bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of annealing atmosphere on the properties of thin-film CdSe
Authors: Szabo Jeffery P. , Cocivera Michael
Published: July 2002

Thin-film CdSe has been deposited electrochemically from an aqueous solution of selenosulfite. The light-to-electric power conversion efficiency depends on the annealing atmosphere with air (oxygen) giving the highest value. The power conversion efficiency correlates with the donor density, the quantum efficiency, and the minority-carrier diffusion length. Surface analytical techniques have been used to detect the presence of oxygen in the film. Various mechanisms for oxygen activation of the film are considered.
Etching of tungsten with XeF2: An x-ray photoelectron spectroscopy study
Authors: Bensaoula A. , Grossman E. , Ignatiev A.
Published: July 2002

I n situ x-ray photoelectron spectroscopy measurements of both W(100) crystals and sputter-deposited tungsten films exposed to a molecular beam of XeF2 with and without an accompanying argon ion beam have yielded the fluorine coverage and the chemical states of the adsorbed fluorine as a function of temperature, exposure, and ion dose. WF, WF2, WF3, and WF4 were found to exist on the tungsten surfaces. Room and elevated temperature exposures of clean tungsten resulted in the surface population of mainly WF species with WF4 observed on nonannealed samples. Ion dose promoted the formation of higher fluorine coordination species from the WF leading to the formation of volatile WF6 and thus resulting in ion-enhanced etching of tungsten.
Optical detection of injected charge carriers in polymer films via Stark spectroscopy
Authors: Kador L. , Haarer D.
Published: July 2002

Persistent spectral holes in a thin film of poly(vinyl butyral) doped with a substituted Zn-tetrabenzoporphin show irreversible broadening effects when high electric field strengths (some hundred kV/cm) are applied. A small field (a few kV/cm) with opposite sign, however, cancels the broadening almost completely and restores the original hole width. Therefore, the broadening is not due to field-induced spectral diffusion, but to permanent electric fields generated in the sample. Our interpretation in terms of electret theory shows that the source of these fields is charges near the surface of the sample, which are injected from the electrodes.
Polycrystalline Si under strain: Elastic and lattice-dynamical considerations
Authors: Anastassakis E. , Liarokapis E.
Published: July 2002

Expressions are derived for the effective phonon deformation potentials of polycrystalline materials, based on the properties of tensor invariants and the Voigt–Reuss–Hill averages [Proc. Phys. Soc. London Sect. A 65, 349 (1952)]. These results, and similar ones for the elastic constants, are used to determine the effect of strains on the long-wavelength optical phonons exhibited by such materials. Application to polycrystalline Si shows that under a uniaxial or biaxial stress, the triply degenerate phonons split into singlet and doublet components that shift at different rates and appear indistinguishably in the Raman spectrum. The implications of such effects on the Raman band shape are discussed.
Contribution of holes to the photorefractive effect in n-type Bi12SiO20
Authors: Strohkendl F. P. , Hellwarth R. W.
Published: July 2002

This is the first characterization of both positive and negative charge carriers as well as deep traps in any crystal sufficient to predict photorefractive behavior for all beam geometries on time scales larger than recombination times. By using beam coupling and light-induced grating-erasure measurements, we find the following in a nominally undoped n-type Bi12SiO20 crystal: (1) the mobility-lifetime product (7.7±1.3) 10−7 cm2/V for electrons and (1.21±0.34) 10−6 cm2/V for holes; (2) an effective photorefractive trap density of (1.01±0.18) 1016 cm−3; (3) a ratio of absorption coefficients for holes and electrons of 0.242±0.027; and (4) near unity quantum efficiency. The hole contributions to the photorefractive effect in this electron-dominated crystal are considerable for large grating wave vectors.
Minority-carrier emission effect in deep level transient spectroscopy measurements on Schottky diodes
Authors: Lee W. I. , Borrego J. M.
Published: July 2002

One basic assumption underlying all the deep level transient spectroscopy derivations is the existence of a single dominant emission process during the transient. It is demonstrated in this paper that the use of conventional procedure could result in significant errors in all the calculated defect parameters when the concerned level has comparable emission rates for both carriers in the detected range. A simple verification method and an improved scheme are proposed to determine the accurate defect parameters. By using this scheme, it is confirmed that EL2 in GaAs is an electron trap with a dominant electron emission rate. A level, designated as E3, in Si shows similar electron and hole emission rates in detected range. It is proven for this level that the conventional Arrhenius plot will lead to large errors in all the calculated defect properties, while the improved scheme provides the accurate information.
Light-ion-bombarded p-type In0.53Ga0.47As
Authors: Rao Mulpuri V. , Babu R. Sachidananda , Dietrich Harry B. , Thompson Phillip E.
Published: July 2002

Multiple-energy H-, He-, and B-ion bombardments were performed to obtain uniform high resistivity over the entire thickness of p-type In0.53Ga0.47As. High resistivity, 580 Ω cm, which is close to the intrinsic resistivity limit of ≊103 Ω cm in InGaAs, is observed. The thermal stability of the high-resistance layers depended upon the mass of the implanted ion. The B-ion-implanted layers maintained high resistivity up to ≊200 °C. Photoluminesence measurements were used to obtain the energy of compensating levels produced by light-ion bombardment.
Probe beam size effects in photothermal deflection experiments
Authors: Lasalle E. Legal , Lepoutre F. , Roger J. P.
Published: July 2002

