bluuebunny/crossref_metadata_2025 huggingface dataset:
Secondary ion mass spectrometry and electrical characterization of Zn diffusion in n-type InP
Authors: Blaauw C. , Shepherd F. R. , Eger D.
Published: July 2002

Zn diffusion in InP has been investigated in two configurations: diffusion from an external source into uniformly n-doped substrates and diffusion between layers in n-p-n-p-n structures grown by metalorganic chemical vapor deposition (MOCVD). Alternating layers of p-type InP (0.5 μm, 4×1017<[Zn]<2×1018 cm−3) and n-type InP (0.5 μm, 1016<[Si]<3×1019 cm−3) were grown by low-pressure MOCVD at T=625 °C. The distribution of Si and Zn was determined by secondary ion mass spectrometry, using implanted standards to calibrate the data. For undoped spacer layers (n∼1016 cm−3) the diffusion profiles depended strongly on the Zn-doping level; little out-diffusion of Zn was observed for [Zn]=4×1017 cm−3, but for [Zn]>1018 cm−3, the Zn completely diffused across the spacer layers during growth (1–2 h). For doped spacer layers, the doping level (Si) had a dramatic effect on the Zn diffusion profiles. The total Zn diffusion across the grown dopant interface was not substantially affected, but an accumulation of Zn took place in the Si-doped layers, with the formation of Zn spikes, for which the increase in Zn level, as compared to that in the Zn-doped layer (∼1018 cm−3) was similar to [Si]. No diffusion of Si across the grown dopant interface was detected. Electrochemical C-V profiling indicated that the Si and Zn were electrically active. The results have been explained in terms of a model in which mobile Zn species diffusing into the Si-doped layers are immobilized by the formation of Zn-donor pairs. The model is shown to be consistent with diffusion profiles obtained for diffusion of Zn into n-InP from an external ZnGaCdIn source. The effect of this diffusion on as-grown junctions, is to displace the location of the junction. These results can have important consequences for device fabrication.
Effects of isovalent substitutions on lattice softening and transition character of BaTiO3 solid solutions
Authors: Lin J. N. , Wu T. B.
Published: July 2002

The transition character of paraelectric-to-ferroelectric phase transition and its relation to lattice soft-mode behavior in the solid solutions of (Ba,Sr)TiO3, (Ba,Ca)TiO3, Ba(Ti,Zr)O3, and Ba(Ti,Sn)O3 were investigated by the measurements of dielectric constants in the temperature range from −30 to 150 °C, and x-ray line breadth of 100, 110, and 111 Bragg reflections at temperatures above the transition point. From the result of dielectric measurement, it was found that the specimens containing Ti-site substituents have shown a diffuse phase transition character, whereas those having Ba-site substituents have preserved a sharp phase transition. It was also noted from the change of line breath of Bragg reflections that for the solutions of Ba(Ti,Zr)O3 and Ba(Ti,Sn)O3, the high dynamic strain at the 〈1 0 0〉 axis, induced from the soft-mode of lattice vibration, become repressed when a large amount of solutes is contained, but for the solutions of (Ba,Sr)TiO3 and (Ba,Ca)TiO3, it is recovered at temperatures near the transition point. Moreover, the dynamic strain at the 〈1 1 1〉 axis can be rapidly repressed by the addition of Ti-site substituents. The different effects on the dynamic strains are attributed to the disparate distortions of oxygen octahedra in the perovskite lattice, induced by the solutes of different site substitution. It is also believed that the influence on the soft-mode of lattice vibration has caused the different phase transition characters of the four BaTiO3 solid solutions.
Lateral displacement of a light beam at a ferrite interface
Authors: Singh I. J. , Nayyar V. P.
Published: July 2002

The lateral Goos–Hänchen1 beam shift of a monochromatic plane wave incident from a denser magneto-optical surface medium to a rarer magneto-optical surface medium is theoretically investigated using Maxwell’s theory. We found a large but nondivergent enhancement in the beam displacement as the angle of incidence approaches critical angle.
Electron mobility in InAs1−xSbx and the effect of alloy scattering
Authors: Chin Vincent W. L. , Egan R. J. , Tansley T. L.
Published: July 2002

The significance of alloy scattering on the electron mobility of InAs1−xSbx is studied in detail. Since little experimental data are available for the InAs1−xSbx compound, calculations of the electron mobility of the binary InAs and InSb semiconductors are first evaluated from the average momentum relaxation times, 〈τm〉 of the individual mechanisms (comprising ionized and neutral impurities, acoustic phonon deformation potential and piezoelectric, optical phonon deformation potential and polar) so that they can be compared to available experimental results. Having ensured that the calculated data are in good agreement with the experimental results in these binary compounds, the electron mobility of the ternary, InAs1−xSbx as a function of x at 77 K and 300 K is calculated by introducing an alloy scattering term. At 77 K, for a total doping concentration of ≂1017 cm−3, the electron mobility of InAs1−xSbx is limited by ionized impurity scattering and alloy scattering is insignificant. However, for doping concentration of ≂1015 cm−3, alloy scattering is found to be the most significant process from about x=0.1 to 0.8. At 300 K the alloy scattering reduces the total electron mobility from between a few percent at x=0.15 to a maximum of about 11% for x=0.60. It is observed that alloy scattering is greatest at x=0.15 but it still is about 11% of the total electron mobility at this composition. The alloy mobilities at 77 K for x=0.6 and x=0.15 are 1.28×106 cm2 V−1s−1 and 7.96×105 cm2 V−1s−1, respectively. The mobilities at x=0.60 are further evaluated as a function of the doping carrier concentration at 77 and 300 K. It is found that alloy scattering becomes increasingly significant for lightly doped semiconductors at low temperatures.
Grating-coupled quantum-well infrared detectors: Theory and performance
Authors: Andersson J. Y. , Lundqvist L.
Published: July 2002

