bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-dimensional focal spots related to two-photon excitation
Authors: Sun Hong-Bo , Tanaka Tomokazu , Kawata Satoshi
Published: July 2002

We used a photopolymer consisting of urethane acrylate monomer and free-radical-type initiators as a media for recording two-photon-excited (TPE) focal spots in three dimensions. Mathematically, the initiator plays a role of “k ” operator, transforming the quadratic light intensity distribution to a linear radical concentration distribution. Then, the monomer acts as a three-dimensional (3D) “film,” fixing the focal volume where radical concentration is above a threshold as a voxel. An ascending scan method was utilized to avoid substrate truncation and to achieve isolated complete 3D voxels, from which accurate forms of TPE focus were obtained. The 3D photographing technology led to a clear explanation of the origin of pronounced contradictions in reported spatial resolutions, and would be critical for laser precision microfabrication in various materials.
Ion chemistry in silane dc discharges
Authors: Chatham H. , Gallagher A.
Published: July 2002

The ion production and reactions in dc silane discharges are calculated. It is noted that almost all ion production and reaction occur in the cathode sheath region for typical low-pressure silicon-deposition discharges, so that the calculation considers ion production, drift, and reactions in the sheath region. Sheath models, for inert gas discharges, that utilize local and nonlocal electron energy distributions are compared, and one is adapted to silane conditions. The distribution of ion species (SilH+m) arriving at the cathode is calculated for a range of discharge pressures and currents, for comparison to mass spectrometer measurements. However, we do not make quantitative comparisons to reported observations due to sampling-bias issues that have not been taken into account.
Comparative study of time-resolved conductivity measurements in hydrogenated amorphous silicon
Authors: Kunst M. , Werner A.
Published: July 2002

It is shown that photoconductivity transients in a-Si:H measured in a coplanar electrode configuration and by microwave reflection are identical. This proves that transients obtained with these techniques reflect free charge carrier kinetics where the influence of contacts can be neglected. Evidence is given that charge carrier transport is different in the surface layer and in the bulk. The influence of the temperature and the exciting light intensity on transient photoconductivity data are explained by a tentative recombination model which takes dispersive transport of holes and electrons into account.
Real time optical profilometry as a probe of rates of laser-induced chemical vapor deposition
Authors: Comita Paul B. , Kodas Toivo T.
Published: July 2002

An optical method for the measurement of rates of laser induced chemical vapor deposition processes is described. Vertical growth rates are obtained from measurements of the power attenuation by a growing deposit of a probe beam of known diameter directed parallel to the substrate and intersecting the axis of the deposit. A theoretical analysis is presented that relates the growth rate of deposits with various geometries to the attenuation of the probe beam as a function of time and the probe laser beam diameter. The method can be used to obtain growth rates for deposits as small as a few μm in height, as long as the surface of the deposit is smooth relative to the diameter of the probe beam. The method is used to determine rates of photothermal gold deposition as a function of gold precursor partial pressure. Measured deposition rates ranged from 0.2 to 5.5 μm/s for dimethyl gold hexafluoroacetylacetonate partial pressures from 0.03 to 0.62 Torr.
A new technique for crystallographic characterization of heteroepitaxial crystal films
Authors: Itoh Nobuo , Okamoto Keiichi
Published: July 2002

A new x-ray diffraction technique is proposed which enables us to characterize heteroepitaxial semiconductor films, including a mosaic structure and a variation in lattice spacing, in detail. The equipment can be constructed with a double-crystal spectrometer, a microfocus x-ray tube, and a narrow detector slit. The respective full widths at half maximum (FWHM) of an orientation distribution (Δθ1 ) and broadening due to a spacing distribution (Δθ2 ) are precisely measurable, while the conventional double-crystal method yields their superposition (Δθ<Δθ1+Δθ2 ). The FWHM Δθ1 is determined from a rocking curve obtained by the ω mode with a narrow detector slit admitting x rays diffracted from a given spacing to pass through exclusively. The FWHM Δθ2 is measured from a diffraction profile by the 2θ/θ mode with the slit. Here correction should be made for the spatial and spectral dispersion of the incident characteristic x rays, and hence the relation of observed values to broadening due to the spacing distribution (true value) is calculated. Thus Δθ2 can be determined with errors less than 5 s of arc at FWHMs above 15 s, which has never been attained by the use of the conventional diffractometer. The availability of the present method is explained with examples of the measurements for a silicon perfect crystal, a GaN film, a ZnSe film, and ZnSx Se1−x alloy films.
Defect structure in Ba2YCu3O7
Authors: Nakahara S. , Boone T. , Yan M. F. , Fisanick G. J. , Johnson D. W.
Published: July 2002

Transmission electron microscopy is used to characterize the defects in high Tc superconductor, Ba2YCu3O6.9 produced by two different ceramic processing procedures. In both samples the microstructure is dominated by lamellar domains. These have been identified as reflection twins by observations both of rotations of the diffraction patterns and of δ-fringe contrast. The reflection plane is (110). The characteristic width associated with the twins correlates with annealing time. Occasionally within the twinned domain, substructure on a 50-Å scale is visible, which is attributed to further subdivision into twinned and matrix domains. The twin structure does not interrupt the continuity of the Cu-O planes between Y and Ba layers, but does terminate Cu-O chains in the inter-Ba Cu-O planes. Convergent beam electron diffraction results are consistent with a Pmmm space group. Microcracks and disloction loops are also observed in the (001) basal plane, indicating that the (001) plane is both the slip and cleavage plane in this material.
Modeling of an electron cyclotron resonance heated mirror plasma for highly charged ion and soft x-ray sources
Authors: Booske J. H. , Aldabe F. , Ellis R. F. , Getty W. D.
Published: July 2002