Important effects of the finite size of the probe beam occur in optical beam deflection experiments when the probe beam radius is of the order of magnitude of the thermal diffusion length. The calculation of this effect is checked by experiments in air at frequencies around 10 kHz and in liquids around 100 Hz. In this last case, for certain values of the modulation frequency and of the probe beam-sample surface distance, the amplitude of the deflection exhibits a sharp minimum and its phase varies drastically.
Temperature dependence of photoluminescence from GaAs single and multiple quantum-well heterostructures grown by molecular-beam epitaxy
Authors: Jiang D. S. , Jung H. , Ploog K.
Published: July 2002

The photoluminescence (PL) from GaAs/AlxGa1−xAs single and multiple quantum well (QW) heterostructures grown by molecular-beam epitaxy (MBE) has been studied in the temperature range 10<T<300 K. The temperature dependence of the PL peak energy and of the integrated intensity observed under both direct and indirect excitation of the GaAs QWs provides insight into the capture mechanism of photoexcited carriers. The temperature dependence of the QW emission energy follows the band-gap shrinkage of bulk GaAs. Deviations from this behavior are caused by lateral monolayer thickness fluctuations of the GaAs quantum well. Under direct excitation the QW luminescence intensity decreases with increasing temperature first due to a partial dissociation of excitons and finally due to thermally activated nonradiative centers. Free carriers generated in the AlxGa1−xAs barrier layers under indirect excitation diffuse rapidly into the GaAs well where they recombine radiatively. This diffusion is enhanced with the increase of temperature. A simple model allows the quantitative description of this diffusion process and of the quantum efficiency of the GaAs quantum well as a function of temperature. In the calculation of the temperature dependence of the luminescence intensity under indirect excitation we use a temperature-independent radiative recombination term and a temperature-dependent nonradiative recombination term to interpret the experimental data.
Molecular-beam epitaxial growth and electrical properties of lattice mismatched InAs1−xSbx on (100) GaAs
Authors: Yen M. Y.
Published: July 2002

Lattice mismatched InAs1−xSbx films have been grown on (100) GaAs substrates by molecular-beam epitaxy under growth conditions far from equilibrium. The growth mechanism of the Sb related compound is found to be different from that of the As related compound. The surface morphology, the crystallinity, and the electrical properties are consistent with each other and indicate the materials to be of high quality.
Photothermal ionization spectroscopy of donors in high-purity germanium
Authors: Darken L. S.
Published: July 2002

The results of narrow linewidth (0.10 cm−1 FWHM) photothermal ionization spectroscopy (PTIS) investigations of shallow donors in high-purity germanium are reported. The donors observed include phosphorus, arsenic, lithium, a hydrogen-oxygen complex, and three lithium-related complexes. One lithium-related complex designated D(Li,Y) is reported here for the first time. Within experimental accuracy, energies of the excited states with respect to the conduction band are the same for all donors. Fourteen different 1S→excited state transitions (five previously unreported, two others seen for the first time in PTIS from the ground state) have been observed. The Zeeman effect was used to help identify these levels. PTIS lines from the ground state to 2P0 and 3P0 were found to be relatively weak but their intensity was in good agreement with the intensity calculated by means of the Cascade theory. In as-grown samples, linewidth broadening of group V donors was observed that depended on the square root of the dislocation density (etch pit density) and with features expected from deformation potential theory.
Initial epitaxial growth of Cu on Mo{011} by low-energy electron microscopy and photoemission electron microscopy
Authors: Mundschau M. , Bauer E. , Świec̢h W.
Published: July 2002

Micrographs are published showing the effect of atomic steps, surface defects, and growth conditions on the submonolayer growth of Cu on Mo{011}. Double-layer growth at various temperatures and multiple-layer growth are examined. At 700 K Cu nucleates at atomic steps and grows two dimensionally and isotropically out onto the terraces. The first layer is pseudomorphic. A complex double layer grows followed by three-dimensional islands. At temperatures below 500 K, islands nucleate on the terraces and further growth is monolayer by monolayer on every terrace. Films were grown without any tendency to form three-dimensional islands. Such films show pronounced quantum-size effects, which appear as large changes of the reflectivity of the electrons with energy and film thickness.
Thermodynamic study on the origin of oval defects in GaAs grown by molecular-beam epitaxy
Authors: Shinohara Masanori , Ito Tomonori
Published: July 2002

The relationship between oval defect density and GaAs growth conditions for molecular-beam epitaxy is investigated. Oval defects are proved to form continuously during growth, with density dependent on Ga cell and substrate temperatures. The correlation between oval defect formation and chemical reactions occurring in the Ga cell and on substrate surfaces, e.g., Ga-Ga2O3, GaAs-Ga2O3, and C-Ga2O3, including the vaporization of Ga and Ga2O3, is investigated thermodynamically. Ga2O formed by the reactions between Ga and Ga2O3 and between C and Ga2O3 in the Ga cell is determined to be related to oval defect formation. Next, substrate surface reactions are studied to investigate the behavior of Ga2O emitted from the Ga cell. The activation energy related to the reaction suggests that only Ga2O adsorption and desorption occur on the substrate. Thus, Ga2O adsorbed on the substrate determines oval defect formation. Prevention of Ga oxidation by chamber baking suggested by the above investigation results in a decrease in oval defect density by a factor of 10. Here, Ga2O is also proved to be the defect origin. It is formed by the reactions between Ga and H2O and between C, Ga, and H2O remaining even in ultrahigh vacuum conditions. These results suggest that both the oxygen and the H2O remaining in the MBE chamber should be reduced to eliminate oval defects.
Cylindrical magnetron discharges. II. The formation of dc bias in rf-driven discharge sources
Authors: Yeom G. Y. , Thornton John A. , Kushner Mark J.
Published: July 2002

