bluuebunny/crossref_metadata_2025 huggingface dataset:
Annealing temperature dependence of Raman scattering in Ge+-implanted SiO2 films
Authors: Wu X. L. , Gao T. , Bao X. M. , Yan F. , Jiang S. S. , Feng D.
Published: July 2002

We have examined the Raman spectra of violet and infrared emitting Ge+-implanted SiO2 films with special emphasis upon annealing temperature (AT) dependence of Raman scattering. We found that the complete spectrum mainly consists of three bands at 220–280, 300, and 430 cm−1, corresponding to scattering of Ge-related components, Ge nanocrystallites, and localized Si–Si optical phonons in the Ge neighborhoods, respectively. The Ge crystalline band shows an obvious AT dependence. The theoretical result from the phonon confinement model can predict its linewidth change with AT, but cannot explain its constant peak frequency. Based on the experimental result from x-ray diffraction, we attributed the discrepancy mainly to the compressive stress exerted on Ge nanocrystallites, which leads to the upshift of Ge crystallite peak thereby basically compensating the downshift caused by the confinement on phonon frequency.
Desorption behavior of antimony multilayer passivation on GaAs (001)
Authors: Zinck J. J. , Tarsa E. J. , Brar B. , Speck J. S.
Published: July 2002

A systematic study of the desorption behavior of Sb multilayers applied as a passivant to (001) GaAs is presented. Reflection high-energy electron diffraction, reflectivity, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and thermal desorption data reveal unique and complementary information which can be used to monitor the progress of passivant desorption and substrate preparation for subsequent process steps. The data confirm that Sb acts as a robust barrier to surface contamination.
Role of internal loss in limiting type-II mid-IR laser performance
Authors: Bewley W. W. , Vurgaftman I. , Felix C. L. , Meyer J. R. , Lin C.-H. , Zhang D. , Murry S. J. , Pei S. S. , Ram-Mohan L. R.
Published: July 2002

We report an experimental and theoretical investigation of internal losses in optically pumped type-II lasers with InAs/GaSb/Ga1−xInxSb/GaSb superlattice active regions. Whereas the losses are found to be moderate at 100 K (11–14 cm−1), they increase rapidly with increasing temperature (to 50–120 cm−1 at 200 K). Comparison with a detailed numerical simulation shows that the internal losses play a much more important role than Auger recombination or carrier/lattice heating in limiting the laser performance at high temperatures. Calculations of the temperature-dependent intervalence absorption cross sections show that losses of the magnitude observed experimentally can easily occur if one does not take special care to avoid resonances in all regions of the Brillouin zone. Practical design guidelines are presented. The superlattice lasers yield maximum peak output powers of up to 6.5 W per facet at 100 K and 3.5 W per facet at 180 K, threshold incident pump intensities as low as 340 W/cm2 at 100 K, and Shockley–Read lifetimes >30 ns at 100 K. The Auger coefficients are suppressed (⩽1.6×10−27 cm6/s at T=260 K) despite the intervalence resonances which produce the high internal losses.
Temperature dependent broadband impedance spectroscopy on poly-(p-phenylene-vinylene) light-emitting diodes
Authors: Scherbel J. , Nguyen P. H. , Paasch G. , Brütting W. , Schwoerer M.
Published: July 2002

Using temperature dependent impedance spectroscopy in a broad frequency range (10−1–107 Hz), we have found that the ac behavior of indium-tin oxide (ITO)/poly-(p-phenylene-vinylene) (PPV)/aluminum light-emitting diodes shows several features which cannot be described by the usual simple double RC circuit representing a depleted junction region and an undepleted bulk. Instead, our measurements in combination with a theoretical modeling suggest that the PPV bulk is composed of a highly doped region at the ITO interface and a region with lower doping at a higher distance to the ITO. Moreover, the boundary between these two regions is not sharp but there is a gradual change in dopant concentration. The large frequency range allowed us to identify two distinct processes corresponding to the PPV bulk and a third one to the junction. The bulk relaxation frequencies correspond to the characteristic dielectric relaxation frequencies of charge carriers in the high and low conducting sublayers and are proportional to the respective conductivities. The magnitude and activation energy of the relaxation time correlates well with results obtained from temperature dependent DC conductivity measurements. For ITO substrates we obtain activation energies of 0.4 eV and room temperature conductivity of about 10−7 and 10−9 S/cm for the high and low conducting sublayers, respectively. On gold substrates only one bulk process and no junction process with an activation energy of about 0.6 eV and a corresponding conductivity of 3×10−11 S/cm at room temperature is observed. The Schottky junction has been studied by temperature dependent capacitance–voltage spectroscopy at a low frequency of 0.16 Hz. The obtained acceptor dopant concentration from 1/C2 plots varies from 1.4×1017 at room temperature to 6.9×1016 cm−3 at 200 K. Assuming a density of states between 5×1020 and 5×1021 cm−3 for the valence band the temperature dependent acceptor dopant density can be described with an acceptor ionization energy between 0.16 and 0.2 eV.
Ultraviolet laser ablation of polymers: spot size, pulse duration, and plume attenuation effects explained
Authors: Schmidt H. , Ihlemann J. , Wolff-Rottke B. , Luther K. , Troe J.
Published: July 2002