A model is described for predicting equilibrium ion charge state distributions and soft x-ray emissions for highly charged ion plasmas confined in magnetic mirrors. Such a plasma would be generated and sustained with electron cyclotron resonance (ECR) heating. The model is then used to analyze a typical laboratory-sized ECR mirror plasma such as MIMI [Phys. Fluids 28, 3116 (1985)]. Computations suggest that an experiment of this size should readily achieve a highly charged ion operating regime of relevance to studies of ECR highly charged ion plasma. In particular, these devices may be promising candidates as steady-state soft x-ray sources for x-ray lithography.
Influence of dislocations on electrical properties of large grained polycrystalline silicon cells. II. Experimental results
Authors: El Ghitani H. , Martinuzzi S.
Published: July 2002

The predictions of the model given in part I are tested by means of a set of diode arrays which allow us to measure the electron effective diffusion length (Leff) and the short circuit photocurrent (Jsc) in large grained polycrystalline silicon cells. The experimental dependencies of Leff and Jsc on the average of the dislocation density (Ndis) fit well with the computed curves, and then the mean value of the recombination strength of dislocations (Sd) may be evaluated. As predicted by the model, the reduction of the value of Sd by a passivation technique like hydrogenation, significantly increases the values of Leff and Jsc. The good agreement between computed and experimental results indicate that dislocations are the most harmful defects.
The production of nanocrystalline powders by magnetron sputtering
Authors: Hahn H. , Averback R. S.
Published: July 2002

Magnetron sputtering has been used for the production of nanoscale particles of pure metals, binary alloys, intermetallics, and ceramics. Al, Mo, Cu91Mn9, Al52Ti48, and ZrO2 particles with diameters of 7–50 nm were synthesized. The structures and grain sizes of the particles were examined by means of x-ray diffraction and both transmission and scanning electron microscopy. The particles were collected and compacted in situ in the sputtering chamber. The structure of these nanocrystalline materials is also described. This novel application of magnetron sputtering at high gas pressures is of interest for both the production of nanocrystalline materials and isolated clusters.
Improved analysis of ionic conductivity relaxation using the electric modulus with a Cole–Davidson distribution
Authors: Pathmanathan K. , Stevens J. R.
Published: July 2002

The empirical response function introduced by Cole and Davidson for the analysis of dielectric relaxation in the frequency domain has been used in an analogous way to reanalyze ionic conductivity relaxation using the electric modulus representation. The fitting of data for ionic relaxation in AgPO3, 0.1AgI-0.9AgPO3, and 0.4Ca(NO3)2-0.6KNO3 originally fit using a Kohlrausch–Williams–Watts response function has been considerably improved.
Electron optical characterization of cubic boron nitride thin films prepared by reactive ion plating
Authors: McKenzie D. R. , Cockayne D. J. H. , Muller D. A. , Murakawa M. , Miyake S. , Watanabe S. , Fallon P.
Published: July 2002

Boron nitride films prepared by reactive ion plating from a boron evaporation source were characterized structurally using three independent electron optical techniques: energy filtered electron diffraction with radial distribution function analysis; electron energy-loss spectroscopy of the near-edge structure of the boron K edge; and spectroscopy of the plasmon region of the energy-loss spectrum. Both specimens had a graded BNx layer between the BN layer and the silicon substrate and in addition one specimen had a titanium bonding layer underneath the BNx layer. The presence of c-BN in both specimens was confirmed by all techniques. The specimen with the titanium bonding layer was examined in cross section and showed essentially pure c-BN on the surface. A model for the formation of c-BN assisted by the compressive stress generated during deposition is proposed.
Coherent millimeter-wave generation by heterodyne conversion in low-temperature-grown GaAs photoconductors
Authors: Brown E. R. , Smith F. W. , McIntosh K. A.
Published: July 2002

An analysis has been carried out of optical heterodyne conversion with an interdigitated-electrode photomixer made from low-temperature-grown (LTG) GaAs and pumped by two continuous-wave, frequency-offset pump lasers. The analytic prediction is in excellent agreement with the experimental results obtained recently on a photomixer having 1.0-μm-wide electrodes and gaps. The analysis predicts that a superior photomixer having 0.2-μm-wide electrodes and gaps would have a temperature-limited conversion efficiency of 2.0% at a low difference frequency, 1.6% at 94 GHz, and 0.5% at 300 GHz when connected to a broadband 100 Ω load resistance and pumped at hν=2.0 eV by a total optical power of 50 mW. The predicted 3-dB bandwidth (193 GHz) of this photomixer is limited by both the electron-hole recombination time (0.6 ps) of the LTG-GaAs material and the RC time constant (0.5 ps) of the photomixer circuit.
Effect of rf power on remote-plasma deposited SiO2 films
Authors: Hattangady S. V. , Alley R. G. , Fountain G. G. , Markunas R. J. , Lucovsky G. , Temple D.
Published: July 2002