A theory of the grating-coupled quantum-well infrared detector, based on the modal expansion method (MEM) is presented together with numerical simulation results. MEM is a near-exact way of determining the electromagnetic-field pattern in a reflection grating-coupled quantum-well infrared detector, and is suitable for the calculation of absorption quantum efficiencies. Both lamellar gratings and crossed (doubly periodic) gratings are dealt with. Numerical simulations show that quantum efficiencies integrated with respect to wavelength ηint equal to 1 μm may be obtained in a detector equipped with a crossed grating of square symmetry, and a waveguide. The waveguide is defined by the metal grating on one side of the infrared absorbing quantum wells, and a thick aluminum arsenide layer on the other. This implies a factor of 4 higher ηint than in a 45° polished edge detector with the corresponding quantum-well characteristics. Finally, a sensitivity analysis shows that a change in grating dimensions of the order of 0.1 μm results in relative changes of ηint of about 1%.
Temperature and concentration distribution of H2 and H atoms in hot-filament chemical-vapor deposition of diamond
Authors: Chen Kuei-Hsien , Chuang Mei-Chen , Penney C. Murray , Banholzer William F.
Published: July 2002

The temperature and concentration distributions of hydrogen in a hot-filament chemical-vapor deposition reactor of diamond have been measured simultaneously by coherent anti-Stokes Raman scattering (CARS). The bright background from the filament was rejected by using CARS and gating on the detector as well as spatial filtering. The CARS spectra provided direct and accurate measurements of the H2 temperature and concentration distributions. The concentration distribution of atomic hydrogen was also determined by assuming a constant pressure condition and equilibrium between translational and rotational degrees of freedom in the system. These temperature and concentration distributions are essential for the understanding and modeling of the diamond growth processes. It was found that the H-atom distribution departed substantially from the thermal equilibrium prediction except very near the filament; however, a diffusion-controlled model predicted the slope of this distribution throughout the measured region.
Thermal wave probing of pyroelectric distributions in the surface region of ferroelectric materials: A new method for the analysis
Authors: Ploss Bernd , Emmerich Rudolf , Bauer Siegfried
Published: July 2002

For the investigation of polarization distributions in pyroelectric materials, the laser intensity modulation method (LIMM), which is based on thermal waves, is widely used. With this method, the sample under investigation is heated by the absorption of intensity modulated light at one surface, while the pyroelectric current is measured. The thermal excitation generates a thermal wave penetrating into the sample. The penetration depth is varied with the modulation frequency. A new procedure for the reconstruction of the polarization distribution from a measured pyroelectric current spectrum is introduced. This procedure is especially well suited for polarization probing near the sample surface. An approximation for the polarization distribution is calculated from a measured pyroelectric spectrum in a very simple and direct way, avoiding mathematical instabilities. The calculation can be performed during the measurement of a pyroelectric current spectrum. This makes LIMM an on-line procedure. The new technique of analysis is applied to the measurement of thin depolarized layers near the surface of homogeneously poled ferroelectric polymer films.
Initial crystallization stage of amorphous germanium films
Authors: Edelman F. , Komem Y. , Bendayan M. , Beserman R.
Published: July 2002

The incubation time for the crystallization of amorphous Ge (a-Ge) films was studied as a function of temperature between 150 and 500 °C by means of both in situ transmission electron microscopy and Raman scattering spectroscopy. The temperature dependence of the incubation time for free-sustained a-Ge films follows an Arrhenius curve with an overall (nucleation+growth) crystallization process activation energy of 2.0 eV. In the case where the a-Ge films were on Si3N4 substrates, an earlier stage of the crystallization was observed (nucleation), having an activation energy of 1.3 eV. In addition, it was found that a thin metallic layer of Al or Au, deposited on the a-Ge films, induces a very fast crystallization in the mode of dendritic growth, as reflected by a low activation energy (0.9 eV) for the incubation time temperature dependence.
Magnetoelastic coupling and ΔE effect in TbxDy1−x single crystals
Authors: Clark A. E. , Restorff J. B. , Wun-Fogle M. , Lindberg J. F.
Published: July 2002

The elastic moduli of the highly magnetostrictive TbxDy1−x alloys (x=0.5, 0.6, and 0.67) were measured at 77 K under conditions of constant magnetic field and constant magnetic induction. From these values the magnetic contribution to the moduli (intrinsic ΔE effect) and magnetoelastic coupling factor k were calculated. For Young’s moduli measured under constant flux density (magnetically blocked conditions), it was found that EB∼20 to 50 GPa. For measurements made while maintaining a constant magnetic field (magnetically free conditions), it was found that Young’s moduli EH minima range from ∼3 to 5 GPa. Such large differences between EB and EH yield magnetoelastic coupling factors in excess of 0.9. Theoretical expressions of the magnetic contribution to the elastic compliance, (1/EH−1/EB), were derived using the single vector magnetization rotation model.
Dependence of absorption spectrum and responsivity on the upper state position in quantum well intersubband photodetectors
Authors: Liu H. C.
Published: July 2002

