictions are obtained only when an In.15Ga.85As piezoelectric constant of e14=−0.11(3)  C/m2 is used in the analysis. This value for e14 is 79% of what one would obtain from a linear interpolation of the binary e14s and is significantly different from results obtained by others. Comparisons are made with other work, and speculations are made regarding the cause for the differences.
Characterization of excess carbon in cubic SiC films by infrared absorption
Authors: Sun Yong , Miyasato Tatsuro , Wigmore J. Keith
Published: July 2002

The behavior of excess carbon in cubic SiC films was investigated using infrared absorption spectroscopy of modes which were optically activated by defect-induced strain in the film. The results show that the excess carbon which is formed interstitially in the region of SiC grain boundaries as an interstitial phase, consists of both crystalline and amorphous phases. The crystalline phase declines and the amorphous phase grows when the growth temperature is increased above 700 °C.
Measurements and modelling of the barrier heights and ideality factors in the metal/conducting polymer composite Schottky device
Authors: Bandyopadhyay S. , Bhattacharyya A. , Sen S. K.
Published: July 2002

Metal/polymer Schottky contacts have been fabricated using electrochemically prepared free standing thin films of conducting polyaniline/polycarbonate composite as well as conducting polyaniline pellets with various metals such as Al, In, Pb and Sn. The current–voltage characteristics have been studied from room temperature down to 100 K. The data have been analyzed and interpreted on the basis of the thermionic emission mechanism. The barrier height varies from 0.6 to 0.7 V for pellet and from 0.7 to 0.8 V for composite films. There is little dependence of metal on the work function. The ideality factor is dependent on the amount of polyaniline incorporation in polycarbonate, as indicated by the shift in the carbonyl peak in the Fourier transform infrared spectrum. The abnormal decrease in barrier height and increase of ideality factor with decrease in temperature have been interpreted assuming a Gaussian distribution of barrier heights at the interface. This takes account of the nonuniformity and inhomogeneities at the interface. As in the case of inorganic semiconductor diodes, the analysis indicates an apparent decrease in zero bias height and nonlinearity in activation plots. The bias dependence of barrier height and standard deviation causing an increase in ideality factor at low temperature has also been explored.
Coulomb blockade thermometry using a two-dimensional array of tunnel junctions
Authors: Bergsten Tobias , Claeson Tord , Delsing Per
Published: July 2002

We have measured current–voltage characteristics of two-dimensional (2D) arrays of small tunnel junctions at temperatures from 1.5 to 4.2 K. This corresponds to thermal energies larger than the charging energy. We show that 2D arrays can be used as primary thermometers in the same way as one-dimensional (1D) arrays, and even have some advantages over 1D arrays. We have carried out Monte Carlo simulations, which agree with our experimental results.
Diffuse interface model for electromigration and stress voiding
Authors: Bhate Deepali N. , Kumar Ashish , Bower Allan F.
Published: July 2002

We report a diffuse interface or phase field model for simulating electromigration and stress-induced void evolution in interconnect lines. Our approach is based on the introduction of an order parameter field to characterize the damaged state of the interconnect. The order parameter takes on distinct uniform values within the material and the void, varying rapidly from one to the other over narrow interfacial layers associated with the void surface. The evolution of this order parameter is shown to be governed by a form of the Cahn–Hilliard equation. An asymptotic analysis of the equation demonstrates, as intended, that the zero contour of the order parameter tracks the motion of a void evolving by diffusion under the coupled effects of stresses and the “electron wind” force. An implicit finite element scheme is used to solve the modified Cahn–Hilliard equation, together with equations associated with the accompanying mechanical and electrical problems. The diffuse interface model is applied to simulate a range of problems involving void evolution in interconnect lines. Results obtained are shown to be in excellent agreement with those computed previously using equivalent sharp interface models.
Temperature study of trap-related photoluminescence decay in CdSxSe1−x nanocrystals in glass
Authors: Nĕmec P. , Malý P.
Published: July 2002