When a steady-state capacitive rf discharge is sustained between two electrodes whose surfaces in contact with the plasma have different areas, a negative dc self-bias usually develops on the smaller electrode. For ratios of electrode areas greater than about three, the self-bias is typically 80%–90% of the zero-to-peak potential of the applied rf voltage. However, in cylindrical-post magnetron discharges which are driven by rf power, the self-bias is often only 10%–20% of the zero-to-peak applied voltage. Since these discharges are of interest to both sputtering and plasma-assisted etching, we have investigated the formation of the dc self-bias as a function of applied magnetic field strength. We studied discharges using different diameter post electrodes at rf frequencies of 1.8 and 13.56 MHz. The self-bias voltages were indeed found to be considerably less than those observed in nonmagnetron discharges. Electrostatic probes were used to measure the interelectrode potential distribution. From these results we conclude that the reduced dc self-bias in rf driven cylindrical magnetron discharges can be explained in terms of the effect of the axial magnetic field on electron transport to the cylindrical-post electrode during that portion of the rf cycle when the post electrode is serving as an anode.
Analysis of the composition of surface layers in high-temperature superconducting materials by the combination of Rutherford backscattering spectrometry and nuclear resonance reaction
Authors: Freire F. L. , Barros Leite C. V. , Patnaik B. K. , Baptista G. B. , Naugle D. , Pandey R. K. , Kirk W.
Published: July 2002

Rutherford backscattering spectrometry combined to the 16O(α, α) nuclear resonance (E=3.045 MeV) are used to determine the stoichiometry of Y-Ba-Cu-O superconducting samples. The oxygen depth profile is also measured at thin surface layers of the compound.
Pulsed, high-current, in-line reversal electron attachment detector
Authors: Bernius Mark T. , Chutjian Ara
Published: July 2002

A new, pulsed, high-current, in-line reversal electron attachment ionizer/detector is described. The ionizer is capable of delivering a beam of electrons into an electrostatic mirror field to form a planar wall of electrons having zero kinetic energy. Electron attachment to a molecular target at the reversal point produces either parent or fragment negative ions through a zero-energy (s-wave) state. The atomic or molecular ion is pulsed out of the attachment region approximately 2 μs after the electrons are pulsed off, and focused onto the entrance plane of a quadrupole mass analyzer. The sensitivity of the apparatus is preliminarily assessed, and its higher-energy behavior with regard to molecular attachment and ionization is described.
Phase formation sequence for the reaction of multilayer thin films of Nb/Al
Authors: Barmak K. , Coffey K. R. , Rudman D. A. , Foner S.
Published: July 2002

We have investigated the phase formation sequence for the reaction of Nb and Al in multilayer films using cross-sectional transmission electron microscopy and x-ray diffraction. NbAl3 is the first intermetallic phase to form. Contrary to previous reports, we find evidence from cross-sectional transmission electron microscopy that the sigma phase, Nb2Al, is not bypassed in the reaction sequence. Instead, its formation is concurrent with the formation of the superconducting A15 phase, Nb3Al. However, depending on the periodicity and the composition of the film, the Nb2Al phase can be consumed by the Nb3Al phase for long annealing times. The significance of this phase formation sequence to powder metallurgically processed magnet wire is discussed.
Recombination and dissociation of H2+ and H3+ ions on surfaces to form H2(v″): Negative-ion formation on low-work-function surfaces
Authors: Hiskes J. R. , Karo A. M.
Published: July 2002

The recombination and dissociation of H+2 and H+3 ions incident upon metal surfaces leads to H, H2(v″), and H− products rebounding from the surface. A four-step model for H+2 -ion recombination generates H2(v″) via resonant electron capture through the b 3Σ+u and X 1Σ+g states. A molecular trajectory analysis provides final-state H2(v″) distributions for incident energies of 1, 4, 10, and 20 eV. The calculated H2:H+2 yields compare favorably with the observed yields. A similar four-step model for incident H+3 proceeds via resonant capture to form the H3(2p 2E′→2p 2A1) ground state, in turn dissociating into H+H2(v_″), with the fragment molecule rebounding to give the final H2(v″) distribution. Comparing the final populations v″≥5 for incident H+2 or H+3 shows that the H+3 ion will be more useful than H+2 for H− generation via dissociative attachment. Molecular ions incident upon low-work-function surfaces generate additional H2(v″) via resonant electron capture through excited electronic states and provide two additional sources of H− production: Direct H− production by H dissociation products rebounding from the surface and H− production through the formation of H−2 in the surface selvage that in turn dissociates into H+H−. The H−2 in the selvage is formed by resonant capture to the low-lying vibrational levels of H2(v″), and complements dissociative attachment to high-lying levels in the discharge. The H, H2(v″), and H− yields are inventoried for H+3 incident upon barium surfaces.
Thermally induced structural modification of Mo-Si multilayers
Authors: Stearns D. G. , Stearns M. B. , Cheng Y. , Stith J. H. , Ceglio N. M.
Published: July 2002