A versatile model for ultraviolet (UV) laser ablation of polymers is presented, which is very successfully applied to the calculation of a variety of different properties of this process, including the influence of plume attenuation dynamics. The polymer is described as a system of chromophores with two possible electronic states. The model is based on the combination of photothermal decomposition and photodissociative bond breaking in the electronically excited state. Laser induced chemical modifications are incorporated via different absorption coefficients for the initial and for the modified polymer after absorption of UV light. Dynamic attenuation of the expanding ablation plume and heat conduction are taken into account. The results of the theoretical calculations are compared with the results of three different series of experiments performed with polyimide (PI) and polymethylmethacrylate at the excimer laser wavelength 248 nm and with PI also at 308 nm: (1) Measurement of the ablation rate as a function of fluence for four different pulse durations between 20 and 250 ns; (2) Measurements of the ablation rate as a function of fluence for five different laser irradiation spot radii between 10 and 150 μm, and (3) Time resolved measurement of the dynamic plume attenuation at the ablating laser wavelength as a function of fluence for four different pulse durations between 20 and 250 ns. The model leads to a prediction of etch rates, ablation thresholds, plume attenuation, and surface temperatures during the ablation process, which is in good agreement with the experimental results. The observed increase of the ablation rate with increasing pulse length and with decreasing laser spot size can be explained by the model as a consequence of laser induced modified absorption in combination with the dynamic shielding of the expanding plume.
Electrical and thermal transient during dielectric breakdown of thin oxides in metal-SiO2-silicon capacitors
Authors: Lombardo S. , Crupi F. , La Magna A. , Spinella C. , Terrasi A. , La Mantia A. , Neri B.
Published: July 2002

The dielectric breakdown of gate oxide layers with thickness of 35 and 9.3 nm in metal-oxide-semiconductor capacitors with a n+ polycrystalline Si/SiO2/n− Si stack was investigated. Breakdown was characterized in a particular circuit configuration by following the time evolution of voltage, current, and power through the capacitor with a time resolution of the order of 2 ns. A detailed morphological characterization of the damaged samples by emission and transmission electron microscopy is shown and discussed. The results of the morphological analysis and of the electrical measurements are quantitatively discussed by simulating, through heat-flow calculations, the time evolution of the temperature in the regions interested to the breakdown phenomenon.
Nonspecular x-ray scattering in a multilayer-coated imaging system
Authors: Stearns D. G. , Gaines D. P. , Sweeney D. W. , Gullikson E. M.
Published: July 2002

We present a rigorous theoretical treatment of nonspecular x-ray scattering in a distributed imaging system consisting of multilayer-coated reflective optics. The scattering from each optical surface is obtained using a vector scattering theory that incorporates a thin film growth model to provide a realistic description of the interfacial roughness of the multilayer coatings. The theory is validated by comparing calculations based on measured roughness to experimental measurements of nonspecular scattering from a Mo–Si multilayer coating. The propagation of the scattered radiation through the optical system is described in the context of transfer function theory. We find that the effect of nonspecular scattering is to convolve the image with a point spread function that is independent of the coherence of the object illumination. For a typical soft x-ray imaging system, the scattering within the image field from the multilayer coatings is expected to be slightly greater than for single surfaces (as normalized to the reflectivity). This is because the roughness of the coatings includes both replication of the substrate roughness and the intrinsic roughness of the multilayer growth process. Our analysis indicates that the current multilayer coating technology is capable of producing soft x-ray imaging systems that have acceptably low levels of scattering, provided that the optical substrates are sufficiently smooth.
Deep centers and their spatial distribution in undoped GaN films grown by organometallic vapor phase epitaxy
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Shin M. , Skowronski M. , Greve D. W.
Published: July 2002

Deep traps in undoped n-GaN layers grown by organometallic vapor phase epitaxy on sapphire substrates were studied by temperature dependent conductivity, photoinduced current transient spectroscopy (PICTS), thermally stimulated current, electron beam induced current (EBIC), and band edge cathodoluminescence (CL) methods. Presence of electron traps with energy levels 0.1–0.2 eV below the conduction band and hole traps with energy levels of about 0.25, 0.5, and 0.85 eV above the valence band edge was detected. CL and EBIC measurements show that the deep recombination centers in GaN are distributed inhomogeneously with well defined cellular pattern. Both carrier lifetime and luminescence intensity are enhanced at cell walls indicating lower density of recombination centers. However, the density of main hole trap (0.85 eV) is enhanced in these regions as determined by local PICTS measurements. Photoconductivity in many GaN samples exhibits very long decay times at temperatures between 100 and 300 K. The effect most probably is not related to shallow donors such as silicon, but rather is associated with unidentified deep centers with a 0.2 eV barrier for electron capture.
Rise time of electroluminescence from bilayer light emitting diodes
Authors: Nikitenko V. R. , Tak Y.-H. , Bässler H.
Published: July 2002

An analytic theory has been developed to calculate the rise of electroluminescence from bilayer light emitting diodes with internal energy barriers for both majority and minority carriers upon applying a rectangular voltage pulse. Unless the mobility of the minority carriers—usually electrons, instead, is ⩽2×10−7 cm2/V s, onset of electroluminescence is governed by the growth of the interfacial charge densities and the concomitant redistribution of the electric field inside the device rather than by charge carrier transport. The crucial system parameters turn out to be the energy barriers that control hole and electron injection. Good agreement with experiment is found.
Solution of the radiative transfer equation in an absorbing and scattering Nd:YAG laser-induced plume
Authors: Lacroix D. , Jeandel G. , Boudot C.
Published: July 2002

