bluuebunny/crossref_metadata_2025 huggingface dataset:
Study of indium antimonide metal-oxide-semiconductor structure prepared by direct photochemical-vapor deposition
Authors: Su Y. K. , Liaw U. H.
Published: July 2002

Silicon dioxide (SiO2) insulator layers on indium antimonide (InSb) have been prepared by direct photochemical-vapor deposition at low temperature below 200 °C using 2537 Å UV light. Ellipsometric studies prove that the refractive index and deposition rate of the photo-oxide films depend on the substrate temperature and gas ratio. The films evaluated by Auger electron spectroscopy (AES) depth profile showed that composition atoms were distributed uniformly throughout the oxide film. The AES analysis found the dominant components of the oxide film are silicon and oxygen. Fourier transform infrared spectroscopy absorption shows that the grown film has strong Si—O bonds with few Si—H bonds. The chemical x-ray photoelectron spectroscopy depth profile shows that the constituents of the semiconductors’ outdiffusion into the oxide are few. Metal-oxide-semiconductor (MOS) capacitors were constructed on InSb substrates. Capacitance voltage (C-V) characteristics of the MOS capacitors were measured at 77 K. The interface-state density is of the order of 1011 cm−2 eV−1, and distributed in a very good U shape within the midgap. C-V curves showed almost no hysteresis and smaller flatband voltage. The current-voltage curve shows the leakage current is about 1 nA at 0.8 V, and the breakdown voltage is about 0.8 MV/cm.
Effect of tension on R lines in ruby crystals shocked along crystal c axis
Authors: Gupta Y. M. , Horn P. D. , Burt J. A.
Published: July 2002

An experimental method has been developed to measure the wavelength shift of R lines in ruby samples subjected to well-defined, large tensile stresses. Plate impact experiments, conducted at 77 K, were used to produce uniaxial strain tension along the crystal c axis. Longitudinal stresses ranged from 105 kbar in tension to 110 kbar in compression. Both R lines remained sharp and showed a reversible, nonlinear blueshift in tension; the R1-R2 splitting increases with increasing tension. The observed data are in excellent agreement with predictions using crystal field theory [Sharma and Gupta, Phys. Rev. B 43, 879 (1991)]. The present results also permit an examination of the tensile response of sapphire at these large stresses.
Optical transition energies of GaInP quantum wells with GaInP/AlInP superlattice barriers
Authors: Seko Y. , Fukatsu S. , Shiraki Y. , Fuse M.
Published: July 2002

A systematic study on the optical transition energies of GaInP quantum wells with GaInP/AlInP superlattice barriers was carried out. It was found that the commonly used simple approach to regard the short-period superlattice barrier as a bulk barrier with a height of its miniband edge brought about a substantial error in calculating the quantized levels in these structures. Instead, it was revealed that an excellent agreement between theory and experiment can be obtained when the wave function of each layer in the superlattice barriers is exactly calculated, demonstrating the validity of the effective-mass approximation for the quantum-well structures with superlattice barriers.
Long-wavelength HgCdTe photodiodes: n+-on-p versus p-on-n structures
Authors: Rogalski A. , Ciupa R.
Published: July 2002

The performance of long-wavelength n+-on-p and p-on-n HgCdTe photodiodes is reexamined theoretically. It is assumed that the performance of photodiodes is due to thermal generation governed by the Auger mechanism. The influence of junction position on the R0A product, photoelectrical gain, and noise for both types of HgCdTe photodiodes operated at 77 K, with 0.1 eV base material, is considered. Especially, the R0A product as a function of cutoff wavelength and temperatures is analyzed in detail for both type of structures. For assumed doping concentrations in the base region of homojunctions (Na=5×1015 cm−3 for the n+-on-p structure and Nd=5×1014 cm−3 for the p-on-n structure), the influence of a p-type cap layer on the effective R0A product is more serious for p-on-n structures in comparison with influence of an n+-type layer on the R0A product for n+-on-p junctions. Therefore, to suppress the deleterious influence of cap layers (especially in the case of p-on-n junctions), a wider band-gap cap layer is necessary. For a given cutoff wavelength, the theoretical values of the R0A product for p-on-n photodiodes in the temperature range below 77 K are a little greater than for n+-on-p photodiodes, which is due to lower doping. In the higher temperature range for very long-wavelength photodiodes, p-type base devices are more advantageous. Results of calculations are compared with experimental data reported by other authors.
Field and high-temperature dependence of the long term charge loss in erasable programmable read only memories: Measurements and modeling
Authors: Herrmann M. , Schenk A.
Published: July 2002