Significant improvement in material and electrical properties of SiO2 films deposited by low-temperature remote plasma-enhanced chemical vapor deposition take place as the rf power to the plasma discharge is increased. The deposition rate increases, and the index of refraction at 632.8 nm, the frequency of the dominant Si—O—Si bond-stretching vibration in the infrared absorption spectrum, the etch rate, and the static dielectric constant of the remote-plasma deposited films all approach those of thermally grown SiO2 films with increasing rf power to the plasma discharge. The total compressive stress in the oxides deposited at high power, ∼300 W, is about 20% higher than that in oxides deposited at lower power, ∼30 W. Comparison of film properties with those of plasma-deposited substoichiometric oxides (SiOx, x≤2) and thermally grown stoichiometric oxides SiO2 leads us to conclude that (i) the films deposited at rf levels from 10 to 300 W are homogeneous stoichiometric oxides SiO2, (ii) correlated variations in the film properties at the higher powers are consistent with a densification of the film, and (iii) systematic changes in the material properties take place along a linear path between two previously identified network morphologies on an n vs ν phase diagram.
A neutron diffraction and Mössbauer effect study of the magnetic properties of Pr2Fe17 and Pr2Fe17N2.6 (invited) (abstract)
Authors: Long Gary J. , Pringle O. A. , Grandjean F. , Yelon W. B. , Buschow K. H. J.
Published: July 2002

The neutron diffraction powder patterns of Pr2Fe17 and Pr2Fe17N2.6 have been measured at 295 K and refined within the Th2Zn17 structure type to give the structural changes which occur upon nitrogenation; changes which include a 6.4% expansion of the unit cell volume. The Mössbauer spectra of Pr2Fe17 and Pr2Fe17N2.6 have been measured between 85 and 295 K and analyzed with a model which is based on the Wigner–Seitz cell environment of each iron site, the basal orientation of the magnetization, and a comparison with the Mössbauer spectra of Nd2Fe17 and Nd2Fe17N2.6. Upon nitrogenation of Pr2Fe17, the 85-K-weighted average isomer shift increases from 0.049 to 0.156 mm/s, and the isomer shifts of the four crystallographically distinct iron sites increase in agreement with the increase found in their Wigner–Seitz cell volumes and the presence of a nitrogen near-neighbor for the 18f and 18h sites. The temperature dependence of the isomer shifts indicates an increase in covalency upon the formation of the nitride. Upon nitrogenation of Pr2Fe17, the 85-K-weighted average hyperfine field increases from 286.2 to 331.9 kOe, however, as is the case for the nitrogenation of Nd2Fe17, the increases on the 6c and 18f sites are much smaller than those observed on the 9d and 18h sites. These changes provide support for the band structure calculations of changes in the magnetic moments upon nitrogenation of Y2Fe17, Nd2Fe17, and Gd2Fe17.
Optical properties of CdTe: Experiment and modeling
Authors: Adachi Sadao , Kimura Toshifumi , Suzuki Norihiro
Published: July 2002

The optical response of CdTe in the 1.1–5.6 eV photon-energy range at room temperature has been studied by spectroscopic ellipsometry. The measured ε(E) data reveal distinct structures at energies of the E0, E1, E1+Δ1, and E2 critical points. These data are analyzed using various theoretical models, namely the model dielectric function (MDF), harmonic-oscillator approximation (HOA), and standard critical-point (SCP) model. Results are in satisfactory agreement with the MDF calculations over the entire range of the photon energies. The HOA (five oscillators) cannot successfully explain the peculiar experimental ε(E) data. The SCP model can provide satisfactory fits to the derivatives of ε(E), but does not yield good fits to ε(E). Dielectric-related optical constants of CdTe, such as the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity, are presented and analyzed. Results of the chemical surface-treatment effect on the pseudodielectric functions have also been presented.
Electroelastic equations for electroded thin plates subject to large driving voltages
Authors: Tiersten H. F.
Published: July 2002

Existing rotationally invariant electroelastic equations are reduced to the case of large electric fields and small strain. These latter equations are specialized to the case of thin plates with completely electroded major surfaces, and it is shown that in this case the charge equation of electrostatics is satisfied trivially to lowest order. It is further shown that for the thin stress-free polarized ferroelectric ceramic plate subject to large electric fields, the resulting equations readily account for experimental data in existence in the literature
Electron distribution in pseudomorphic Al0.30Ga0.70As/ In0.15Ga0.85As/GaAs δ-doped heterostructures
Authors: Fernández J. M. , Lazzouni M. E. , Sham L. J. , Wieder H. H.
Published: July 2002

We have investigated the electronic properties of the δ-modulation-doped Al0.30Ga0.70As/In0.15Ga0.85As heterojunction for a variety of different configurations. Experimental findings are compared with theoretical predictions based on the two-band effective mass approximation including strain, many-body, finite temperature, and DX center effects. We have also examined the case where the δ doping is placed in an embedded GaAs well within the Al0.30Ga0.70As. This is intended to reduce the effects of DX centers. In this instance, as the doping concentration is increased, experimentally determined channel density, ns, suggests higher saturation values, which may leave a residual electron density in the GaAs well. This residual charge density is parallel with the electron density in the InGaAs well and represents an additional conducting path. The features of the theoretical channel density versus doping density curves are confirmed by 300 and 1.6 K resistivity and Hall measurements, and Shubnikov–de Haas measurements made at 1.6 K.
Microwave interferometry of shock waves. II. Reacting porous media
Authors: Glancy Brian C. , Sandusky Harold W. , Krall Albert D.
Published: July 2002