We analyze the scattering effect in a laser-induced plasma plume in the case of a Nd:YAG laser welding process using a discrete ordinate scheme and the radiative transfer equation. The optical properties of plume vapors from stainless steel are analyzed at various temperatures as different parameters are varied (laser wavelength, plasma pressure and composition, type of shielding gas). The scattering coefficients and the phase function due to the presence of small particles in the plume are calculated using the Mie theory for different particle diameters and several densities of particles in the medium. With our model we estimate the temperature in the plasma plume.
Laser-induced back ablation of aluminum thin films using picosecond laser pulses
Authors: Bullock Anthony B. , Bolton Paul R.
Published: July 2002

A study of laser-induced back ablation of aluminum thin film targets with picosecond laser pulses is reported. Ablated plume edge velocities are studied as a function of film thickness, laser pulse width, and incident laser fluence. Edge velocity results are compared to a model of total transmitted fluence incident at the substrate/film interface. A model including laser-induced avalanche ionization and multiphoton ionization mechanisms in the substrate shows a transmitted fluence limit which is consistent with observed edge velocity limits.
Metal–insulator and magnetic transition in Nd1−xSrxMnO3 solid solutions
Authors: Pattabiraman M. , Senthilkumaran Narayani , Murugaraj P. , Rangarajan G.
Published: July 2002

In this article we report the results of electrical resistivity, thermopower and ac magnetic susceptibility measurements on Nd1−xSrxMnO3 with x=0.40, 0.33 and 0.30. The room temperature paramagnetic to low temperature ferromagnetic transition temperature TC and electrical resistivity ρ(T) are observed to depend on the processing conditions. ρ(T) exhibits a shoulder at TC and a peak below TC. The latter peak is likely to be of intrinsic origin rather than due to lattice defects/grain boundaries, etc. The resistivity decreases with increasing x as well as upon Ar annealing. Upon Ar annealing the thermopower [S(T)] below TC shows no appreciable change in magnitude, while above TC |S| decreases. S(T) exhibits considerable differences between x=0.33 and x=0.3, which we attribute to a dopant induced insulator–metal (I–M) transition. Our thermopower data along with the resistivity and magnetic measurements show that the changes in the observed features in the ρ(T) and S(T) cannot be related to magnetic ordering alone but the electronic transitions also have to be taken into consideration.
Formation of etch pits during carbon doping of gallium arsenide with carbon tetrachloride by metalorganic vapor-phase epitaxy
Authors: Begarney M. J. , Li L. , Han B.-K. , Law D. C. , Li C. H. , Yoon H. , Goorsky M. S. , Hicks R. F.
Published: July 2002

Scanning tunneling microscopy was used to examine the effects of carbon tetrachloride concentration and temperature on the morphology of carbon-doped gallium arsenide films grown by metalorganic vapor-phase epitaxy. Deposition was carried out at 505–545 °C, a V/III ratio of 75, and IV/III ratios between 0.5 and 5.0. The growth rate declined monotonically with increasing carbon tetrachloride concentration. Step bunching and pinning was observed at a IV/III ratio of approximately 2.5. Increasing this ratio further resulted in the formation of pits ranging from 20 to 50 nm in diameter. These results can be explained by two competing processes that occur at the step edges: (1) the reaction of chlorine with adsorbed gallium from the group III precursor, and (2) the reaction of chlorine with gallium arsenide. Both reactions desorb gallium chlorides and reduce the growth rate, but only the latter reaction produces pits.
Infrared 45° reflectometry of very thin films
Authors: Silva-Castillo A. , Pérez-Rodríguez F.
Published: July 2002

It is shown that the spectra given by the difference between the reflectivity for p-polarized light and the squared reflectivity for s-polarized light (Rp−Rs2) with an angle of incidence of 45° can be used for directly determining the frequencies of zone-center transverse optical (ωTO) and longitudinal optical (ωLO) phonons in very thin polar-semiconductor films on substrates. These spectra are also useful for measuring resonance frequencies of both transverse and longitudinal optical confined phonons in ultrathin layers of superlattices.
Optical characterization of low-energy electron-beam-colored LiF crystals by spectral transmittance measurements
Authors: Montecchi M. , Nichelatti E. , Mancini A. , Montereali R. M.
Published: July 2002

The complex refractive index of a LiF crystal surface layer irradiated by low-energy electrons is modified by the stable formation of color centers embedded in it. A simplified dipole-electromagnetic field interaction model has been adopted in order to estimate the dispersion curves of colored LiF from a single optical transmittance measurement. The excellent agreement with the corresponding experimental curves (obtained by means of spectrophotometry and ellipsometry) demonstrates this to be a promising approach for LiF-based optical waveguide characterization.
Determination of diffusion in polycrystalline platinum thin films
Authors: DiBattista Michael , Schwank Johannes W.
Published: July 2002