Analytical results are derived for the intersubband absorption quantum efficiency, which take into account contributions from both bound-to-bound state and bound-to-continuum state transitions. Including the final state lifetime broadening, the absorption spectrum gradually decreases in peak strength and becomes broader from the pure bound-to-bound case to the pure bound-to-continuum situation. The physical reason for the divergence in absorption strength when the upper state is in resonance with the top of the barrier and in the absence of broadening is discussed. The detector current responsivity as a function of well width (and hence upper state position) is estimated. Calculated examples are given, covering the crossover region from the bound-to-bound case to the bound-to-continuum case. Our results compare well with experiments.
X-ray diffraction characterization of highly strained InAs and GaAs layers on InP grown by metalorganic vapor-phase epitaxy
Authors: Liu Q. , Lindner A. , Scheffer F. , Prost W. , Tegude F. J.
Published: July 2002

Using high resolution x-ray diffraction highly strained extremely thin InAs and GaAs layers grown on InP substrates by low-pressure metalorganic vapor-phase epitaxy have been studied. In order to determine the growth rate and the layer quality of extremely thin InAs and GaAs different kind of test structures are developed. InAs/In0.53Ga0.47As, GaAs/In0.53Ga0.47As, and InAs/GaAs/In0.53Ga0.47As superlattice structures were prepared, providing independent informations about InAs and GaAs growth rate under high strain, layer quality, and strain compensation effects. A relation was derived, which allows the direct calculation of the layer thicknesses of individual layers within a superlattice, avoiding the time consuming computer fitting. The thicknesses of very thin InAs and GaAs layers can be determined by extracting parameters from the rocking curve. Samples grown with various strained layer thicknesses and periods were analyzed and compared. Dramatic broadening of satellite peaks with increasing InAs thickness was observed, and this is attributed to the three-dimensional growth of InAs highly strained layers and the onset of strain relaxation. Smooth GaAs surfaces seem easier to form, although GaAs layers are under a similar amount of strain as InAs. The effect of period number on the rocking curve quality was also studied. Only in case of coherent growth with good interfaces and small fluctuations the satellite-peak intensities increased with increasing the period number, as theoretically predicted. In addition, strain compensation in the InAs/GaAs/InGaAs SLS’s and InAs/GaAs interface quality was also discussed.
Solid-source molecular beam epitaxy growth of GaInP and GaInP-containing quantum wells
Authors: Mowbray D. J. , Kowalski O. P. , Skolnick M. S. , DeLong M. C. , Hopkinson M. , David J. P. R. , Cullis A. G.
Published: July 2002

The growth and characterization of high quality epitaxial layers of GaInP and GaInP-containing quantum wells grown by solid-source molecular beam epitaxy (MBE) is reported. Bulk GaInP shows photoluminescence linewidths as small as 6.7 meV and double-crystal x-ray diffraction linewidths as narrow as 12.5 arcsec. Evidence for the presence of long-range ordering in MBE-grown GaInP is discussed. GaAs-GaInP quantum wells show good structural and optical quality. A comparison of the measured transition energies with the predictions of a simple, finite depth square well model suggests a very small value for the conduction band offset in this system. GaInP-(Al0.37Ga0.64)0.51In0.49P quantum wells show good optical properties with emission at energies as high as 2.15 eV (≡6000 Å at 300 K) at 4.2 K for a 12 Å well.
Tin nitride thin films prepared by radio-frequency reactive sputtering
Authors: Maruyama Toshiro , Morishita Tomonori
Published: July 2002

Tin nitride thin films were obtained by the reactive sputtering method. The metallic tin target was sputtered by nitrogen gas with rf magnetron sputtering equipment. To prevent the re-evaporation of atomic nitrogen from the substrate, the depositions were made at the low substrate temperature of 60 °C. Polycrystalline films were obtained at an rf power lower than 90 W. The resistivity of polycrystalline film was 3–14×10−2 Ω cm, while the resistivity of amorphous film increased monotonically with decreasing sputtering pressure. For amorphous film, the change in resistivity is attributable to the change in carrier concentration. The decrease in carrier concentration is associated with an increase of –N=O combination in the film. The optical energy gap of tin nitride was 1.5 eV.
Germanium partitioning in silicon during rapid solidification
Authors: Brunco D. P. , Thompson Michael O. , Hoglund D. E. , Aziz M. J. , Gossmann H.-J.
Published: July 2002

Pulsed laser melting experiments were performed on GexSi1−x alloys (x≤0.10) with regrowth velocities ranging from 0.25 to 3.9 m/s. Analysis of post-solidification Ge concentration profiles, along with time-resolved melt depth measurements, allowed determination of the liquid-phase diffusivity Dl for Ge in Si and the dependence of the Ge partition coefficient k on interface velocity v. A Dl of 2.5×10−4 cm2/s was measured. The k vs v data were analyzed using various models for partitioning, including both the dilute and nondilute Continuous Growth Models (CGM). Extrapolating to zero velocity using the partitioning models, an equilibrium partition coefficient of approximately 0.45 was obtained. Best fitting of partitioning data to the nondilute CGM yields a diffusive speed of 2.5 m/s. These measurements quantify previous indications of partitioning observed in other studies of pulsed laser processed GexSi1−x alloys.
Triggered breakdown in low-pressure hollow cathode (pseudospark) discharges
Authors: Pitchford L. C. , Ouadoudi N. , Boeuf J. P. , Legentil M. , Puech V. , Thomaz J. C. , Gundersen M. A.
Published: July 2002