Shocked porous ball powders have been investigated by means of microwave interferometric techniques. Equations developed for interferometric measurements on inert materials apply for energetic materials until reaction at the shock front begins. As reaction begins, the microwave interferometer (MI) output exhibits characteristic changes in both the absorption and reflection of the microwave signal. These changes have been related to hot spot development at the shock front. The hot spots in a reacting bed have been experimentally approximated to a first order by including metallic particles in unreacting beds and measuring their effects on the propagation of microwaves. With the aid of dielectric measurements of the metallized beds, hot spot concentrations as a function of time were predicted from the MI output of reacting beds.
Dielectric function spectra of strained and relaxed Si1−xGex alloys (x=0–0.25)
Authors: Pickering C. , Carline R. T.
Published: July 2002

Dielectric function spectra for strained and relaxed Si1−xGex alloys with x∼0.13 and 0.20 are presented in numerical form. The effect of strain is shown to cause a modification of the spectra in the E1 critical point region, resulting in a decrease in refractive index at 1.96 eV, amounting to 0.06 at x=0.22. The spectral dependence of the refractive index is presented for a series of strained layers. An overview is given of spectral databases and the single-wavelength ellipsometry data available in the literature.
Magnetic properties of the two-dimensional ‘‘triangles-in-triangles’’ Kagomé lattice Cu9X2(cpa)6 (X=F,Cl,Br)
Authors: Maruti Sanchit , ter Haar Leonard W.
Published: July 2002

Magnetic properties of the isostructural series of compounds known as Cu9X2(cpa)6 (cpa=carboxypentonic acid; X=F,Cl,Br) are reported as a function of the μ3-halide ion. The interconnection topology within each layer is a hexagonal arrangement of trimeric units, i.e., a ‘‘triangles-in-triangles’’ Kagomé lattice. Magnetic field dependent susceptibility data in the range 1.7–250 K demonstrate that all three compounds have a magnetic ground state which is highly field dependent at lower temperatures, but that phase transitions to long-range order are not observed down to 1.7 K. Development of 2-D antiferromagnetic models appropriate for the chemical nature and symmetry of the Cu9X2(cpa)6 lattice will require a two J-value solution (intratrimer and intertrimer), and although theoretical results for this type of lattice are as yet unknown, analogies can be clearly drawn to the Kagomé problem. The Curie–Weiss temperatures in the range of −250 K strongly suggest spin frustration in the absence of any traditional long-range order. Magnetization studies exhibit an intermediate saturation level corresponding to a ground state of one unpaired spin per unit cell at field values well below 1.0 kG, supporting the conclusion that the two J values are antiferromagnetic.
Numerical investigation of the self-focusing of broad-bandwidth laser light with applied angular dispersion
Authors: McKenty P. W. , Skupsky S. , Kelly J. H. , Cotton C. T.
Published: July 2002

The self-focusing of phase-modulated broad-bandwidth (BBW) laser light, in which the spectral components are angularly dispersed to produce smoothing of laser nonuniformities, is analyzed. A multifrequency, multidimensional computer code has been developed to simulate the propagation of angularly dispersed BBW light within a nonlinear medium. Earlier work has shown that the self-focusing behavior of phase-modulated BBW light without angular dispersion is insignificantly different from that of monochromatic beams. With the addition of angular dispersion, pure phase modulation cannot be maintained during propagation, and the beam becomes subject to the development of spatial amplitude modulation which moves across the beam in time. Instantaneously, this amplitude modulation will begin to self-focus in a nonlinear medium. However, when examined over one period of the phase modulation, the time-integrated intensity (TII) due to the angularly dispersed BBW beam is consistently smooth, regardless of the amount of propagation modulation or self-focusing growth present in the instantaneous profile. Similarly, the angular dispersion will produce a time-dependent deflection of any amplitude modulation on the beam produced by laser defects. Because the focus of the modulation is enlarged due to the deflection, the TII of this amplitude structure is found to be less than that for a monochromatic beam.
Raman study of low temperature phase transitions in bismuth titanate, Bi4Ti3O12
Authors: Idink H. , Srikanth V. , White William B. , Subbarao E. C.
Published: July 2002

An anomalous line narrowing along with the appearance of several new lines on cooling Bi4Ti3O12 ceramics, indicates a subtle phase transition in the 150–200 K region. In the same temperature range, anomalies in dielectric properties (dielectric constant, coercive field, and spontaneous polarization) of single-crystal Bi4Ti3O12 were reported. A possible subtle monoclinic-orthorhombic phase change, combined with a continuous ordering of dipoles associated with Bi3+ lone pair electrons and TiO6 octahedra, may be responsible for these results.
Laser ablation deposition of Cu-Ni and Ag-Ni films: Nonconservation of alloy composition and film microstructure
Authors: van Ingen R. P. , Fastenau R. H. J. , Mittemeijer E. J.
Published: July 2002