Grain boundary diffusion of titanium through platinum thin films has been carried out in the temperature range from 200 to 600 °C. Five different platinum/titanium bilayer thicknesses, from 35 to 800 Å Pt, were annealed in 5% O2/95% N2. The accumulation of titanium at the platinum surface layer was measured by x-ray photoelectron spectroscopy (XPS) to determine the grain boundary diffusion coefficient (Db). Diffusivity values were calculated based on two different analysis methods assuming type C kinetics. For Pt layers thicker than 200 Å, the activation energy (Qb) for titanium diffusion was found to be 118±15 kJ/mol (1.22±0.16 eV). For Pt layers thinner than 200 Å, there was a thickness dependence on the diffusion kinetics, resulting in activation energies as low as 20±4 kJ/mol (0.21±0.04 eV). XPS results gave no evidence for any Pt-Ti alloy formation in these layers. The suppression of alloy formation may be attributed to the presence of oxygen at the Pt/Ti interface during layer deposition. The quantitative analysis of titanium interdiffusion in platinum provides valuable information regarding Pt/Ti surface concentrations in thin-film chemical sensors, and for understanding changes in operational characteristics of platinum electrodes.
Production and evolution of composition, morphology, and luminescence of microcrystalline arsenic oxides produced during the anodic processing of (100) GaAs
Authors: Finnie C. M. , Li X. , Bohn P. W.
Published: July 2002

GaAs when exposed to a 7 V anodic bias in aqueous HCl, forms pitted structures from which visible photoluminescence has been observed. Previous work in our laboratory identified the source of the luminescence to be arsenic oxide microcrystals, formed during the electrochemical oxidation, which evolve in composition and morphology during the course of the anodic processing. The density and size of pits is dependent on the solution conditions as well as the applied potential program (stepped vs swept to 7 V. At early times (t<30 min) the pits are composed of a mixture of small (∼1 μm) and larger (10’s of μm) faceted microcrystals identified by x-ray photoelectron spectroscopy and energy-dispersive x-ray analysis to be a combination of As(III) and As(V) oxides. At longer times As(III) oxides predominate, and the smaller microcrystals are no longer observed, suggesting that the smaller microcrystals are As(V) oxide, and that they evolve chemically into As(III) oxide over the course of the oxidation. A suitable reaction pathway which explains the observed predominance of As(III) species is suggested. The luminescence properties of these electrochemically produced structures were investigated by near-field excitation. The luminescent properties evolve in parallel with the morphology and composition although the form of the spectrum is invariant in time and consistent with that obtained from bulk As2O3 and As2O5. The similarity of emission obtained from the two types of microcrystals suggests an impurity-related origin.
Space charge effects in carrier escape from single quantum well structures
Authors: McFarlane S. C. , Barnes J. , Barnham K. W. J. , Tsui E. S. M. , Button C. , Roberts J. S.
Published: July 2002

Recently published data on the variation with applied bias and temperature of steady-state photoluminescence and photoconductivity from a series of GaAs/AlGaAs single quantum well p-i-n structures are subjected to detailed theoretical analysis, using phenomenological variables introduced in connection with these results. The data are interpreted as revealing the presence in the well of a space charge, which causes band bending and hence indirectly modifies carrier escape lifetimes. It is shown that the thermionic escape of holes can affect the electron tunneling escape lifetime so that the latter displays a thermal activation energy which is quantitatively similar to the hole well depth.
Photo-oxidation studies of porous silicon using a microcalorimetric method
Authors: Salonen J. , Lehto V-P. , Laine E.
Published: July 2002

We have used an isothermal microcalorimetric method to study photoinduced effects in porous silicon (PS). In the photo-oxidation investigations using constant wavelength, sharp threshold behavior with threshold energies 3.9 eV for n-PS and 4.7 eV for p+-PS were observed. The two discussed origins for the different threshold energies are based either on the energy gap transitions and enhanced electron transfer from the conduction band to the electron-affinity level of oxygen molecules, or the Si–O bond energy. Also nonlinear dependence on the irradiation intensity was found. Surprisingly, high exothermic signals were observed in measurements made under an inert perfusion. It is proposed that this is associated with relaxation of PS structure, which seems to be more efficiently induced by illumination than thermal treatment.
Transient enhanced diffusion of aluminum in SiC during high temperature ion implantation
Authors: Usov I. O. , Suvorova A. A. , Sokolov V. V. , Kudryavtsev Y. A. , Suvorov A. V.
Published: July 2002

6H–SiC wafers were implanted at room temperature (RT) and at 1700 °C high temperature (HT) with 50 keVAl+ ions to doses from 1.4×1014 to 1.4×1016 cm−2. Compared to samples implanted at RT, the samples implanted at high temperature display considerable aluminum redistribution. The diffusion of Al is shown to be a transient effect with different decay times in the near-surface region and in the bulk. Investigation of the crystalline structure indicated that in the near-surface region dislocation loops grow in size and Al precipitates are formed as the dose of Al implanted at HT is increased. Changes in the structure of the implanted layer may have a strong effect on the redistribution of Al. The observed redistribution can be explained by a dissociative diffusion mechanism during the high-temperature implantation.
Local order and magnetic behavior of amorphous and nanocrystalline yttrium iron garnet produced by swift heavy ion irradiations
Authors: Costantini J. M. , Desvignes J. M. , Pérez A. , Studer F.
Published: July 2002