The trap-related photoluminescence dynamics in CdSxSe1−x nanocrystals (for x=0.24 and x=0.74) in glass were studied in the temperature interval 10–300 K. A close link between the temperature behavior of the photoluminescence decay rate and that of the photoluminescence efficiency was found, which indicates a dominant role of nonradiative recombination. The trap-related photoluminescence was interpreted as the recombination of a shallowly trapped electron with a hole in a deep trap. An exponential decrease (with a constant in the range 0.2–0.4 eV) in the density of the deep traps with increasing energy above the valence band was found. Apart from the Arrhenius type of temperature behavior of decay rate with the activation energy ≈50 meV, the Berthelot type (characteristic temperature ≈160 K) was also observed. The latter was modeled by carrier tunneling between localized sites and the energetic distribution of the tunneling distances was obtained.
Ion induced magnetization reorientation in Co/Pt multilayers for patterned media
Authors: Weller D. , Baglin J. E. E. , Kellock A. J. , Hannibal K. A. , Toney M. F. , Kusinski G. , Lang S. , Folks L. , Best M. E. , Terris B. D.
Published: July 2002

Co/Pt multilayer films with perpendicular magnetic anisotropy and large out-of-plane coercivities of 3.9–8.5 kOe have been found to undergo a spin reorientation transition from out-of-plane to in-plane upon irradiation with 700 keV nitrogen ions. X-ray reflectivity experiments show that the multilayer structure gets progressively disrupted with increasing ion dose, providing direct evidence for local atomic displacements at the Co/Pt interfaces. This effectively destroys the magnetic interface anisotropy, which was varied by about a factor of 2, between KS≅0.4 erg/cm2 and KS≅0.85 erg/cm2 for two particular films. The dose required to initiate spin-reorientation, 6×1014 N+/cm2 and 1.5×1015 N+/cm2, respectively, scales with KS. It is roughly equal to the number of Co interface atoms per unit interface area contributing to KS.
Element specific spin-resolved densities of states in amorphous Fe75B25 probed with a synchrotron radiation source
Authors: Xu Y. B. , Greig D. , Seddon E. A. , Matthew J. A. D.
Published: July 2002

The partial spin-dependent densities of states (DOS) of both the iron and the boron in Fe–B amorphous magnetic alloys have been determined from spin-resolved photoemission using a synchrotron radiation source. The spin-integrated energy distribution curves (EDCs), polarization spectra and the spin-resolved EDCs show distinct differences between 15 and 40 eV photon energies due to strikingly different photon energy dependencies of the cross sections of Fe-d and B-p states. The B-p states were found to hybridize with Fe-d states and occupy a binding energy range of about 1–5.5 eV with a negative polarization; i.e., the B moment is “antiferromagetically” coupled with that of the Fe-d moment.
InGaN/GaN multiple quantum well light-emitting diodes with highly transparent Pt thin film contact on p-GaN
Authors: Huh Chul , Kim Hyun-Soo , Kim Sang-Woo , Lee Ji-Myon , Kim Dong-Joon , Lee In-Hwan , Park Seong-Ju
Published: July 2002

The fabrication and characterization of an InGaN/GaN multiple quantum well light-emitting diode (LED) with a highly transparent Pt thin film as a current spreading layer are described. The room temperature electroluminescence exhibits a strong emission at 453 nm. Pt-contacted LEDs show good electrical properties and high light-output efficiency compared to Ni/Au-contacted ones. The light transmittance and the specific contact resistance of a Pt thin film with a thickness of 8 nm on p-GaN was determined to be 85% at 450 nm and 9.12×10−3 Ω cm2, demonstrating that a Pt thin film can be used as an effective current spreading layer with high light transparency.
Preparation and magnetic properties of Ba2Co2Fe28O46 nanocrystals
Authors: Xiong Gang , Mai Zhen Hong
Published: July 2002

Ba 2 Co 2 Fe 28 O 46 nanocrystals were synthesized through a stearic acid sol–gel method. The reaction temperatures were notably lower than that of the conventional ceramic method. The nanocrystalline particles obtained at 750 °C were spherical in shape with grain sizes ranging from 10 to 25 nm. Their morphology became more cubic as the heat-treatment temperature was increased. The surface composition of the nanocrystals was found to be different from that of the bulk Ba2Co2Fe28O46, exhibiting a higher content of Ba and Co. The magnetic properties of these samples are clearly size dependent. The specific saturation magnetization is lower than that of bulk Ba2Co2Fe28O46 and inversely proportional to the grain size D. This can be explained by assuming the existence of a nonmagnetic layer on the surface of the particles. The coercivity of Ba2Co2Fe28O46 nanocrystals reaches a maximum when the particle sizes are approximately equal to 70 nm and then decreases by a factor of 3 as the particle size is further reduced. These results are attributed to superparamagnetic effects in a random distribution of single domain ferromagnetic particles.
Collisional broadening of rotation–vibration lines for asymmetric-top molecules. II. H2O diode laser measurements in the 400–900 K range; calculations in the 300–2000 K range
Authors: Hartmann J. M. , Taine J. , Bonamy J. , Labani B. , Robert D.
Published: July 2002