Laser ablation deposition was used to grow polycrystalline Cu-Ni and Ag-Ni thin films on amorphous substrates at room temperature. X-ray diffraction was employed to determine the phases present and the residual macrostress and to analyze the structural imperfection in terms of crystallite size and microstrain. For confirmation and complementary microstructural data transmission electron microscopy was applied. Analysis of the gross composition was achieved by electron probe microanalysis and x-ray fluorescence. The films contained substantially less Cu and Ag than the targets, which was caused by preferential scattering of ablated Cu and Ag species upon incidence at the growing films. The Cu-Ni films were entirely composed of a CuxNi1−x solid solution. The Ag-Ni films were composed of a AgxNi1−x solid solution and of pure Ag and pure Ni. The nonequilibrium AgxNi1−x solid solution could contain up to 44 at. % Ag. The residual macrostress in the Cu-Ni films was compressive, whereas it was tensile in the Ag-Ni films. The occurrence of these stresses could be interpreted as due to the combined effects of atomic peening and cooling after deposition and, in the case of the Ag-Ni films, of stress relaxation by partial decomposition of the AgxNi1−x solid solution during film growth. The microstrains in the AgxNi1−x solid solutions were higher than in similarly prepared pure elemental Ag and Ni films. Compositional inhomogeneity of the AgxNi1−x solid solution crystallites contributed in particular to this effect. The strain-free lattice parameters of the solid solutions were found to be in fair agreement with those predicted by Vegard’s law.
On the shear strength of shock loaded brittle solids
Authors: Rosenberg Z.
Published: July 2002

We propose a new approach to analyze the response of brittle materials to shock loading. Our analysis is based on assuming that these materials obey Griffith’s criterion for failure under pressure. This criterion leads to an explicit relationship between the shear strength of the shocked solid and the pressure behind the shock front. The agreement between this relation and direct measurements of shear strength, on shock loaded titanium-diboride, is very good. Moreover, several discrepancies between different sets of data, concerning shock loaded alumina, are removed by applying the new interpretation for the response of brittle solids to shock waves.
Elastic constants of boron nitride
Authors: Grimsditch M. , Zouboulis E. S. , Polian A.
Published: July 2002

Using Brillouin scattering the three independent elastic stiffness constants cij of single-crystal cubic boron nitride have been measured: c11=820, c12=190, and c44=480 GPa. The resulting bulk modulus, 400 GPa, is in reasonable agreement with two independent determinations obtained from x-ray measurements in a diamond anvil cell. Using the bulk modulus it is found that the x-ray results are best fitted with a value of dB/dP=3.0.
The photonic band edge optical diode
Authors: Scalora Michael , Dowling Jonathan P. , Bowden Charles M. , Bloemer Mark J.
Published: July 2002

Using numerical methods, we study pulse propagation near the band edge of a one-dimensional photonic band gap material with a spatial gradiation in the linear refractive index, together with a nonlinear medium response. We find that such a structure can result in unidirectional pulse propagation. That is, the field will be transmitted for, say, a left-to-right direction of propagation, while for right-to-left nearly complete reflection occurs. This behavior constitutes the operational mechanism for a passive optical diode.
Very large Stark shift in three-coupled-quantum wells and their application to tunable far-infrared photodetectors
Authors: Huang Yimin , Lien Chenhsin
Published: July 2002

The quantum-confined Stark effect in the three-coupled-quantum-well (TCQW) structure is studied theoretically in this paper. The basic TCQW structures are composed of three quantum wells separated by two thin barriers. Coupled one-dimensional Schrödinger and Poisson equations are solved self-consistently to find the sub-band eigenenergies and the envelope wave functions for the TCQW structures. Results indicate that the GaInAs/AlGaAs/GaAs two-depth TCQW structure exhibits both a very large Stark shift and a high absorption coefficient for the 1→3 intersub-band transition. By using a 1→3 intersub-band Stark shift in the two-depth TCQW structure, a highly sensitive tunable far-infrared photodetector is proposed. This photodetector is ideal for device applications in the 8–14 μm atmospheric window region. The operation of this device is based on the infrared absorption by electrons in the ground state transited from the ground-state sub-band E1 of the TCQW to the second-excited-state sub-band E3. A very large variation of eigenenergy spacing ΔE31 between E3 and E1 under an applied electric field can be achieved. Since the infrared radiation is absorbed via the intersub-band resonance absorption (ℏω=E3−E1), the detected infrared wavelength can be tuned by the ΔE31 which can be adjusted by an applied electric field. Based on the theoretical calculations, a tuning range from 7.4 to 14 μm is predicted for the two-depth TCQW structure. This tuning capability is achieved by varying the applied electric field in the 60 to −60 kV/cm range.
Optical properties of PbSe
Authors: Suzuki Norihiro , Sawai Katsuyuki , Adachi Sadao
Published: July 2002

The real (ε1) and imaginary (ε2) parts of the dielectric function, ε(E)=ε1(E)+iε2(E), of PbSe have been measured by spectroscopic ellipsometry in the 1.15–5.4 eV photon-energy range at room temperature. The measured spectroscopic-ellipsometry data reveal distinct structures at three critical-point energies (E1, E2, and E3). These data are analyzed using the two theoretical models, namely, the model dielectric function and standard critical-point model. It is found that both the model dielectric function and standard critical-point model successfully explain the measured derivative spectra, d2ε/dE2. It is also found that the model dielectric function shows excellent agreement with the experimental ε(E) spectra, but the standard critical-point model does not. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of PbSe are also presented.
Band gap of GaN films grown by molecular-beam epitaxy on GaAs and GaP substrates
Authors: Lacklison D. E. , Orton J. W. , Harrison I. , Cheng T. S. , Jenkins L. C. , Foxon C. T. , Hooper S. E.
Published: July 2002