Thin epitaxial films of gallium or scandium-doped and undoped yttrium iron garnet (Y3Fe5O12 or YIG) on nonmagnetic Gd3Ga5O12 substrates were irradiated with swift heavy ions (50 MeV 32S, 50 MeV 63Cu, and 235 MeV 84Kr) in the electronic slowing down regime. The mean electronic stopping power in the films was always larger than the threshold for amorphous track formation in YIG which is around 4.5 MeV/μm in this low ion-velocity range. The local order and magnetic properties of the damaged films were then studied at room temperature by Fe57 conversion electron Mössbauer spectroscopy (CEMS) and x-ray absorption spectroscopy (XAS) at the iron K edge in the fluorescence mode. In the case of paramagnetic gallium or scandium-substituted films (YIG:Ga, YIG:Sc) irradiated with S32 or Cu63 ions, the CEMS data show that the tetrahedral Fe3+ sites are preferentially damaged, while the octahedral sites are conserved. This is confirmed by the decrease of the pre-edge peak in the XAS data of the ferrimagnetic undoped YIG films showing that the number of tetrahedral iron sites is decreased in the amorphous phase obtained with Kr84 ion irradiation, due to the formation of fivefold-coordinated pyramidal sites, as already found in a previous study on undoped YIG sinters amorphized by 3.5 GeV Xe132 ion irradiation. In the case of the nanophase induced by ion-beam recrystallization of the tracks with S32 or Cu63 irradiations, a further decrease of the pre-edge peak is found. This is interpreted by (i) an increase of the fivefold-coordinated pyramidal sites and/or (ii) a probable decomposition of the garnet into orthoferrite (YFeO3) and haematite (α-Fe2O3) under the high-pressure and high-temperature conditions in the thermal spike generated by the ions. The CEMS data of irradiated undoped YIG also show that both the amorphous and nanocrystalline phases have a paramagnetic behavior at room temperature. The nanophase magnetic behavior is analyzed on the basis of a superparamagnetic relaxation above the blocking temperature, whereas the amorphous phase behavior is ascribed to a speromagnetic state.
High permittivity thin film nanolaminates
Authors: Zhang H. , Solanki R. , Roberds B. , Bai G. , Banerjee I.
Published: July 2002

Thin (∼10 nm) films comprising of Ta2O5–HfO2, Ta2O5–ZrO2, and ZrO2–HfO2 nanolaminates were deposited and characterized for possible gate dielectric applications. These films were deposited on silicon substrates using atomic layer deposition. The dielectric constants of these films were in 12–14 range and the leakage currents in 2.6×10−8–4.2×10−7 A/cm2 range at 1 MV/cm electric field. It was found that as these films were made thinner, the value of their dielectric constant dropped compared to their bulk values. The dominant leakage current mechanism at low electric fields was determined to be Schottky emission, whereas Poole–Frenkel emission dominated at higher fields.
Analysis of electric field distribution in GaAs metal–semiconductor field effect transistor with a field-modulating plate
Authors: Hori Yasuko , Kuzuhara Masaaki , Ando Yuji , Mizuta Masashi
Published: July 2002

Electric field distribution in the channel of a field effect transistor (FET) with a field-modulating plate (FP) has been theoretically investigated using a two-dimensional ensemble Monte Carlo simulation. This analysis revealed that the introduction of FP is effective in canceling the influence of surface traps under forward bias conditions and in reducing the electric field intensity at the drain side of the gate edge under pinch-off bias conditions. This study also found that a partial overlap of the high-field region under the gate and that at the FP electrode is important for reducing the electric field intensity. The optimized metal–semiconductor FET with FP (FPFET) (LGF∼0.2 μm) exhibited a much lower peak electric field intensity than a conventional metal–semiconductor FET. Based on these numerically calculated results, we have proposed a design procedure to optimize the power FPFET structure with extremely high breakdown voltages while maintaining reasonable gain performance.
Magnetic heat capacity in lanthanum manganite single crystals
Authors: Lin P. , Chun S. H. , Salamon M. B. , Tomioka Y. , Tokura Y.
Published: July 2002

The heat capacity of single crystal La0.7D0.3MnO3, where D=Ca, Sr, has been measured through the Curie point in fields up to 70 kOe. The magnetic contribution of the Ca sample exhibits a sharp heat capacity peak at TC≃218 K in zero field. The peak broadens and decreases in height with increasing field but, unlike an ordinary ferromagnet, the peak shifts substantially in temperature. As a consequence, the heat capacity data cannot be collapsed into a single scaling function. These features indicate that the transition is not an ordinary second-order ferromagnetic transition. Preliminary heat capacity data from the Sr-doped single crystal, with TC≃360 K, do not exhibit the same shift in peak position with applied field. We attribute the difference in behavior between Ca- and Sr-doped samples to a change in the nature of the phase transition as TC lowers.
Tomography of integrated circuit interconnect with an electromigration void
Authors: Levine Zachary H. , Kalukin Andrew R. , Kuhn Markus , Frigo Sean P. , McNulty Ian , Retsch Cornelia C. , Wang Yuxin , Arp Uwe , Lucatorto Thomas B. , Ravel Bruce D. , Tarrio Charles
Published: July 2002