Triggered breakdown in hollow cathode discharges in geometries similar to those used for pseudospark switches and pseudospark pulsed electron beams has been investigated experimentally and with a two-dimensional model previously developed. A systematic study of the influence of the discharge conditions (applied voltage and pressure), geometry, and trigger conditions (trigger intensity and position) on the time to breakdown in helium is presented, and some data are also shown for argon. Excellent qualitative agreement is found between the model predictions and the experimental results. The relation between the time to breakdown and the geometrical distribution of injected charge is discussed, and the understanding gained from these model results is used to suggest guidelines for trigger optimization. Conditions wherein significant oscillations in the current—a ‘‘current quenching’’ effect—are observed in the prebreakdown current wave form are discussed.
Passivation of porous silicon by wet thermal oxidation
Authors: Chen Huajie , Hou Xiaoyuan , Li Gubo , Zhang Fulong , Yu Mingren , Wang Xun
Published: July 2002

A wet thermal oxidation method is proposed to passivate the electrochemically etched porous silicon. Bright and stable photoluminescence is achieved by wet oxidation at relatively low temperatures of 400–500 °C. The Fourier transform infrared absorption shows that the formation of SiH(O3), SiH(SiO2), SiH2(O2) bonds may be responsible to the stabilization of luminescence under the laser illumination.
Debye-temperature–elastic-constants relationship for materials with hexagonal and tetragonal symmetry
Authors: Siethoff Hans , Ahlborn Karl
Published: July 2002

In the literature a relation is often used that correlates Debye temperature and bulk modulus by a square-root law. It was recently shown that, for different cubic crystal structures, such a law is only fulfilled within relatively large error limits. If one takes, however, the average of the elastic constants of the transversal acoustic phonon modes as elastic modulus instead of the bulk modulus, the square-root law is established with high precision. It is demonstrated that the same procedure may also be applied successfully to materials with hexagonal crystal symmetry such as hexagonal close-packed metals and semiconducting compounds with the wurtzite structure, and to different structures of the tetragonal system. The adequate moduli are Gh={c44[c44(c11−c12)/2]1/2} 1/2 and Gt=[c44c66(c11−c12)/2]1/3 for materials with hexagonal and tetragonal symmetry, respectively. The difference between the various structures of a crystal system is quantitatively described by the different number of atoms in the crystallographic unit cell.
Effect of different preparation conditions on light emission from silicon implanted SiO2 layers
Authors: Ghislotti G. , Nielsen B. , Asoka-Kumar P. , Lynn K. G. , Gambhir A. , Di Mauro L. F. , Bottani C. E.
Published: July 2002

Visible light emission from Si+ implanted SiO2  layers as a function of different annealing conditions (temperature, time and ambient) is studied. It is shown that a 560 nm band, present in as implanted samples, increases its intensity for increasing annealing temperatures and is still observed after annealing at 1000 °C. The emission time is fast (0.5–2 ns). A second band centered at 780 nm is detected after annealing at 1000 °C. The intensity of the 780 nm band further increases when hydrogen annealing was performed. The emission time is long (1μs–0.3 ms). Based on the annealing behavior and on the emission times, the origin of the two bands is discussed.
X-ray photoemission studies of W 4f core levels of electrochromic HxWO3 films
Authors: Jeong J. I. , Hong J. H. , Moon J. H. , Kang J.-S. , Fukuda Y.
Published: July 2002

Using x-ray photoelectron spectroscopy (XPS), W 4f core level spectra of electrochromic HxWO3 films have been investigated as a function of the injected charge. W 4f peaks become broader with increasing injected charge. The line shape analysis of W 4f XPS spectra of amorphous HxWO3 films reveals that each spin-orbit split component consists of two peaks. It is also found that, as the injected charge increases, the intensity of the low binding energy (BE) peaks, the asymmetry factor of core lines, and the intrinsic lifetime width increase systematically. Composite structures of W 4f peaks have been attributed to the existence of the different final states, screened by different numbers of W 5d electrons. The low BE peaks are considered to correspond to the locally screened final states, with one more W 5d electrons than in the ground state, which arise from midgap state conduction electrons near EF. There is also a possibility that a mixture of fine scale phases contributes to composite structures and broader linewidths in HxWO3.
Influence of the spatial extent of the most probable potential well on the distribution of electronic states in disordered semiconductors
Authors: O’Leary Stephen K. , Lim P. K.
Published: July 2002

We examine how the spatial extent of the most probable potential well influences the form of the distribution of electronic states in disordered semiconductors. We show that, over most of the relevant energy spectrum, the spatial extent of this well does not vary greatly. A simple model, in which all potential wells are taken to be of the same spatial scale, is demonstrated to have the same physical content as a model in which variations of the spatial scale of the most probable potential well are taken into account.
Formation and decay kinetics of light-induced dye radicals on spectrally sensitized AgBr microcrystals measured by electron spin resonance spectroscopy: A quantitative model
Authors: Ceulemans T. , Schoemaker D. , Vandenbroucke D. , De Keyzer R.
Published: July 2002