There has been controversy concerning the energy gap of the cubic (zinc-blende) form of the wide-gap semiconductor GaN. Measurements are reported of the band gaps of both hexagonal (wurtzite) and cubic thin films deposited by a modified molecular-beam-epitaxy process on (001) GaAs and GaP substrates. The important difference from conventional MBE lies in the method of supplying nitrogen to the growing film. Here a rf nitrogen plasma source operating at 13 MHz is used. The structure of the films was monitored by x-ray diffraction and controlled by the addition of an As beam which results in growth of the cubic form—otherwise films grow with the hexagonal structure. The band gaps were measured at room temperature by optical reflectivity, as evidenced by the sharp reduction in interference oscillations as the photon energy approached the band edge, and confirmed by the observation of band-edge photoluminescence. The results can be summarized as Eg=3.42±0.02 eV for the hexagonal and 3.22±0.02 eV for the cubic form. The observation of films containing mixed hexagonal and cubic phases, which may have led to earlier errors in band-gap measurements, is also reported.
Optimal control of force microscope cantilevers. I. Controller design
Authors: Garbini J. L. , Bruland K. J. , Dougherty W. M. , Sidles J. A.
Published: July 2002

In magnetic resonance force microscopy (MRFM) experiments, magnetic forces couple to the motion of microscale cantilever beams. Extension of MRFM to the detection of single electrons will require both unprecedented force sensitivity and motional stability of the cantilever. We describe the principles and performance of optimal cantilever motion control. The method accounts for inherent noise processes and practical application of control forces. We show that active feedback control improves cantilever motional stability, enabling instrument designs of much higher sensitivity and faster imaging than passive designs. Experimental results of implemented cantilever control systems are presented in Part II.
Poled polymers for sensors and photonic applications
Authors: Bauer Siegfried
Published: July 2002

A brief survey of the electrical and optical properties of poled polymer electrets for sensors and photonic applications is given. Semicrystalline ferroelectric polymers are highly suitable for piezo- and pyroelectric applications, while amorphous polymers containing molecular dipoles with acceptor and donor groups linked by delocalized π electrons (A-π-D) are interesting for photonic applications. The large variety of poling techniques, such as electrode, corona, electron-beam, and photothermal poling, is discussed in detail together with specifically developed poling techniques for ferroelectric or amorphous polymers. Methods for the experimental investigation of the polar order are based on the piezo-, and pyroelectric effect, birefringence, the electro-optical effect and second-harmonic generation. Newly developed thermal analysis techniques and dipole relaxation spectroscopies complement traditional techniques, such as thermally stimulated depolarization and broadband linear and nonlinear dielectric relaxation spectroscopy. The compatibility of polymers with semiconductor technology is illustrated with selected applications in hybrid integrated thermal and acoustical imaging devices, electro-optical modulators and second-harmonic generators.
Flash annealing and magnetic interactions in Pr4Fe78B18
Authors: Villas-Boas V. , Romero S. A. , Missell F. P.
Published: July 2002

Enhanced-remanence Pr4Fe78B18 alloys were produced by using a high heating rate/short time anneal, known as flash annealing. Partially amorphous ribbons were produced by melt spinning and then annealed for 10–20 s at temperatures 600–820 °C by passing current through the sample. Coercivity increases of up to 15% were systematically observed in relation to furnace-annealed material. Mössbauer spectra showed that flash annealing resulted in significant amounts of Pr2Fe23B3 and reduced quantities of α-Fe. Our best flash-annealed material (600 °C/20 s) had Hc=3.5 kOe, Mr/Ms=0.77, and (BH)max=11.1 MG Oe. The magnetic remanence curves Mr(H), obtained for different initial demagnetized states (thermal, ac, dc+, dc−), and the remanence Md(H), obtained for an initially saturated state, were determined for the construction of Henkel plots (HP). The HP for ac and thermally demagnetized Pr4Fe78B18 are very similar and show both magnetizing and demagnetizing interactions. There is no significant difference between the HP of furnace and flash-annealed samples.
Structural and magnetic properties of CoFe alloy films
Authors: Kim T. H. , Jeong Y. H. , Kang J.-S.
Published: July 2002

The investigation of the structural and magnetic properties of bcc Co1−xFex (0.1<x<0.4) alloy films prepared on MgO(001) substrates using e-beam evaporation and pulsed laser ablation deposition (PLAD) methods is reported. Both structural and magnetic properties seem to be strongly correlated to the deposition conditions, i.e., substrate temperatures (Ts) and deposition rates (DR). To identify the stability limits of the bcc structure in the phase diagram, our study is limited to the Co-rich compounds. For x⩾0.3 at high Ts (500 °C), alloy films deposited by e-beam evaporation exhibit a bcc (001) growth, showing the magnetic anisotropy with easy axis parallel to the MgO [100] crystal axis. In the PLAD technique, the excellent bcc (001) film growth is observed at Ts=300 °C for x=0.25 which is close to the concentration limit of the bcc regime in the equilibrium bulk phase diagram, whereas the same compound films deposited at high Ts reveal the mixed phase of fcc (001) and bcc (001) structures. Magnetic hysteresis measurements of the film grown by the PLAD technique exhibit similar results to those of the films deposited by e-beam evaporation. These results show that, for the well-crystallized films with a bcc (001) texture, the magnetocrystalline anisotropy develops in-plane.
Raman and photoluminescence studies of biaxial strain in GaN epitaxial layers grown on 6H–SiC
Authors: Davydov V. Yu. , Averkiev N. S. , Goncharuk I. N. , Nelson D. K. , Nikitina I. P. , Polkovnikov A. S. , Smirnov A. N. , Jacobson M. A. , Semchinova O. K.
Published: July 2002