An integrated circuit interconnect was subject to accelerated-life test conditions to induce an electromigration void. The silicon substrate was removed, leaving only the interconnect test structure encased in silica. We imaged the sample with 1750 eV photons using the 2-ID-B scanning transmission x-ray microscope at the Advanced Photon Source, a third-generation synchrotron facility. Fourteen views through the sample were obtained over a 170° range of angles (with a 40° gap) about a single rotation axis. Two sampled regions were selected for three-dimensional reconstruction: one of the ragged end of a wire depleted by the void, the other of the adjacent interlevel connection (or “via”). We applied two reconstruction techniques: the simultaneous iterative reconstruction technique and a Bayesian reconstruction technique, the generalized Gaussian Markov random field method. The stated uncertainties are total, with one standard deviation, which resolved the sample to 200±70 and 140±30 nm, respectively. The tungsten via is distinguished from the aluminum wire by higher absorption. Within the void, the aluminum is entirely depleted from under the tungsten via. The reconstructed data show the applicability of this technique to three-dimensional imaging of buried defects in submicrometer structures relevant to the microelectronics industry.
The effects of microstructural transitions at width transitions on interconnect reliability
Authors: Hau-Riege C. S. , Thompson C. V.
Published: July 2002

Interconnect reliability is usually assessed through lifetime tests on straight stud-to-stud lines with fixed widths. However, real interconnects often have junctions with narrow-to-wide transitions. We have carried out experimental and modeling-based studies of interconnects with narrow-to-wide transitions to test the Korhonen model for electromigration-induced failure and determine parameters for failure. Two-level Al-0.5% Cu via-to-via electromigration test structures with narrow-to-wide width transitions have been studied as a function of the ratio of linewidths and as a function of the ratio of lengths of the wide and narrow line segments. The narrow-to-wide transition was found to be a site of atomic flux divergence due to the discontinuity in diffusivities between the narrow and wide segments, which have bamboo and polygranular microstructures, respectively. Consequently, it was found that the electromigration failure rate increases as the position of width transition is decreased below a critical distance from the electron–source via. This occurs due to an interaction between evolution of the stress field at the electron–source via and the evolution of the stress field at the width transition. Narrow-to-wide transitions with positions greater than a critical distance from the electron–source via have much higher lifetimes which are not a function of the transition location, due to a lack of interaction between the electron–source via and the width transition. Because the mode of failure was identical for all structures (i.e., failure by voiding at the electron–source via), use of experimental results with simulations allows determination that the critical stress range for void-nucleation failure in these structures is 600 MPa±108. It was experimentally found that changing linewidth ratios did not affect interconnect reliability when the transition was 150 μm away from the electron–source via. However, simulations indicate that if the width transition is close to the electron–source via, the lifetime should decrease with the width of the polygranular region. Additional electromigration studies have been conducted on constant-width interconnects in which microstructural transitions were created using a scanned laser annealing. Analyses of the experiments are consistent with those on microstructural transitions caused by width transitions.
Ionization energies and electron mobilities in phosphorus- and nitrogen-implanted 4H-silicon carbide
Authors: Capano M. A. , Cooper J. A. , Melloch M. R. , Saxler A. , Mitchel W. C.
Published: July 2002

Comparisons are made between the carrier concentrations, ionization energies, and electron mobilities in 4H–SiC samples implanted with similar doses of nitrogen or phosphorus and annealed at 1300 or 1700 °C for 10 min in argon. The objective of the research is to determine which element may yield lower resistance 4H–SiC. Ionization energies of 53 and 93 meV are measured from phosphorus-implanted 4H–SiC, and are assigned to the hexagonal and cubic lattice positions in 4H–SiC, respectively. The corresponding ionization energies for nitrogen-implanted 4H–SiC are 42 and 84 meV, respectively. Phosphorus is not activated to the same extent that nitrogen is, and the carrier concentrations are about a factor of five lower for phosphorus-implanted 4H–SiC annealed at 1300 °C than for nitrogen-implanted 4H–SiC annealed at the same temperature. A higher mobility for phosphorus-implanted 4H–SiC is observed, but is not sufficiently high to offset the lower carrier concentration of this material. For the doses considered in this study, the resistivity of nitrogen-implanted 4H–SiC is lower than the resistivity of phosphorus-implanted 4H–SiC following anneals at either 1300 or 1700 °C.
Elucidation of the layer exchange mechanism in the formation of polycrystalline silicon by aluminum-induced crystallization
Authors: Nast Oliver , Wenham Stuart R.
Published: July 2002

Aluminum-induced crystallization of amorphous silicon is studied as a promising low-temperature alternative to solid-phase and laser crystallization. Its advantages for the formation of polycrystalline silicon on foreign substrates are the possible usage of simple techniques, such as thermal evaporation and dc magnetron sputtering deposition, and relatively short processing times in the range of 1 h. The overall process of the Al and Si layer exchange during annealing at temperatures below the eutectic temperature of 577 °C is investigated by various microscopy techniques. It is shown that the ratio of the Al and a-Si layer thicknesses is vitally important for the formation of continuous polycrystalline silicon films on glass substrates. The grain size of these films is dependent on the annealing temperature and evidence is given that grain sizes of 20 μm and more can be achieved. The poly-Si films are described as solid solutions containing 3×1019 cm−3 Al atoms as solute. Only a fraction of the solute is located at substitutional sites, and therefore, electrically active leading to the p-type character of the polycrystalline silicon films. Additional Al is trapped in the form of small clusters between the continuous Si layer and the substrate. The interaction of the Al and Si layers is discussed as a diffusion-controlled process where the depletion of Si solute in the Al matrix around growing Si grains is of major importance for the overall crystallization process and the size of the resulting Si grains.
Applied voltage effect on the electron delocalization of excited Ce3+ in SrS:Ce electroluminescent devices
Authors: Barthou C. , Benoit J. , Benalloul P. , Polamo K. , Soininen E.
Published: July 2002