Numerous comparisons between predictions of the model presented in part I of this paper and experimental H2O infrared linewidths are presented. It is shown that our model, contrary to those used up to now, gives accurate results for H2O room-temperature line broadening by O2 and Ar, and for high rotational quantum-number lines by N2. First accurate experimental widths and intensities of some H2O ν2-band lines in the 400–900 K temperature range are also presented. Detailed analysis of the data demonstrates the great influence of a ‘‘resonance overtaking’’ mechanism. The latter results from the modifications of both the perturber rovibrational population distribution and kinetic energy with temperature; it strongly enhances the contributions of the collision-induced rotational transitions involving significant energy jumps. This mechanism is well accounted for by our model and quantitatively explains the unusually slow decrease of some linewidths with temperature.
Absolute quenching cross section for collisions between Ar(3P0,2 ) and H2 O
Authors: Novicki Stephen , Krenos John
Published: July 2002

We have developed a new method for measuring quenching cross sections for metastable atom reactions yielding luminescent products. Application of our method to the Ar*+H2 O system yields an absolute quenching cross section with a strong E−0.5 dependence over the energy range of 0.045–0.130 eV.
Equilibrium constants for the formation of van der Waals dimers: Calculations for Ar–Ar and Mg–Mg
Authors: Dardi Peter S. , Dahler John S.
Published: July 2002

A variety of calculations of the equilibrium constant for the formation of atom–atom van der Waals complexes have been performed. Several approximate classical and quantum methods are compared with ‘‘exact’’ numerical calculations. Included are approximations which involve uncoupling, to varying degrees, of the vibrational and rotational motions. These approximate techniques can be generalized to larger systems for which exact calculations are infeasible. Calculations are performed for both Ar2 and the somewhat more strongly bound Mg2, using realistic analytic potentials and more approximate Lennard-Jones potentials. Our goal is to understand better the accuracy of approximate calculations of the equilibrium constants for weakly bound systems.
An exact expression for the transverse nuclear magnetic resonance relaxation of a dynamic scale invariant polymer chain governed by a single relaxation time
Authors: Brereton M. G.
Published: July 2002

An exact analytic calculation of the transverse nuclear magnetic resonance (NMR) relaxation function, due to dipolar interactions, is presented for a polymer chain considered at the scale invariant level of description. The calculation is possible for the particular case where the dynamics of the bond vectors are governed by a single relaxation time. This exact result is used to check the accuracy of two approximation methods when they are applied to the single relaxation time case. The well known second moment approximation is shown to be seriously wrong when the time scale of the bond dynamics is comparable or greater than the NMR time scale set by the dipolar interactions. A more recent method, based on a representation of the chain dynamics in terms of hybrid Fourier components defined over the experimental time interval, is shown to give excellent agreement with the exact result over the entire range of relaxation times.
Silicon nitride formation by low energy N+ and N+
2 ion beams
Authors: Park K. H. , Kim B. C. , Kang H.
Published: July 2002

Reactions of N+ and N+2 ions with Si(100) surface are examined as a function of both ion kinetic energy and dose using a low energy ion beam instrument. The Si surface is exposed to low energy (1–300 eV) ion beams in an ultrahigh vacuum environment and the resulting surface species are characterized by Auger electron spectroscopy and ultraviolet photoelectron spectroscopy. The absolute reaction probability Pr is measured for nitridation processes. Pr(N+) has a value of ∼0.25 and stays constant in the energy range of 1–25 eV. Pr(N+2) increases from zero to ∼0.25 in the same range. Continued exposure of the ion beams to a dose ≳5×1015 ions/cm2 leads to a saturation and formation of a dense and stable silicon nitride layer. Variation of Pr with energy and dose is explained in terms of elementary reaction steps such as charge neutralization of the projectile ion, collisional dissociation of N+2, nitridation reaction, and chemically induced desorption of surface nitrogen species. A mechanism is proposed to explain enhancement in the stability and ordering of the nitride layer during the low energy ion–surface interaction. At saturation the processes of both nitridation and chemically induced desorption simultaneously occur, which, under hyperthermal environments, effectively remove defects and interstitial nitrogen atoms from the layer.
Pressure-induced amorphization of wollastonite (CaSiO3) at room temperature
Authors: Serghiou George C. , Hammack William S.
Published: July 2002