The effect of elevated temperature on the structural stability and performance of Mo-Si multilayer mirrors is investigated. Mo-Si multilayers deposited by magnetron sputtering are annealed at temperatures ranging from 200 to 800 °C. A detailed and consistent picture of the thermally induced changes in the microstructure is obtained using an array of complementary measurement techniques including small- and large-angle x-ray scattering and high-resolution electron microscopy. The first significant structural changes are observed at 400 °C, characterized by an increase in the width of the amorphous interlayer regions, as well as the nucleation of microcrystallites of silicide in these regions. At higher temperature the Mo layers transform completely into polycrystalline mixtures of Mo5 Si3 and MoSi2 in both the hexagonal and tetragonal phase. The layers of silicide remain intact but exhibit a structural instability, resulting in severely warped layers surrounded by pockets of amorphous Si and voids. By 800 °C the layered structure is completely destroyed and the composition is predominately tetragonal MoSi2 . The performance of the multilayers as normal-incidence x-ray mirrors is measured and correlated with the observed structural modifications. Finally, our results are compared and contrasted with other annealing studies of the Mo-Si system.
Optical and electrical properties of ytterbium-doped GaAs grown by metalorganic chemical vapor deposition
Authors: Taguchi Akihito , Nakagome Hiroshi , Takahei Kenichiro
Published: July 2002

Optical and electrical properties of ytterbium-doped GaAs are investigated. The samples are grown by low-pressure metalorganic chemical vapor deposition using tris-cyclopentadienyl ytterbium as the ytterbium doping source. Yb concentrations in the GaAs epitaxial layers, which were measured by secondary ion mass spectroscopy, are well controlled by the source temperature up to 1.4×1019 cm−3, but GaAs:Yb samples do not show Yb intra-4f-shell photoluminescence. Hall effect and deep-level transient spectroscopy measurements reveal that the Yb doping forms deep electron traps or acceptor levels. The depth of such levels is likely to be responsible for the absence of Yb 4f photoluminescence in GaAs:Yb, in contrast to the strong Yb luminescence observed in InP:Yb.
Phase shift nonlinearity at resonance in a piezofilm-based fiber-optic phase modulator
Authors: Sudarshanam V. S. , Srinivasan K.
Published: July 2002

The PVF2 film-based fiber-optic phase modulator exhibits a resonance around 3.5 kHz. Existing reports of its characterization have considered only the linearity at nonresonance frequencies and the frequency response at a single drive voltage. A large nonlinearity of phase shift with applied voltage at frequencies near the resonance has been measured. The factors that influence the resonance have been identified on the basis of the frequency response for different drive voltages. These measurements were facilitated by the three new methods of phase detection we recently reported.
A quantitative analysis of strain relaxation by misfit dislocation glide in Si1−xGex/Si heterostructures
Authors: Tuppen C. G. , Gibbings C. J.
Published: July 2002

Misfit dislocation glide velocities have been measured in Si1−xGex/Si heterostructures. Dislocations were deliberately introduced at sites of crystalline damage, the samples were then annealed, and dislocation propagation distances measured using defect selective chemical etching. A number of different sample configurations were investigated with different layer thicknesses and alloy compositions. The measured velocities were found to depend on a number of factors including anneal temperature, an activation energy (which was found to depend on the Ge mole fraction), the effective misfit stress (which is a function of the Ge mole fraction and layer thickness), and the length of the threading arm of the misfit dislocation. Si/Si1−xGex/Si buried-layer structures typical of the heterojunction bipolar transistor were also studied. Two possible relaxation mechanisms, involving two- and three-segment dislocation configurations, are considered and an evaluation of the most likely mechanism for a range of different structures is presented. A complete quantitative analysis is made of all the results and expressions have been derived for the misfit dislocation glide velocity as a function of layer thickness and alloy concentration for all types of layer configuration.
Circularly symmetric distributed feedback semiconductor laser: An analysis
Authors: Erdogan T. , Hall D. G.
Published: July 2002

We analyze the near-threshold behavior of a circularly symmetric distributed feedback laser by developing a coupled-mode theory analysis for all azimuthal modes. We show that the equations that describe the low-order azimuthal modes are, to a very good approximation, the same as those for the one-dimensional (linear) distributed feedback laser. We examine the behavior of higher-order azimuthal modes by numerically solving the exact coupled-mode equations. We find that while a significant amount of mode discrimination exists among radial (longitudinal) modes, as in the one-dimensional distributed feedback laser, there is a much smaller degree of discrimination among azimuthal modes, indicating probability of multimode operation. Despite the multimode behavior, we find that the frequency bandwidth associated with modes that do lase ought to be smaller than the spacing between Fabry-Perot modes of a typical semiconductor laser. This laser is an excellent candidate for a surface-emitting laser—it should have a superb quality output beam and is well-suited for array operation.
Enhanced ac Josephson effect
Authors: Monaco R.
Published: July 2002

The amplitude of the current steps induced by a rf field in a Josephson junction is strictly related to the stability of the phase-locked state. We show how the ac Josephson effect can be enhanced by means of a proper drive: when two phased ac fields of angular frequencies ω and 2ω are added together, the rf-induced steps have maximum normalized amplitudes larger than the Bessel functions as predicted for a sinusoidal drive. Furthermore, in the limiting case of a driving signal that consists of an impulse train, the stability of the phase-locked state can reach its maximum value, i.e., rf-induced steps as large as the critical current can be obtained. Experimental evidence of the enhanced ac Josephson effect is reported for a highly hysteretic Nb/Al-Al2O3/Nb Josephson tunnel junction. The enhanced ac Josephson effect may open new ways for the understanding of the nonlinear dynamics of Josephson junctions and for the design of more efficient Josephson voltage standard devices.
Laser chemical vapor deposition direct patterning of insulating film
Authors: Hiura Y. , Morishige Y. , Kishida S.
Published: July 2002