During optical excitation of spectrally sensitized silver bromide microcrystals, dye radicals are generated at the surface of the crystals. For the dyes studied in this work the radicals are oxidized dye molecules, which have a hole trapped at their doubly degenerate highest occupied molecular orbital. The dye positive holes can be detected using electron spin resonance (ESR) spectroscopy. After optical excitation the oxidized form of the dye starts to decay. A new method to acquire kinetic data of formation and decay of dye positive holes is introduced, taking into account the variation in linewidth of the ESR signal. By comparing measurements in ambient air, dry air and dry nitrogen gas it is found that moisture and not oxygen influences the kinetic behavior of the radicals. A quantitative model is suggested which is able to describe the formation of dye positive holes during optical excitation. This model allows one to predict the decay of the oxidized dye molecules, using the fitting parameters of the formation curves.
Multiscale modeling of chemical vapor deposition
Authors: Rodgers Seth T. , Jensen Klavs F.
Published: July 2002

An integrated method for modeling chemical vapor deposition on length scales ranging from microns to meters has been developed. The macroscale problem of flow and transport in a single wafer, low pressure chemical vapor deposition reactor is solved using the finite element method. However, on the feature scale, continuum models of flow and transport are not valid and discrete particle transport models must be employed. The two transport regimes, continuum and discrete particle, are linked by an effective reactivity function ε which includes both effects of multiscale surface heterogeneity and microscale transport resistance. A hybrid ballistic transport, Monte Carlo method, is developed permitting calculation of ε for any set of reaction pathways occurring over microelectronic device features of any geometry. Surface topography, that is, feature scale calculations, are combined to yield an effective reactivity map over the surface of the substrate. This map is subsequently used to formulate a flux boundary condition on the macroscopic model. Iteration between macroscopic and microscopic models is used to assure a consistent set of conditions at the macro–micro interface. The resulting linked models are used for investigation of local and reactor wide loading effects. Matched macroscopic solutions provide inputs for profile evolution calculations during fill of trenches and vias.
Effects of cell structure on the reflection of cholesteric liquid crystal displays
Authors: Xu Ming , Xu Fudong , Yang Deng-Ke
Published: July 2002

Cholesteric liquid crystal displays employ cells consisting of parallel glass plates which have indium-tin-oxide coating as the electrode and polyimide as the alignment layer. We have performed a systematic study of the reflections from the interfaces and the liquid crystal. We measured the reflection spectra using various incident light, various polarizations, and detection polarizations. Using the Berreman 4×4 method, we simulated the reflection spectra without fitting parameters; the results agreed well with the experimental results.
Conduction mechanisms in amorphous and crystalline Ta2O5 thin films
Authors: Ezhilvalavan S. , Tseng Tseung-Yuen
Published: July 2002

Electrical properties and leakage current mechanisms of amorphous and polycrystalline tantalum pentoxide (Ta2O5) films were studied. Ta2O5 thin films were deposited on Pt/SiO2/n-Si substrate by reactive magnetron sputtering and then annealed at temperatures ranging from 500 to 800 °C for 30 min in O2. The as-deposited films were amorphous and showed good electrical properties in terms of a dielectric constant of about 31 and leakage current density of 10−10 A/cm2 at 100 kV/cm. The leakage current flowing through Ta2O5 film increases from 10−10 to 10−7 A/cm2 following the annealing. The conduction mechanism at low electric fields (100 kV/cm) is due to Ohmic conduction. On the other hand Schottky mechanism dominates at the intermediate fields (100–350 kV/cm) and Poole–Frenkel becomes predominant at high fields (>350 kV/cm). The increase in leakage current density in the crystallized film is due to Si penetrated into the Ta2O5 grain and grain boundary from the underlying SiO2/n-Si substrate.
Structural, magnetotransport, and optical properties of sputtered Co/Cu multilayers examined as a function of Co layer thickness at the second antiferromagnetic maximum
Authors: Christides C. , Logothetidis S. , Gioti M. , Stergioudis S. , Stavroyiannis S. , Niarchos D.
Published: July 2002

A series of {[Co(tCo)/Cu(2.1 nm)]30/Co(tCo)}30 multilayers have been deposited under specific magnetron sputtering deposition conditions that lead to giant magnetoresistance (GMR) curves with technological interest. X-ray reflectivity, magnetic, magneto-transport, and spectroscopic ellipsometry measurements were used together to examine the dependence of their properties upon the Co layer thickness (tCo). Remarkably, the obtained film density and roughness, the saturation and coercivity fields, the reduced remnant magnetization, the GMR ratios, and the plasma frequency exhibit a significant divergence as a function of tCo in the range between 1.3–1.6 nm. The observed microstructural, magneto-transport, and magnetic relative differences, induced in (111) textured Co/Cu multilayers by varying the tCo, were correlated with changes of the optical electronic states of the constituents in the electronic density of states near the Fermi level.
Electronic and optical properties of carbon nitride thin films synthesized by laser ablation under ion beam bombardment
Authors: Lu Y. F. , Ren Z. M. , Song W. D. , Chan D. S. H.
Published: July 2002