The effect of biaxial strain on optical phonons in high-quality GaN epitaxial layers grown on 6H–SiC substrates by metal organic chemical vapor deposition has been studied. The deformation potential constants for the E2(1), A1(TO), E1(TO), and E2(2) optical phonon modes in hexagonal GaN have been obtained. A method for calculating strain in hexagonal GaN layers from Raman data alone is suggested. A comparative analysis of the strain measured by x-ray diffraction and Raman spectroscopy shows that these data agree well. It is found that the biaxial stress of 1 GPa results in a shift of the excitonic photoluminescence lines of 20±3 meV.
Thermodynamics of polydomain heterostructures. I. Effect of macrostresses
Authors: Roytburd Alexander L.
Published: July 2002

The thermodynamic principles of the formation of a new class of materials, polydomain heterostructures, are formulated. The polydomain heterostructures can be formed as a result of phase transformations in constrained layers, composed of epitaxial couples or multilayers. Due to the elastic interaction between the layers of a heterostructure, these layers transform into sets of periodically alternating lamellae, or elastic domains. A polydomain layer can consist of either differently oriented domains of the same phase (twins) or domains of different phases. The goal of this article is to determine the parameters of the polydomain heterostructures and the conditions for their formation, i.e., their dependence on the characteristics of phase transformations, lattice misfits, and the film thickness, as well as external fields. Heterostructures containing ferroelectric and superconductor oxide layers will be considered as examples. In part I the necessary conditions for the formation of polydomain heterostructures have been established on the basis of the analysis of thermodynamic effects of internal and external macrostresses on phase transformations. The thermodynamic criteria are formulated in terms of a few energy parameters: the misfit energies of the phases that form the heterostructure and the energies of interaction between domains. Together with the free energies of the unconstrained phases, these parameters give a quantitative thermodynamic description of polydomain heterostructures provided that the interface effects are negligible. It is shown that polytwin structures are always more stable than single domain structures and therefore, their formation is always possible at some external stresses and temperatures. The stability of heterophase polydomain structures depends on the parameter of incompatibility between the domains, i.e., the ratio of the energy of direct interactions between the domains through their interface to the energy of indirect interaction between domains through the elastic surrounding matrix. If the interdomain interfaces are mobile, their movement may result in superelastic deformation and essential softening of the effective elastic modulus.
Analysis and design of an interdigital cantilever as a displacement sensor
Authors: Yaralioglu G. G. , Atalar A. , Manalis S. R. , Quate C. F.
Published: July 2002

The interdigital (ID) cantilever with two sets of interleaving fingers is an alternative to the conventional cantilever used in the atomic force microscope (AFM). In this paper we present a detailed analysis of the interdigital cantilever and its use as a sensor for the AFM. In this study, we combine finite element analysis with diffraction theory to simulate the mechanically induced optical response of the ID. This model is used to compare this system with the optical lever detector as used in conventional instruments by analyzing the ratio of signal to noise and overall performance. We find that optical detection of the cantilever motion with interdigital fingers has two advantages. When used in conjunction with arrays of cantilevers it is far easier to align. More importantly, it is immune to laser pointing noise and thermally excited mechanical vibrations and this improves the sensitivity as compared to the optical lever.
Stress-relief behavior in chemical-vapor-deposited diamond films
Authors: Fan Qi Hua , Fernandes A. , Pereira E. , Grácio J.
Published: July 2002

Biaxial stress in diamond film deposited on titanium coated copper substrate is investigated. Raman spectra show that with an increase in the film thickness, the diamond Raman line shifts from higher wave numbers to lower, approaching 1332 cm−1. Fitting the experimental plot of the in-plane stress σx versus the film thickness y yields a linear function σx=(0.186y−5.776) GPa. On the other hand, the biaxial stress is modeled from Airy stress theory, showing also that the stress changes linearly along the film-growth direction. A critical film thickness hc=31 μm is found at which the compressive stress is completely released. In addition, it is shown that the variation of the biaxial stress is less than 5% (∼0.28 GPa) throughout the thickness of a film thinner than 1.5 μm. Therefore, the stress in such a thin diamond film can be considered uniform according to the Raman measurement due to the spectrum resolution.
Stopping of energetic light ions in elemental matter
Authors: Ziegler J. F.
Published: July 2002

The formalism for calculating the stopping of energetic light ions (H, He, and Li) at energies above 1 MeV/u, has advanced to the point that stopping powers may now be calculated with an accuracy of a few percent for all elemental materials. Although the subject has been of interest for a century, only recently have the final required corrections been understood and evaluated. The theory of energetic ion stopping is reviewed with emphasis on those aspects that pertain to the calculation of accurate stopping powers.
Semiconducting properties of magnesium vanadate glasses
Authors: Sen S. , Ghosh A.
Published: July 2002