The 10 μm scale patterned SiO2 films with excellent insulating quality has been directly deposited by laser chemical vapor deposition projection printing. Leakage current through the SiO2 film was kept within permissible limits for large scale integration (LSI) functioning. This new local insulating scheme substantially extends the potential of laser direct writing circuit restructuring technology, such as in locally insulating two crossing interconnections on LSIs.
Application of low-temperature ions in the nonequilibrium state plasma of a Penning source
Authors: Sato Y. , Ogawa H. , Yamada S. , Yamada T. , Kohno T.
Published: July 2002

We present the high-output properties during the early 100–200 μs of a pulsed Penning source. The peak intensity, I, is higher than that of a dc source by a factor of more than 10. This phenomenon is due to the long confinement time of ions, τ, at the rising of the plasma, since within such a short time the ion temperature, T, does not reach thermal equilibrium and is still low. These characteristics are expressed as I∝τ∝T−1/2. The ratio of T−1/2 before and after equilibrium has a maximum of 1/18.
A model for the Zn diffusion in GaAs by a photoluminescence study
Authors: Ky Nguyen Hong , Pavesi L. , Araújo D. , Ganière J. D. , Reinhart F. K.
Published: July 2002

To study the mechanism of zinc diffusion in GaAs, we diffused zinc from a ZnAs2 source into Si-doped GaAs samples (n ≊ 1.3 × 1018 cm−3) at different temperatures (from 575 °C up to 700 °C) in sealed evacuated quartz tubes. The samples are characterized by the depth profile of the photoluminescence at different temperatures. The photoluminescence spectra show characteristic emission associated to deep levels of gallium and arsenic vacancies. A detailed analysis of the spectra demonstrates the role played by vacancies in the Zn diffusion process. The spatial correlation between the luminescence spectra and the Zn concentration obtained from secondary ion mass spectroscopy measurements has been demonstrated.
Samarium-iron based magnet materials with the Th2Zn17-type structure
Authors: Weitzer F. , Hiebl K. , Rogl P.
Published: July 2002

Substitution mode and magnetic behavior in Sm2Fe17 have been investigated as a function of (a) the metal substitution (Fe replaced by Mn, Co, Al, Ga) and (b) the accommodation of interstitial carbon atoms. X-ray powder analysis proved in all cases a consistency with the rhombohedral Th2Zn17-type structure and with a random distribution of the Al-atoms on the iron-sublattice sites. Substitution of iron in Sm2Fe17 by Mn,Co,Al,Ga, and/or carbon leads to an anisotropy change from easy plane to easy axis geometry. In correspondence with the volume expansion, Curie temperatures tend to increase as a result of an enhanced ferromagnetic coupling mechanism. Magnetization values, however, are strongly reduced due to a change of collinear ferrimagnetism to a ferromagnetic type of order of the various magnetic sublattices. The magnetic anisotropy phase diagram at 300 K has been derived for the iron-rich region of the quaternary system Sm2Fe17−x−yCoxAly (0≤x≤8, 0≤y≤3) revealing a wide region of spin reorientation between the fields of easy plane and easy axis anisotropy.
Internal heat transfer in Czochralski grown silicon crystals
Authors: Wallace John P. , Tien John K. , Stefani Jerry A. , Choe Kwang Su
Published: July 2002

Silicon crystals grown by the CZ process have recently been studied with eddy current techniques to determine thermal profiles within the growing crystal. A key question concerning heat transfer in this semiconductor system during CZ growth is whether optical semitransparency in the infrared is important in affecting the high-temperature thermal distribution within the crystal. From normal Fourier’s law calculations, well behaved profiles with rather flat radial isotherms are predicted in CZ growth. Eddy current data, though, show abrupt temperature changes near the crystal outer surface, indicating sharp radial thermal gradients. It is proposed that these sharp gradients are due in part to the onset of optical semitransparency in the crystal in the infrared. It is expected that such a transparency phenomenon will occur below a transition temperature. Any sharp gradients can be responsible for creep damage during growth.
Magnetic and magneto-optic properties of rapid thermally crystallized garnet films (invited)
Authors: Suzuki Takao
Published: July 2002

A new technique to improve the morphology of sputter-deposited bismuth garnet films for optical storage is discussed. This technique employs high ramp-up rate annealing in a nitrogen atmosphere, which leads to a smaller grain size and smoother surface than a conventional oven annealing or in situ crystallization process during film growth. By utilizing this technique, the films can be successfully crystallized to the grain size of about 200–300 Å, and a high coercivity (≳kOe). Multilayer films of garnets with other additions such as Cr and SiOx are also discussed. These multilayer films can provide better morphology and high coercivity.
A one-dimensional model for wall motion coercivity in magneto-optic media
Authors: Della Torre E. , Perlov C. M.
Published: July 2002