High-temperature charge transport across an oxide-nitride-oxide sandwich of erasable programmable read only memories is mainly governed by the oxide conductivity as experimentally determined. It was verified in the examined devices that charge loss is not due to mobile ions. Since hole injection from the control gate into the nitride can be blocked by a 70-Å-thick top oxide we conjecture that charge loss is due to leakage of electrons; however, the observed leakage current is too large to be explained by pure electrode-limited charge transport (Richardson emission and direct tunneling). It was also verified that field gain on asperities and along edges cannot increase the charge loss current to the required range. Numerical evaluation of trap tunneling and resonant tunneling indicated that both mechanisms are weakly temperature dependent while charge loss has a typical activation energy of 1.2 eV in the range of 250–350 °C. Consequently, a multiphonon-assisted tunneling mechanism is proposed where electrons stored on the floating gate tunnel to oxide traps, then are emitted into the nitride. The coupling of the trap level to oxide phonons results in virtual energy levels in the oxide which allow for more effective transition paths. As a consequence of the electron-phonon coupling, the emission occurs close to the oxide conduction-band edge at temperatures between 250 and 350 °C, producing a strong temperature dependence for the mechanism.
The effects of ion species and target temperature on topography development on ion bombarded Si
Authors: Carter G. , Vishnyakov V. , Martynenko Yu. V. , Nobes M. J.
Published: July 2002

The production of periodic ripple or wavelike structure by Si+, Ne+, Ar+ and Xe+ high-fluence ion bombardment at 20 and 30 keV of Si at 45° incidence angle and at target temperatures from 120 K to room temperature and observed by scanning electron and atomic force microscopy is described. Different species exhibit different behaviors at different temperatures with Xe+ producing transverse wave structures at room temperature and below, Ar+ producing more patchy ripple structures, and Ne+ and Si+ only producing wave structures at low temperatures. Possible reasons for the different behavior include implant ion incorporation which mediates flow and stress relief in the amorphized near-surface layer.
Detection of AlO molecules produced by KrF laser-ablated Al atoms in oxygen gas and plasma environments
Authors: Ching C. H. , Gilgenbach R. M. , Lash J. S.
Published: July 2002

Experiments have been performed to measure, in real time, the formation of AlO molecules from laser-ablated Al atoms in oxygen gas and plasma environments. The Al atom plume is generated by focusing a KrF laser (4 J/cm2) on Al metal targets or polycrystalline Al2O3 (alumina) ceramic. AlO molecule formation has been characterized by emission spectroscopy at 464.82 and 484.22 nm molecular bandheads. Time-integrated and time-resolved optical emissions have been measured of laser-ablated Al atoms interacting with oxygen or argon neutral-gas versus plasma backgrounds generated by a high-voltage capacitive discharge. Results indicate that gas/plasma-phase reactions occur between laser-ablated Al atoms and oxygen. Optimal enhancement of AlO optical emission is measured in oxygen plasmas at about 200 mTorr fill pressure.
Native defects in low-temperature GaAs and the effect of hydrogenation
Authors: Pritchard R. E. , McQuaid S. A. , Hart L. , Newman R. C. , Mäkinen J. , von Bardeleben H. J. , Missous M.
Published: July 2002