Energy dispersive x-ray diffraction and Raman measurements show that the mineral wollastonite (CaSiO3) becomes amorphous at a pressure of 25.6 GPa at 300 K. A consideration of the high pressure/high temperature behavior of CaSiO3 shows that the amorphization occurs because the phase transition wollastonite→perovskite CaSiO3 is kinetically impeded. We suggest that the amorphous phase may be viewed as a defective long-period modulated wollastonite phase.
Can the density maximum of water be found by computer simulation?
Authors: Billeter S. R. , King P. M. , van Gunsteren W. F.
Published: July 2002

Path integral molecular dynamics (PIMD) simulations of the extended simple point charge (SPC/E) model of liquid water (both H2O and D2O) have been performed in order to find the temperature at which the maximum liquid density is obtained. For comparison, purely classical SPC and SPC/E water have also been simulated over long periods (more than 200 ps). Structural properties and various temperature dependent quantities are reported. Special attention has been given to the fluctuations of pressure and volume in simulations run in the canonical (constant NVT) and isothermal–isobaric (constant NPT) ensembles, respectively. Although a density maximum is detected by monitoring energy–volume correlations as a function of temperature, the large statistical uncertainty in the correlations reduces the significance of this finding. The main conclusions of this work are threefold. First, the use of the energy–volume correlation moment to determine the density maximum appears to be a useful method, although a number of very long simulations of between 0.5–1.0 ns over a fairly wide temperature range are necessary to obtain accurate results. Second, neither the SPC nor the SPC/E water models are able to reproduce the density of cold water accurately. Finally, a quantum mechanical treatment of the water model is necessary to correctly represent the structure, energy, and fluctuations of the condensed phase.
Grand canonical Monte Carlo simulations of the distribution and chemical shifts of xenon in the cages of zeolite NaA. II. Structure of the adsorbed fluid
Authors: Jameson Cynthia J. , Jameson A. Keith , Lim Hyung-Mi , Baello Bernoli I.
Published: July 2002

The quantitative agreement between the results of a grand canonical Monte Carlo (GCMC) simulation and the various direct experimental measures of the distribution of the Xe atoms between adsorbed phase and gas phase, of intrazeolitic xenon among the alpha cages, and of the distribution of n Xe atoms in a Xen cluster within one alpha cage permit us to consider the structure of the adsorbed fluid in the GCMC simulation as a reasonable description of the actual structure. We provide here the adsorption sites for a single Xe atom in the alpha cage of zeolite NaA, the transition states between these adsorption sites, the one-body distribution functions for the individual clusters Xen inside the alpha cage, the Xe–Xe pair distribution functions for Xe2 through Xe8 at two temperatures, and some of the local minima in the configuration space of the clusters Xe2 through Xe8, i.e., some of the minimum energy configurations of the clusters.
Electron affinity of clusters and liquids of polar molecules: Theory of many-body polarization interactions
Authors: Stampfli P.
Published: July 2002

We examine the attractive interaction between an excess electron and the dipole moments of polar molecules forming a cluster or a liquid. This interaction determines the size dependence of the electron affinity of these clusters. We use the polarizable electropole model for the molecules and include interactions between the permanent and induced dipole moments. Thus, we obtain that the interaction between the excess electron and the molecules is strongly decreased by the induced polarization of the bound electrons, which is opposed to the permanent dipole moments and makes a dielectric screening. Note that previous calculations of the electron affinity of clusters of polar molecules resulted in much larger vertical detachment energies of the excess electron than observed in the experiment. This error is due to the use of an unphysical pair-potential approximation, which neglects the interaction between the permanent dipole moments and induced dipole moments. Thus the attractive interaction between the excess electron and the molecules and the resulting electronic binding energies have been overestimated. The results of our microscopic theory are identical with macroscopic continuum models and agree very well with experimental results. This theory also might be important for the study of solvent effects on oxidation–reduction reactions and on photosynthesis.
Determination of the structure of HBr OCS
Authors: Walker A. R. Hight , Chen Wei , Novick Stewart E. , Bean Brian D. , Marshall Mark D.
Published: July 2002