A numerical one-dimensional model for domain-wall structure in magneto-optic materials has been developed. It is assumed that the source of coercivity is a variation in micromagnetic parameters such as the exchange constant. A classical domain wall is placed at location of the change in the parameters and allowed to relax. Then, the coercivity is computed by slowly increasing a magnetic field until the wall breaks away from this well. In an illustrative example, a decrease in exchange, which permits a higher rate of spin rotation in the wall, distorts the classical domain-wall shape, and this distortion has to be reduced as the wall tries to break away.
Electrical properties of Au/ and YBa2Cu3O7−x/SrTi1−yNbyO3 diodes
Authors: Yoshida A. , Tamura H. , Gotoh K. , Takauchi H. , Hasuo S.
Published: July 2002

Electrical properties of Au/ and YBa2Cu3O7−x/SrTi1−yNbyO3 heterojunctions were studied by measuring their capacitance-voltage, current-voltage, and conductance-voltage characteristics. The heterostructures were made by depositing Au or YBa2Cu3O7−x films on SrTi1−yNbyO3 substrates. The results of the capacitance-voltage measurement indicated that there was an interfacial layer having a dielectric constant lower than that of bulk SrTiO3 at the Au/SrTiO3 and YBa2Cu3O7−x/SrTiO3 interfaces. The current-voltage characteristics of the Au/SrTi1−yNbyO3 diodes with substrate Nb concentrations of 0.05 and 0.005 wt. % matched characteristics normally associated Schottky junctions and had a large ideality factor, n, consistent with the low-dielectric-constant interfacial layers. When the carrier concentration of the n-SrTiO3 substrate was 2×1019 cm−3, the Au and YBa2Cu3O7−x junctions showed interfacial-layer tunneling characteristics. The YBa2Cu3O7−x junctions exhibited two peaks in their conductance-voltage relations whose peak structures relies on the superconducting state density in YBa2Cu3O7−x films.
Microstructural stability of epitaxial CoSi2/Si (001) interfaces
Authors: Rajan K. , Hsiung L. M. , Jimenez J. R. , Schowalter L. J. , Ramanathan K. V. , Thompson R. D. , Iyer S. S.
Published: July 2002

Epitaxial films of cobalt silicide grown on (001) Si by molecular beam epitaxy have been characterized by transmission electron microscopy. Apart from (001) oriented CoSi2 grains, regions of 〈221〉 type orientations were also found. The 〈221〉 oriented domains were found to be associated with pronounced facetted depressions on the (001) Si surface. Empirical observations suggest that the formation of 〈221〉 CoSi2 domains and the formation of other types of silicide stoichiometries may be related. It is demonstrated that these microstructural instabilities may be suppressed by the codeposition of cobalt and silicon rather than simply by depositing cobalt and reacting with the Si substrate to produce (001) CoSi2.
Substrate temperature effects on the preparation of YBa2Cu3O7−x superconducting films on (100) SrTiO3 by laser ablation
Authors: Lee S. H. , Bae S. C. , Kang J. G. , Lee H. J. , Shin H. J. , Ku J. K.
Published: July 2002

In situ high-Tc superconducting thin films of YBa2Cu3O7−x were deposited on (100) SrTiO3 substrate by laser ablation with a glancing angle geometry at different substrate temperatures in the 560–700 °C range. The transition temperatures, x-ray diffraction patterns, and surface morphologies of the films were substantially different from one another. The critical temperatures of the films deposited at below 600 °C were 72–84 K, while the films deposited at above 650 °C showed Tc0 of 85–91 K. The x-ray diffraction patterns and surface morphologies of the films showed that the orientations of the deposited films depended on the substrate temperature. The films deposited at 700 °C showed an exclusive orientation of ‘‘c’’ axis perpendicular to the (100) SrTiO3 substrate surface, but those deposited at below 600 °C exhibited random but a preferential orientation of c axis parallel to the (100) SrTiO3 surface. The dependence of the film orientations versus substrate temperature is explained in terms of lattice misfits as well as the surface mobility of the adatoms or molecules at the deposition temperature. Also, the deposition rates were found to be a function of the substrate temperature, and the activation energy of surface migration of adatoms was measured to be 0.21±0.03 eV.
Role of hydrogen dilution and diborane doping on the growth mechanism of p-type microcrystalline silicon films prepared by photochemical vapor deposition
Authors: Ghosh Sukriti , De Abhijit , Ray Swati , Barua A. K.
Published: July 2002

Boron-doped microcrystalline hydrogenated silicon (μc-Si:H) films were grown from a gas mixture of silane, diborane, and hydrogen employing mercury sensitized photochemical vapor deposition. At a low diborane doping ratio (1.1×10−3) hydrogen dilution of source gases resulted in films exhibiting a maximum conductivity of 7.4 S cm−1. For a higher doping ratio (10−2) the value of conductivity remained almost unchanged (10−5–10−6 S cm−1) with hydrogen dilution indicating nonexistence of microcrystallinity under such conditions. Transmission electron microscopy of B-doped μc-Si:H films revealed formation of crystallites possessing different crystallographic orientations, e.g., (111), (220), and (311) along with other planes. X-ray spectra confirmed a large number of crystallites with (220) orientations.
Determination of the charge carrier concentration across growth striations in n-GaAs by Raman spectroscopy
Authors: Herms M. , Irmer G. , Monecke J. , Oettel O.
Published: July 2002

Using a Raman microprobe, the charge carrier concentration across growth striations in Bridgman n-GaAs is determined both by the analysis of coupled LO-phonon-plasmon modes and their relative intensities compared to that of the uncoupled LO phonon of the depletion layer. It is shown that the charge carrier concentration typically varies by ±10% in the regions both under rough and facetting growth conditions. In the facetting grown region the charge carrier concentration appears to be higher by about 50%.
The influence of particle shape on dielectric enhancement in metal-insulator composites
Authors: Doyle W. T. , Jacobs I. S.
Published: July 2002