The electro-optical behavior of SrS:Ce atomic layer epitaxy thin films electroluminescent devices with Ce3+ ions in different point symmetries is analyzed. Under selective photonic excitation in the lower absorption band and for voltage below the EL threshold, it is shown that the delocalization probability of Ce3+ excited ions is much lower for Ce3+ ions in octahedral sites than in lower symmetry ones.
Electron bombardment fragmentation of Ar van der Waals clusters by scattering analysis
Authors: Buck U. , Meyer H.
Published: July 2002

Arn clusters generated in a supersonic nozzle beam are investigated by exploiting their different kinematical behavior in a scattering experiment. In a crossed molecular beam experiment the Arn clusters are scattered from He atoms at E=69.7 meV. By measuring the angular and velocity distributions of the scattered clusters, the different species can be separated from each other. In this way fragmentation probabilities of small Arn clusters up to n=6 by electron impact ionization are measured. The influence of collisional dissociation of the dimer is accounted for by classical trajectory calculations. The dimer fragmentation is found to be f21=0.40±0.05, while the trimer totally fragments to dimers ( f32=0.7±0.3) and monomers ( f31=0.3). All clusters with n from 4 to 6 appear with a probability of 0.95 on the dimer ion mass.
On the role of vibrational energy in the activated dissociative chemisorption of methane on tungsten and rhodium
Authors: Rettner C. T. , Pfnür H. E. , Auerbach D. J.
Published: July 2002

The vibrational energy dependence of the dissociative chemisorption probability of CH4 on W(110) is investigated with the use of a seeded supersonic molecular beam. By variation of the beam source temperature and seed gas mixture the degree of vibrational excitation of the incident CH4 molecules can be varied while the incident kinetic energy is held constant. The results are consistent with a model in which all vibrational modes are equally effective and vibrational energy and translational energy are approximately equivalent in promoting this highly activated process. Previous unsuccessful attempts to promote CH4 chemisorption on rhodium via vibrational excitation are consistent with our findings; we are also able to account for previous observations of enhancement of dissociative chemisorption in heated effusive beam experiments.
Relativistic kinetic energies and mass–velocity corrections in diatomic molecules
Authors: Thakkar Ajit J.
Published: July 2002

Expectation values of various operators with respect to nonrelativistic, self-consistent-field wave functions of good quality for 46 diatomic molecules are computed to examine the differences between the relativistic kinetic energy 〈Hr〉 and the quasirelativistic kinetic energy 〈Tnr〉+〈Hmv〉 in which 〈Tnr〉 is the nonrelativistic kinetic energy and 〈Hmv〉 is the mass–velocity correction. Then 〈Hrc〉=〈Hr〉−〈Tnr〉=〈H mv〉+〈δE〉 is the full relativistic correction to the kinetic energy. 〈Hrc〉 can differ appreciably from 〈Hmv〉 for molecules containing at least one atom with a moderately large atomic number Z. These differences are greatly amplified when the relativistic corrections to dissociation energies are considered; the mass–velocity contribution to the binding energy is found to be inaccurate even for moderate values of Z. Great care is necessary to ensure that the molecular and atomic calculations are of comparable accuracy. A qualitative argument is provided to explain why 〈Hmv〉 can provide a reasonable approximation to 〈Hrc〉 for small enough Z despite the fact that the two operators are inequivalent for αp≥1 where α is the fine structure constant and p is the momentum. Finally the asymptotic behavior of the pertinent integrands is used to show why the numerical evaluation, in momentum space, of 〈Hrc〉 is easier than that of 〈Hmv〉.
Visible absorption and magnetic-rotation spectroscopy of 1CH2: Analysis of the 1A1 state and the 1A1–3B1 coupling
Authors: Petek Hrvoje , Nesbitt David J. , Moore C. Bradley , Birss F. W. , Ramsay D. A.
Published: July 2002

In methylene, the rotational levels of the ã 1A1 (0,0,0) and (0,1,0) vibronic states are so heavily perturbed by nearby rovibrational levels of the ground triplet state (X̃ 3B1) that standard rotation–vibration Hamiltonians do not provide a satisfactory fit or any indication as to which levels are strongly perturbed and which are less perturbed. Recent spectroscopic and theoretical work gives triplet state term values and the singlet–triplet energy difference with an accuracy of a few tens of wave numbers. Using these term values and ab initio spin-orbit matrix elements it is shown that all Ka=1, 3, and 4 levels of 1A1 (0,0,0) and all Ka=1 levels of 1A1 (0,1,0) are strongly perturbed by 3B1 (0,v2,0) levels with 2≤v2≤4. Individual levels in the other Ka stacks are also perturbed but most can be fit satisfactorily with a Watson Hamiltonian. The shifts between the observed term values and those calculated from the Watson Hamiltonian are reproduced in each Ka stack by a spin-orbit matrix element value only 5%–30% larger than the ab initio value. Over 60% of the measured term values show shifts greater than 0.2 cm−1. Shifts of the 322 and 000 levels can only be explained by interaction with levels in either (1,0,0) or (0,0,1) states; possible values for the vibrational frequencies of ν1 and ν3 of 3B1 are given. Molecular constants for 1A1 (0,0,0) and (0,1,0) are derived and quantitative estimates of spin-orbit mixing for individual levels of 1A1 (0,0,0) and (0,1,0) are given. From a chemical point of view singlet methylene is never in a pure spin state and always has some triplet character in its wave function. These data provide a basis for proper modeling of the kinetics of chemical reactions of ‘‘singlet’’ and ‘‘triplet’’ methylene species and their interconversion by ‘‘intersystem crossing.’’
Tridiagonal Fermi resonance structure in the vibrational spectrum of the CH chromophore in CHF3. II. Visible spectra
Authors: Segall J. , Zare R. N. , Dübal H. R. , Lewerenz M. , Quack M.
Published: July 2002