The structure of the HBr OCS van der Waals complex has been studied by Fourier transform microwave spectroscopy. The ground state complex is hydrogen bound and quasilinear with SCO–HBr atomic ordering. Spectra from five different isotopomers were observed and assigned. The wide amplitude bending angle of the hydrogen bromide was calculated from the nuclear quadrupole coupling constant χaa to be 25.2°. Second order quadrupole effects, centrifugal distortion in the nuclear quadrupole coupling constant Dχ, a spin–rotation interaction, CBr, and a spin–spin interaction, Daa, were all included in the Hamiltonian. The following spectroscopic constants have been determined for the H79Br OCS isotopomer: B̄=488.7948(4) MHz; DJ=2.167(6) kHz; HJ=1.19(3) Hz; χaa=387.14(1) MHz; Dx=6.79(14) kHz; CBr=0.55(13) kHz; and Daa=7.7(1.2) kHz.
The effect of 1s correlation on D
e, r
e, and ωe of first-row diatomics
Authors: Császár Attila G. , Allen Wesley D.
Published: July 2002

A series of De, re, and ωe ab initio spectroscopic data for N2, O2, F2, CO, NO, and HF is presented, as determined via the coupled-cluster singles and doubles approach including a perturbational estimate of connected triple excitations [CCSD(T)], appropriately implemented with correlation-consistent, polarized-valence and -core one-particle basis sets varying in quality from [4s3p2d1f] to [12s6p5d4f1g] and [6s5p4d3f2g]. In these molecules, 1s correlation causes the following (De,re) shifts in (kcal mol−1, Å): N2(+0.78,−0.0021), O2(+0.24−0.0020), F2(−0.09,−0.0014), CO(+0.93,−0.0024), NO(+0.42,−0.0021), and HF(+0.18,−0.0006). Evaluation of quadratic force constants at the precise experimental re distances reveals that valence CCSD(T) treatments exhibit an inherent 0.2%–0.8% overestimation of harmonic frequencies for the closed-shell species as the one-particle limit is approached, the error for O2(X 3Σg−) and NO(X 2Π) being slightly higher. At fixed reference geometries the inclusion of 1s correlation serves to lower harmonic frequencies by 3 to 9 cm−1 and proportionately reduce the deficiency in valence treatments by 0.1%–0.4%.
Subpicosecond photoinduced electron transfer from conjugated polymers to functionalized fullerenes
Authors: Kraabel B. , Hummelen Jan C. , Vacar D. , Moses D. , Sariciftci N. S. , Heeger A. J. , Wudl F.
Published: July 2002

We report time-resolved excited state absorption measurements which demonstrate subpicosecond photoinduced electron transfer using soluble derivatives of poly(p-phenylene vinylene) as donors blended with a functionalized fullerene (methanofullerene) as acceptor. The subpicosecond photoinduced absorption spectra of the polymer/methanofullerene blends show that electron transfer from the donor to the acceptor occurs within a picosecond of photoexcitation of the conjugated polymer. Precise determination of the electron transfer dynamics was obtained by monitoring the dichroic ratio. The charge separated state is metastable and persists into the millisecond time domain, yielding an asymmetry of 10 orders of magnitude between the forward and reverse electron transfer times. The increased miscibility of the functionalized methanofullerene with the conjugated polymer is important for preparation of films with sufficiently high acceptor concentrations for practical devices based on photoinduced charge separation.
Phase space path integrals in Monte Carlo quantum dynamics
Authors: Caratzoulas Stavros , Pechukas Philip
Published: July 2002

In this article we present two phase space path integrals in terms of coherent states. The first one is derived in a standard fashion but using a nonstandard resolution of the identity in terms of coherent states with different width parameters. The second path integral emerges from a novel phase space representation in terms of coherent states distributed on n-dimensional manifolds embedded in the 2n-dimensional phase space of an n-degree-of-freedom system. These states are shown to form locally complete basis sets since we show that fairly smooth and localized functions can be expanded in terms of them in a unique way. In this representation the time evolution operator can be cast in the form of a phase space path integral. Both path integrals can be evaluated by straightforward implementation of Monte Carlo methods. In both cases the probability amplitude between two phase points turns out to be proportional to the average of the phase, eih∫(p dq−H dt), over a Markov process of phase space paths consisting of classical trajectories interrupted at intervals by Gaussian ‘‘quantum jumps.’’ The numerical evaluation of these expressions through importance sampling is demonstrated.