Carbon nitride thin films were deposited by 532 nm Nd:YAG laser ablation of graphite assisted by a nitrogen ion beam bombardment on silicon substrates. Different nitrogen ion beam energies (200, 400, and 600 eV) were used while the laser parameters remained fixed. X-ray photoelectron spectroscopy (XPS) and ellipsometry measurements were carried out to analyze the electronic and optical properties. The XPS C 1s spectrum for the C–N binding is at 286.5 eV while the N 1s spectrum has a corresponding peak of C–N binding at 396.9 eV. The optical gap Eopt is on the order of magnitude of 10−1 eV and increases with the N/C ratio in the deposited film. Linear dependence of the refractive index n and the extinction coefficient k on photon energy E in the range of 1.5–3.5 eV are established.
Tight-binding studies of the tendency for boron to cluster in c-Si. II. Interaction of dopants and defects in boron-doped Si
Authors: Luo Weiwei , Rasband Paul B. , Clancy Paulette , Roberts Bruce W.
Published: July 2002

Clusters containing up to five boron atoms were considered as extended defects within a crystalline Si matrix. Tight-binding calculations suggest that a cluster containing two boron atoms occupying substitutional sites is stable, unlike any other small boron cluster that we studied. The formation energy increases when a third and fourth substitutional boron atom is added to the cluster. Estimates of the equilibrium concentration, using tight-binding-derived formation energies and formation entropies from the Stillinger–Weber model, indicate that B2 clusters become important when the boron doping level is ∼1018 cm−3, well below the solubility limit. In contrast, the formation energy of defect clusters involving an interstitial (BnI clusters, n=1–5, in their preferred charge states) decreases with increasing cluster size, down to 0.6 eV for B5I in a −5 charge state. None had formation energies that would lead to stable bound clusters. Several BnI clusters were found to be considerably more stable than isolated Si self-interstitials (by 1–2 eV), the BSBI cluster, assumed in some continuum modeling codes to be important, was not a particular interesting defect structure (a formation energy in the −2 charge state, EF−2, of 2.8 eV). There seemed to be little energetic penalty for creating clusters larger than about B5I, in good agreement with Sinno and Brown’s Stillinger–Weber studies of self-interstitial clusters in Si [Mater. Res. Soc. Symp. Proc. 378, 95 (1997)]. Some support was found for the suggestion of Pelaz et al. [Appl. Phys. Lett. 70, 2285 (1997)] that BI2 is a nucleation site for boron clustering. Boron clusters involving a boron interstitial were generally found to be less likely to form than analogous clusters involving a Si self-interstitial. B2 clusters involving vacancies are not energetically favored, confirming the known tendency for boron to diffuse via an interstitial mechanism rather than vacancies. These results suggest that boron clusters could serve as traps, which slow the diffusion of self-interstitials under conditions of interstitial supersaturation in highly doped silicon, consistent with experimental evidence.
Quasi-phase-matched harmonic generation through coupled parametric processes in a quasiperiodic optical superlattice
Authors: Qin Yi-qiang , Zhu Yong-yuan , Zhu Shi-ning , Ming Nai-ben
Published: July 2002

We have investigated and fabricated an optical superlattice of LiTaO3, in which two building blocks, each containing a pair of antiparallel 180° ferroelectric domains, were arranged as a Fibonacci sequence. The second- and third-harmonic generation in this structure have been studied and measured. The results verifies that the second-harmonic spectrum and the third harmonic may be generated in a quadric nonlinear medium by a series of quasi-phase-matching processes. The experimental results are in good agreement with theory.
Inductively coupled plasma etching of GaN using Cl2/Ar and Cl2/N2 gases
Authors: Sheu J. K. , Su Y. K. , Chi G. C. , Jou M. J. , Liu C. C. , Chang C. M. , Hung W. C.
Published: July 2002

This work investigates inductively coupled plasma (ICP) etching processes of GaN. Etching behaviors are also characterized by varying the ICP power, Cl2/Ar or Cl2/N2 mixing ratio, radio-frequency (rf) power, and chamber pressure. Experimental results indicate that the etching profiles are highly anisotropic over the range of etching conditions. Maximum etching rates of 8200 Å/min in Cl2/Ar plasma and 8330 Å/min in Cl2/N2 plasma are obtained as well. In addition, pressure, ICP power, Cl2/Ar(N2) flow ratio and rf power significantly influence etching rate and surface morphology. In particular, dc bias heavily influences the etching rates, suggesting that the ion-bombardment effect is an important factor of these etching processes.
High dielectric constant and tunability of epitaxial SrTiO3 thin film capacitors
Authors: Fuchs D. , Schneider C. W. , Schneider R. , Rietschel H.
Published: July 2002

The dielectric and insulating properties of epitaxial SrTiO3(STO) thin film capacitors were studied. The films were grown by inverted cylindrical magnetron sputtering in the radio frequency mode on (100) STO substrates which were covered with a (001)-oriented YBa2Cu3O7−x (YBCO) layer as a ground electrode. As a top electrode we used YBCO or Au thin films. A high dielectric constant, ε, of up to 5000 was observed at T=80 K. The capacitors revealed a large tunability, i.e., a nonlinear ε(E) dependence, with respect to voltage biasing. By applying 3 V, ε decreased to 1000 which was 20% of its maximum value. The frequency dependence of ε, the temperature dependence of the dielectric loss factor, tan δ, and the direct currency conductivity reflected that variable range hopping via localized states was present and dominated the conduction process in the STO films at low temperatures. The field strength for the electrical breakdown amounted to 300 kV/cm even for rather thin films with a thickness of about 40 nm. Below T=90 K, the STO films were ferroelectric with a high polarization of up to 30 μC/cm2 at T=4.2 K. The ferroelectric phase transition was found to be of second order and of the displacive type.
Alkyl-terminated Si(111) surfaces: A high-resolution, core level photoelectron spectroscopy study
Authors: Terry Jeff , Linford Matthew R. , Wigren Christer , Cao Renyu , Pianetta Piero , Chidsey Christopher E. D.
Published: July 2002