Disordered suspensions of conducting particles exhibit substantial permittivity enhancements beyond the predictions of the Clausius–Mossotti equation and other purely dipolar approximations. The magnitude of the enhancement depends upon the shape of the particles. A recently developed effective cluster model for spherical particles [Phys. Rev. B 42, 9319 (1990)] that treats a disordered suspension as a mixture, or mesosuspension, of isolated spheres and close-packed spherical clusters of arbitrary size is in excellent agreement with experiments on well-stirred suspensions of spheres over the entire accessible range of volume loading. In this paper, the effective cluster model is extended to be applicable to disordered suspensions of arbitrarily shaped conducting particles. Two physical parameters are used to characterize a general suspension: the angular average polarizability of an isolated particle, and the volume loading at closest packing of the suspension. Multipole interactions within the clusters are treated exactly. External particle-shape-dependent interactions between clusters and isolated particles are treated in the dipole approximation in two ways: explicitly, using the Clausius–Mossotti equation, and implicitly, using the Wiener equation. Both versions of the model are used to find the permittivity of a monodisperse suspension of conducting spheroids, for which the model parameters can be determined independently. The two versions are in good agreement when the axial ratio of the particles is not extreme.

The Clausius–Mossotti version of the model yields a mesoscopic analogue of the dielectric virial expansion. It is limited to small volume loadings when the particles have an extremely nonspherical shape. The Wiener equation version of the model holds at all volume loadings for particles of arbitrary shape. Comparison of the two versions of the model leads to a simple physical interpretation of Wiener’s equation. The models are compared with experiments of Kelly, Stenoien, and Isbell [J. Appl. Phys. 24, 258 (1953)] on aluminum and zinc particles in paraffin, with Nasuhoglu’s experiments on iron particles in oil [Commun. Fac. Sci. Univ. Ankara 4, 108 (1952)], and with new X-band and Kα-band permittivity measurements on Ni-Cr alloy particles in a polyurethane binder.
Plasma opening switch characterization and optimization for inductive energy storage
Authors: Véron L. , Boivinet R. , Rouillé C. , Etlicher B. , Peugnet C. , Dufour B.
Published: July 2002

Some experimental results on carbon plasma guns are reported. Plasma density, as a function of time, is measured both by microwave interferometry (37.5 GHz) and Faraday cups. Plasma velocity is measured by a time-of-flight technique with the Faraday cups and is, also, deduced from the axial resolution of the density measurement. The plasma composition is determined by time-integrated spectroscopy. The time evolution of line radiation has been correlated with both interferometry and charge collectors. Three different ion species have been observed and studied in time: H+, with velocity VH=7 cm/μs, and C+ and C2+, both with velocity VC=2 cm/μs. Both plasma expansion in space and electrode effects are studied by time-integrated photography.
The dispersion of the refractive index of III-V semiconductors
Authors: Bi Wengang , Li Aizhen
Published: July 2002

The refractive index dispersions of semiconductors at wavelengths longer than the fundamental absorption edge have been analyzed with a theoretical prediction which is based on a modification of the simplified models of the interband transitions by taking into account the contribution of the free-carrier absorptions to the index of refraction. This approach has been applied to III-V semiconductor compounds, and the results exhibit excellent agreement with the previously published experimental data over the entire range of wavelengths concerned, thus not only confirming the validity of the presented model, but also demonstrating the non-negligible role played by the free-carrier absorption on the dispersion of the refractive index of semiconductors at wavelengths longer than the intrinsic absorption edge.
Formation of copper patterns on poly(tetrafluoroethylene) via radiation controlled chemical etching and chemical-vapor deposition
Authors: Rye R. R. , Knapp J. A. , Chi K.-M. , Hampden-Smith M. J. , Kodas T. T.
Published: July 2002

Patterned copper films have been deposited on poly(tetrafluoroethylene) (PTFE) in a three-step additive process. In the first step, a pattern is produced by cross linking the PTFE surface in selected areas by irradiation with either electrons or x rays at dose levels below those that are either visually or spectroscopically apparent. The pattern is then developed by wet chemical etching in the second step in which only the nonirradiated areas are appreciably etched with sodium naphthalenide. In the final step, chemical-vapor deposition using the precursor (hexafluoroacetylacetonato) Cu(I) trimethylphosphine at 200 °C results in Cu deposition only on the nonirradiated areas of the surface. The Cu films are continuous with a resistivity of 4 μΩ cm, high purity as determined by Auger electron spectroscopy and x-ray photoelectron spectroscopy, and are sufficiently adherent to survive a Scotch tape test. Patterned feature sizes as small as 35 μm can be produced.
Numerical simulations of inductive-heated float-zone growth
Authors: Chan Y. T. , Choi S. K.
Published: July 2002