The near IR and visible vibrational absorption spectra of CHF3 were recorded up to wave numbers of 17 500 cm−1 providing complete frequency coverage, together with paper I, from the low frequency fundamentals to the N=6  CH stretching–bending overtone multiplet. All strong bands in the high overtone spectra could be predicted and assigned by means of the tridiagonal Fermi resonance Hamiltonian, including a few combinations with intense CF3 stretching vibrations already observed for the low overtones. Improved vibrational Fermi resonance constants are presented on the basis of a fit to 35 assigned bands. An analysis of the rotational fine structure of the 2ν4 (E) overtone component and several Fermi resonance component bands result in values for αb and αs, which allow us to determine Be. In the high overtone bands no rotational fine structure is observed. The bands can be understood by introducing additional homogeneous rovibrational structures of phenomenological widths Γ≊1 to 10 cm−1. The results are discussed in relation to the separation of time scales for mode selective vibrational redistribution and further evolution. The overtone band strengths are reported and analyzed approximately with the empirical local Mecke dipole function.
Electron tunneling through covalent and noncovalent pathways in proteins
Authors: Beratan David N. , Onuchic José Nelson , Hopfield J. J.
Published: July 2002

A model is presented for electron tunneling in proteins which allows the donor–acceptor interaction to be mediated by the covalent bonds between amino acids and noncovalent contacts between amino acid chains. The important tunneling pathways are predicted to include mostly bonded groups with less favorable nonbonded interactions being important when the through bond pathway is prohibitively long. In some cases, vibrational motion of nonbonded groups along the tunneling pathway strongly inluences the temperature dependence of the rate. Quantitative estimates for the sizes of these noncovalent interactions are made and their role in protein mediated electron transport is discussed.
FTIR spectra of ammonia clusters in noble gas matrices
Authors: Süzer Sefik , Andrews Lester
Published: July 2002

FTIR spectra of ammonia have been studied from 200 to 5200 cm−1 over a wide range of concentration and temperature conditions in solid neon, argon, and nitrogen matrices. Dimer bands appear between monomer and higher aggregate absorptions and exhibit intermediate growth behavior on sample annealing and concentration changes. Comparison of spectra in solid argon at 5 and 12 K shows unrelaxed monomer absorptions at 12 K, which almost completely vanish at 5 K without any difference in the dimer spectrum; this indicates that the NH3 submolecules are relaxed in the matrix-isolated dimer. One antisymmetric and two symmetric N–H stretching modes were observed for the dimer, which follow the 14NH3–15NH3 and NH3–ND3 shifts of their monomer analogs. The dimer N–H stretching modes are intensified by fivefold relative to the dimer umbrella bending mode as compared to the same relative monomer band intensities, which is diagnostic of the hydrogen bonding interaction. The matrix dimer spectra show that one N–H bond from one submolecule and two N–H bonds from the other submolecule are involved in nonlinear, intermolecular hydrogen bonding, and that these inequivalent weakly bonded NH3 submolecules form an asymmetric cyclic dimer structure. The matrix dimer spectra further indicate that classical one-hydrogen bond structures cannot be correct. Finally, trimer and higher clusters probably have similar structures, built from the dimer, based on similar infrared spectra.
Theoretical studies of the transition metal–carbonyl systems MCO and M(CO)2, M=Ti, Sc, and V
Authors: Barnes Leslie A. , Bauschlicher Charles W.
Published: July 2002

A b initio calculations on the transition metal–carbonyl systems MCO and M(CO)2, M=Ti, Sc, and V, have been carried out using large Gaussian basis sets and an extensive treatment of electron correlation. The dissociation energies (De) and geometries of these molecules are given, and the bonding mechanisms are discussed. High-spin ground states are favored for the monocarbonyl molecules, whereas for the dicarbonyl molecules there is a competition between high-, intermediate-, and low-spin states, which are found to be very close in energy. The computed De(Ti–CO) is 0.62 eV whereas for Ti(CO)2 it is 1.02 eV, relative to the ground state Ti atomic asymptote and CO(1Σ+). This suggests that the recent experiment giving a value of ≊1.75 eV for De[Ti–(CO)x] should be interpreted as giving the De for Ti(CO)x, x≥2. For the three metal atoms the binding energy per carbonyl is found to be significantly lower for the dicarbonyl than the monocarbonyl molecules. This is in contrast to the Ni(CO)x molecules, where each CO is bound with approximately the same energy.