The bonding of alkyl monolayers to Si(111) surfaces has been studied with high-resolution core level photoelectron spectroscopy (PES). Two very different wet-chemical methods have been used to prepare the alkyl monolayers: (i) Olefin insertion into the H–Si bond of the H–Si(111) surface, and (ii) replacement of Cl on the Cl–Si(111) surface by an alkyl group from an alkyllithium reagent. In both cases, PES has revealed a C 1s component shifted to lower binding energy and a Si 2p component shifted to higher binding energy. Both components are attributed to the presence of a C–Si bond at the interface. Along with photoelectron diffraction data [Appl. Phys. Lett. 71, 1056, (1997)], these data are used to show that these two synthetic methods can be used to functionalize the Si(111) surface.
Violet and blue light amplification in Nd3+-doped fluoroindate glasses
Authors: Maciel G. S. , de S. Menezes L. , de Araújo Cid B. , Messaddeq Y.
Published: July 2002

We report optical gain at 382 and 414 nm from Nd3+-doped fluroindate glasses after excitation with lasers operating either at 583 or 532 nm. Stimulated emission due to a frequency upconversion process results in increase of the emitted violet and blue light intensity and emission line narrowing. Large optical gain is measured by pump-probe spectroscopy using samples with various Nd3+ concentrations.
Ultrafast ablation with high-pulse-rate lasers. Part I: Theoretical considerations
Authors: Gamaly E. G. , Rode A. V. , Luther-Davies B.
Published: July 2002

We propose a novel ultrafast pulsed laser deposition (PLD) technique, which eliminates the well-known problem of contamination of the films produced by PLD with particulates ejected from the target. The method uses low energy, picosecond duration laser pulses delivered onto a target at rates of several tens of MHz and thus differs from conventional the PLD method which utilizes high energy, nanosecond duration pulses delivered at low (≈10 Hz) repetition rates. In this article we present the theoretical background justifying the method and define the optimal conditions for efficient evaporation of a target with given thermodynamic properties. By reducing the laser pulse energy while maintaining optimum evaporation, the number of atoms evaporated by each pulse is reduced to the point where it becomes impossible for macroscopic lumps of material to be ejected with the available laser energy, thus preventing the source of particle contamination in the film. To achieve high evaporation rate, the laser pulse repetition rate is increased. We compare our theoretical predictions with literature reports of optimal evaporation in a number of experiments, while in part II of this article we describe a specific experimental study where the method is applied to the production of amorphous carbon thin films.
Simultaneous determination of the mechanical moduli and mass of thin layers using nonadditive quartz crystal acoustic impedance analysis
Authors: Etchenique Roberto , Weisz Ariel Dan
Published: July 2002

High frequency rheological properties of polymer films have been measured using a nonadditive quartz crystal resonator impedance approach. The method involves fast measurement of the quartz covered with the film in contact with solutions of different viscosities and the measurement of the equivalent impedance of the bare quartz resonator in contact with the same solutions. Although by using a standard quartz crystal acoustic impedance method it is not possible to determine more than two of the four film parameters (density, thickness, storage modulus G′, and loss modulus G″), we demonstrate that the interaction between the film under study and a semi-infinite layer of liquid gives the additional information needed to get the mechanical moduli G′ and G″ simultaneously with the mass or thickness of the film. An alternative approach to measure G′ and G″, which involves independent and very precise determination of the film thickness, is discussed. Experimental measurements of polystyrene using dioctyl phthalate as a plasticizer show excellent agreement with Martin’s model for quartz resonators covered with viscoelastic nonpiezoelectric layers. The results demonstrate that quartz resonators can be used to measure high frequency rheological properties of films simply without the need of an independent measurement of thickness.
Low-temperature defect chemistry of oxides. I. General aspects and numerical calculations
Authors: Sasaki K. , Maier J.
Published: July 2002

Using oxides as examples, the defect chemistry is systematically analyzed for a low-temperature regime, at which the oxygen exchange equilibrium reaction is no longer reversible, while the internal defect equilibrium reactions (in particular, the electronic transfer processes) may still be reversible. For the partially frozen-in states as well as for the complete equilibrium cases, defect concentrations are numerically calculated for idealized model oxides including pure, acceptor-doped, and donor-doped oxides. Foreign ions (major/minor, shallow/deep, acceptor/donor), oxygen vacancies, and oxygen interstitials are taken into account as redox-active defects. The deep-level (redox-active) defects often dominate defect concentrations in the partially frozen-in states, while the major dopants fix the concentrations in complete equilibrium. The temperature and oxygen partial pressure dependencies of defect concentrations in the partially frozen-in states are discussed. The description does not only allow one to extend the defect chemistry to lower temperatures, such as room temperature, but also offers a quantitative basis for manipulation and prediction of defect concentrations in ionic crystals. Thereby, the physical and chemical performance of such materials may be controlled at temperatures lower than those at which the oxygen nonstoichiometry is established. The results are equally relevant for applications in solid state physics (e.g., compound semiconductors) and in solid state chemistry (e.g., solid electrolytes, mixed conductors).
Using chaos to generate white noise
Authors: Kautz R. L.
Published: July 2002