The present work provides an improved fluid flow and heat-transfer modeling of float-zone growth by introducing a rf heating model so that an ad hoc heating temperature profile is not necessary. Numerical simulations were carried out to study the high-temperature float-zone growth of titanium carbide single crystal. The numerical results showed that the thermocapillary convection occurring inside the molten zone tends to increase the convexity of the melt-crystal interface and decrease the maximum temperature of the molten zone, while the natural convection tends to reduce the stability of the molten zone by increasing its height. It was found that the increase of induced heating due to the increase of applied rf voltage is reduced by the decrease of zone diameter. Surface tension plays an important role in controlling the amount of induced heating. Finally, a comparison of the computed shape of the free surface with a digital image obtained during a growth run showed adequate agreement.
Sensitivity of surface acoustic wave velocity in lithium niobate to electric field or biasing stress
Authors: Gafka Dariusz , Tani Junji
Published: July 2002

Velocity shifts of surface acoustic waves (SAW) for lithium niobate due to homogeneous external static stress or an electric field are presented. Stress sensitivity is defined through six independent components of a second order symmetric tensor, and electric field sensitivity is a vector of three components. Maps for different cuts of LiNbO3 and different directions of SAW propagation have been computed. These maps can be used to find new cuts of lithium niobate, which has large or zero sensitivity either on the stress or the electric field. It could be useful for special applications of LiNbO3 substrate in a SAW devices technique.
Microwave characterization and modelization of magnetic granular materials
Authors: Olmedo L. , Chateau G. , Deleuze C. , Forveille J. L.
Published: July 2002

The use of dispersion techniques for development of microwave absorbers is a convenient way of realization of coatings with good attenuation characteristics in the 1- to 20-GHz range. Particularly, elastomer mixing is a very good technique for dispersion and individualization of each grain up to 60% volume. A first generation of materials, developed with ferrite powders, has been extensively studied. It was found possible to predict properties of composites with classical mixture laws using intrinsic properties of filler and matrix. When using conductive ferromagnetic fillers, the inability to measure intrinsic magnetic properties does not permit prediction of behavior of composites. Therefore, a method of inverse calculation has been developed which, from (ε, μ) parameters measured on composites, allows inferrence of intrinsic properties of each filler (iron, cobalt, permalloy,...). It is shown that this method, employed for several amounts of filler, gives significant accuracy on (μ′, μ″) with intermediate concentrations (typically 25% vol. to 45% vol. with spheres). For permittivity, this method is satisfying in the case of nonpercolating systems. Also studied was the effect of morphology on properties of granular materials, particularly in the case of sphericity and grain size of iron and cobalt particles. Permittivity level is directly correlated to sphericity, and magnetic relaxation frequency to grain size. It is also shown that the chemical nature, particularly for ex-carbonyl origin of iron, does not play a significant role in composite material properties.
Enhanced magnetocaloric effect in Gd3Ga5−xFexO12
Authors: McMichael R. D. , Ritter J. J. , Shull R. D.
Published: July 2002

The working refrigerant material in the majority of magnetic refrigerators has been Gd3Ga5O12 (GGG) which has an upper temperature limit near 15 K. In this paper we report on the field-induced adiabatic magnetic entropy change, ΔSm(H,T), of a series of iron-substituted gadolinium garnets (GGIG) Gd3Ga5−xFexO12 which have the potential to increase the working temperature range or to reduce the field requirements of cryogenic magnetic refrigeration. Depending on Fe concentration, x, the entropy change of these materials at applied fields of 0.9 and 5.0 T is much greater than that of GGG, especially at temperatures above 15 K. At low Fe concentrations, the results are consistent with formation of magnetically ordered clusters of spins at low temperatures. Room temperature electron paramagnetic resonance measurements show that Fe3+ ions mediate exchange interactions which are responsible for clustering at low temperatures.
Pulsed laser deposition of magnetic semiconductor EuS, EuSe, and EuTe thin films
Authors: Mulloy M. P. , Blau W. J. , Lunney J. G.
Published: July 2002

Thin films of the europium monochalcogenides, EuS, EuSe, and EuTe, have been produced using pulsed laser deposition on to heated quartz substrates. The films show a high degree of orientation of the (001) planes parallel to the substrate. In optical absorption, the shift of the band edge on cooling below their Curie/Néel temperatures is clearly seen.
HF- and NH4OH-treated (111)Si surfaces studied by spectroscopic ellipsometry
Authors: Utani Katsuyuki , Suzuki Takahiro , Adachi Sadao
Published: July 2002

Spectroscopic ellipsometry has been used to study HF- and NH4OH-treated (111)Si surfaces. The ellipsometric data indicate that aqueous HF etching results in the removal of the surface oxide and leaves behind Si surfaces terminated by atomic hydrogen. Chemical treatment by aqueous NH4OH solution provides a bare Si surface, but further etching of Si leads to roughening of the sample surfaces. Both the HF- and NH4OH-treated surfaces become hydrophobic as the surface is hydrogen-terminated (HF) or the surface oxide layer is etched completely away (NH4OH).
Real time optical diagnostics of the plume dynamics during laser ablation of germanium in an oxygen environment
Authors: Vega F. , Afonso C. N. , Solis J.
Published: July 2002

The dynamics of the gas phase induced by excimer laser ablation of Ge is investigated by analyzing the light emitted by the plume. Space and time-resolved optical spectroscopy measurements indicate the presence of both, neutral (Gei) and ionized (Geii) species. Two populations of neutrals with different velocities, which are related to the neutral atoms directly ejected from the target and those which are produced by recombination of ions, are observed. The velocities of the species remain unchanged for oxygen pressures up to 1 mbar, which suggest that the expansion of the plume occurs without further collisions with the foreign gas in this pressure range. The spectral emission characteristics are consistent with plume dynamics initiated by charged species.