The electrical conductivity of semiconducting magnesium vanadate glasses has been reported for a wide composition range in the temperature range of 80–500 K. The experimental results have been analyzed in the framework of different hopping models. It has been observed that the multiphonon assisted hopping model of small polarons in the nonadiabatic regime, proposed by Emin, can interpret the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. On the other hand, Mott’s optical phonon assisted hopping model at high temperatures provides smaller values of the localization length. However, Mott’s variable range hopping model is consistent with the low temperature data. Schnakenberg’s model yields higher values of the hopping and the disorder energies than the activation energy obtained at the highest and the lowest temperature ranges.
Band-gap and k.p. parameters for GaAlN and GaInN alloys
Authors: Pugh S. K. , Dugdale D. J. , Brand S. , Abram R. A.
Published: July 2002

Using a semi-empirical pseudopotential method, a set of band-structure calculations are performed on a range of GaInN and GaAlN alloys in both the zinc-blende and wurtzite structures. Pseudopotentials for the bulk materials are described by suitable V(q) functions, and these are used to construct the alloy pseudopotentials. The band gap as a function of alloy composition is studied, and it is found that there is no significant bowing in the case of GaAlN. The bowing is larger for GaInN, although heavily dependent on the strain present. A more detailed study of the wurtzite alloys is carried out for low Al and In fractions. Wurtzite k⋅p parameters for several alloys at concentrations commonly used in devices are obtained from the semi-empirical band structure using a Monte Carlo fitting procedure.
Model of the primary rearrangement processes at liquid phase sintering and selective laser sintering due to biparticle interactions
Authors: Anestiev L. A. , Froyen L.
Published: July 2002

A theoretical description of the rearrangement processes at selective laser sintering (SLS) and liquid phase sintering (LPS) is made. A general solution, based on the dimensional analysis and rigorous analytical solution of the equations governing the rearrangement process is obtained. The influence of the process parameters and the physicochemical properties of the sintered materials on the rearrangement processes at SLS and LPS is studied with the model developed. The obtained results are in good agreement with the available experimental data.
Molecular dynamics simulations of Si etching by energetic CF3+
Authors: Abrams Cameron F. , Graves David B.
Published: July 2002

The development of a Tersoff-type empirical interatomic potential energy function (PEF) for the Si–C–F system is reported. As a first application of this potential, etching of a:Si by CF3+ using molecular dynamics (MD) simulations is demonstrated. Aspects of CF3+ ion bombardment through a fluence of 4×1016 cm−2 are discussed, including overlayer composition and thickness, Si etch yields, and etch product distributions. The formation of a 1-nm-thick steady-state SixCyFz overlayer occurs in the simulation, and this layer is an active participant in the etching of the underlying Si. At an ion energy of 100 eV, a steady state the etch yield of Si is predicted to be 0.06±0.01 Si/ion. A comparison of the simulation findings and experimental results from the literature leads to the conclusion that the new PEF performs well in qualitatively modeling the atomic-scale processes involved in CF3+ ion beam etching of Si. Simulations of this kind yield insight into fluorocarbon etch mechanisms, and ultimately will result in phenomenological models of etching by fluorocarbon plasmas.
Formation of a two-dimensional electron gas in an inverted undoped heterostructure with a shallow channel depth
Authors: Kawaharazuka A. , Saku T. , Hirayama Y. , Horikoshi Y.
Published: July 2002

We investigated the dependence of transport characteristics of a two-dimensional electron gas (2DEG) on channel depth in an inverted undoped GaAs/AlGaAs heterostructure. We succeeded in forming a high-mobility 2DEG in a sample with 70 nm channel depth. We controlled the carrier density by varying the back-gate bias over a wide range. The highest mobility reached 2.3×106 cm2/Vs at 3.4×1011 cm−2. The relation between mobility and carrier density is determined: the mobility decreases in a low-carrier-density region as the channel depth decreases. This result suggests that the scattering due to the remote surface charges plays a more significant role.
Magnetic excitations from the S=1/2 two-leg ladders in La6Ca8Cu24O41
Authors: Matsuda M. , Katsumata K. , Eccleston R. S. , Brehmer S. , Mikeska H.-J.
Published: July 2002

We studied the magnetic excitations from the S=1/2 two-leg ladders in La6Ca8Cu24O41. In order to understand the full dispersion of the magnon branch for the lowest triplet excitation, effects of the four-spin and the next-nearest-neighbor (diagonal) exchange interactions are analyzed. We also compare the parameters obtained from the fit to the full dispersion curve with those obtained in the related compounds.
Magnetism of ultrathin FeCo (001) films on GaAs(001)
Authors: Dumm M. , Zölfl M. , Moosbühler R. , Brockmann M. , Schmidt T. , Bayreuther G.
Published: July 2002

Epitaxial Fe34Co66 films in a thickness range from 3 to 100 monolayers (MLs) were grown by molecular beam epitaxy on GaAs(001) at room temperature. The growth was characterized by reflection high energy electron diffraction and x-ray diffraction. The magnetic properties were investigated by alternating gradient magnetometry magneto-optic Kerr effect, and superconducting quantum interference device magnetometry. The films show a strong interface-induced uniaxial in-plane anisotropy with the easy axis along [110]. In addition, the fourfold anisotropy coefficient changes sign around 6 ML i.e., the easy axis of the fourfold anisotropy switches from 〈110〉 to 〈100〉 with decreasing thickness.