Numerical simulations reveal that a phase-locked loop can be operated in a chaotic regime to generate a high level of noise which is approximately white over a broad frequency range. Heuristic arguments indicate that the low-frequency power spectrum may be accurately uniform.
Pockels effect in photopolymers derived from ferroelectric liquid crystals
Authors: Pereda N. , Etxebarria J. , Folcia C. L. , Ortega J. , Artal C. , Ros M. B. , Serrano J. L.
Published: July 2002

The Pockels effect has been studied in some photopolymers based on chiral smectic C liquid crystals. The materials have different degrees of crosslinking, varying from mesogenic side-chain polymers to anisotropic networks. None of them is switchable but there is an important electrooptic response due to a residual molecular motion at low frequencies. In two of the studied materials, the molecular contribution disappears above several kHz and, in these cases, the electronic electrooptic effect has been fully characterized. The electrooptic coefficients are in the range r=1 pm/V. The actual electronic origin of the measured effect has been confirmed by means of comparison with second-harmonic generation data.
Ferromagnetism in carbon-doped Mn5Si3 films
Authors: Gajdzik M. , Sürgers C. , Kelemen M. , Löhneysen H. v.
Published: July 2002

The effect of carbon doping on the structural and magnetic properties of Mn5Si3Cx films is investigated for different substrate temperatures TS and concentrations x. Samples with x≈0.75 prepared at TS=650–750 K exhibit ferromagnetic order with enhanced ordering temperatures TC well above room temperature in contrast to the undoped antiferromagnetic Mn5Si3 compound. Structural analysis shows that C is incorporated interstitially in the hexagonal Mn5Si3-type structure with a lattice expansion with respect to the undoped compound. The enhanced ferromagnetic order is presumably not simply due to a change of the interatomic Mn–Mn distances but due to a change of the electronic structure and/or exchange interactions.
Dynamics of magnetic domain walls in an oscillating field
Authors: Wang Han-Ting , Chui S. T.
Published: July 2002

The dynamics of domain walls in an ac magnetic field is formulated and solved numerically under free boundary conditions. Previous calculations are applicable only in the limit when the sample size is much less than the bare penetration depth, whereas no such approximation is made in the present approach. Our calculation provides for an interpretation of recent experimental results that observe a peak in the ac susceptibility of soft magnets as a function of the amplitude of the external magnetic field.
Molecular dynamics study of a dipolar fluid between charged plates
Authors: Lee Song Hi , Rasaiah Jayendran C. , Hubbard J. B.
Published: July 2002

Recent experiments and computer simulations of thin films have observed the segregation of nonpolar molecules into layers or sheets parallel to the confining walls. We discuss a molecular dynamics study of a thin film of Stockmayer molecules between Lennard-Jones plates and find that, in the absence of an electric field, the dipoles are mainly oriented parallel to the plates in each layer. The component of the dipole autocorrelation function in this plane decays to zero more slowly than the component perpendicular to the walls. The polarization density profile, with an electric field perpendicular to the plates, is also studied, and is found to oscillate from layer to layer, with a magnitude that is in excess of what is predicted by the Debye theory of dielectric saturation by a factor nearly equal to the ratio of the local density to the average bulk density.
Statistical mechanics of the packing of rods on a lattice: Cluster expansion for systematic corrections to mean field
Authors: Bawendi M. G. , Freed Karl F.
Published: July 2002

We introduce a lattice spin field theory which formally provides an exact description of the statistics of nonoverlapping, nonintersecting rods on regular lattices. The theory is applicable to arbitrary rod length distributions and arbitrary volume fractions. The mean field approximation reproduces Flory–Huggins theory for rods on a strict lattice, but our theory permits the rigorous and systematic evaluation of corrections to the mean field entropies. We present those corrections as a cluster expansion for the entropy density which involves a double expansion in powers of the volume fraction ψ and the inverse of the lattice coordination number z−1. Our results are compared for specific cases with those of DiMarzio’s theory which is more accurate than mean field. For dimers which are equally distributed among the possible lattice directions of a hypercubic lattice, we find very good agreement between our theory and DiMarzio’s with ours giving a slightly larger entropy density when corrections to mean field to order ψ4 are included. Through order ψ3, our entropy for longer rods is identical to that of DiMarzio’s expanded similarly. Our dimer results, however, demonstrate that the ψ4 term for rods contains contributions which we attribute to locally ordered configurations and which are absent in DiMarzio’s theory. Our results are compared with the exact entropy density for dimers fully covering an infinite square planar lattice; the inclusion of corrections to mean field up to order z−2 narrows the error from 33.8% for mean field to 7.85%. Our theory, however, also describes the dimer–monomer problem which has not been very amenable to analytic theories.
Excited state electron affinities of NaF, LiCl, and NaCl
Authors: Adamowicz Ludwik , Bartlett Rodney J.
Published: July 2002

Numerical Hartree–Fock and coupled-cluster (CCSD) predictions of the existence of excited states of dipole bound anions are reported for NaF, LiCl, and NaCl. The calculations use a composite numerical and Slater orbital basis set. The second dipole states are found to be bound by 0.012, 0.009, and 0.021 eV for NaF, LiCl, and NaCl, respectively, and should be amenable to experimental observation.