A range of experimental techniques has been used to measure point defect concentrations in GaAs layers grown at low temperatures (250 °C) by molecular-beam epitaxy (LT-GaAs). The effects of doping on these concentrations has been investigated by studying samples containing shallow acceptors (Be) or shallow donors (Si) in concentrations of ∼1019 cm−3. Material grown under As-rich conditions and doped with Be was completely compensated and the simultaneous detection of As0Ga by near-band-edge infrared absorption and As+Ga by electron paramagnetic resonance confirmed that the Fermi level was near the midgap position and that compensation was partly related to AsGa defects. There was no evidence for the incorporation of VGa in this layer from positron annihilation measurements. For LT-GaAs grown under As-rich conditions and doped with Si, more than 80% of the donors were compensated and the detection of SiGa–VGa pairs by infrared localized vibrational mode (LVM) spectroscopy indicated that compensating VGa defects were at least partly responsible. The presence of vacancy defects was confirmed by positron annihilation measurements. Increasing the Si doping level suppressed the incorporation of AsGa. Exposure of the Be-doped layer to a radio-frequency hydrogen plasma, generated a LVM at 1997 cm−1 and it is proposed that this line is a stretch mode of a AsGa–H–VAs defect complex. For the Si-doped layer, two stretch modes at 1764 and 1773 cm−1 and a wag mode at 779 cm−1 relating to a H-defect complex were detected and we argue that the complex could be a passivated As antisite. The detection of characteristic hydrogen-native defect LVMs may provide a new method for the identification of intrinsic defects.
Effects of copper and oxygen precipitation during thermal oxidation of silicon: An electron-beam-induced current study
Authors: Correia A. , Ballutaud D. , Boutry-Forveille A. , Maurice J.-L.
Published: July 2002

The effects on minority carrier diffusion length Ln of the oxidation of p-type silicon in a copper-contaminated ambient have been analyzed using an electron-beam-induced current. The experiments were carried out on Czochralski (Cz) and float-zone silicon, and on samples with damaged and undamaged surfaces, in order to evaluate the role of oxygen supersaturation in the starting material, and the influence of the formation of oxidation-induced stacking faults on oxygen and copper precipitates during the oxidation anneal. The microstructure of the interface silicon underlayers was controlled using transmission electron microscopy and secondary ion mass spectrometry. The diffusion length Ln was drastically decreased in regions free of copper colonies, showing that a noticeable concentration of copper existed in the form of pointlike recombinant defects in the bulk. This effect was more pronounced in Cz silicon, where it was assigned to the presence of oxygen-based clusters acting as copper traps, and in the cases of damaged surfaces, where it indicated that the growth of the copper-related pointlike defects occurred with the emission of silicon self-interstitials.
Intermediate phases produced in growth of Bi2Sr2Ca2Cu3O10+δ
Authors: Xu Ming , Finnemore D. K. , Balachandran U. , Haldar Pradeep
Published: July 2002

High-temperature in situ x-ray measurements have been undertaken during the growth of Bi2Sr2Ca2Cu3O10+δ in order to determine the sequence of phases that form and the kinetics of growth. With a Ag-coated thin pellet of the starting mixture of Pb-doped Bi2Sr2Ca1Cu2O8+δ, Ca-cuprates, and CuO in an air atmosphere, major changes occur in the x-ray spectrum at temperatures as low as 790 °C. A triplet of lines associated with Ca2PbO4 forms as soon as the pellet reaches 790 °C and then disappears after 20 or 30 minutes. The Bi2Sr2Ca2Cu3O10+δ phase begins to grow at about 810 °C. For these rather porous pellets, about 20% of the sample transforms to Bi2Sr2Ca2Cu3O10+δ, as indicated by both x-ray intensities and superconducting magnetization studies.
Origin of compositional modulation of InGaAs in selective area metalorganic vapor phase epitaxy
Authors: Fujii Takuya , Ekawa Mitsuru
Published: July 2002

We investigated the in-plane compositional modulation of InGaAs epilayers in selective area metalorganic vapor phase epitaxy using a stagnant layer model. The growth pressure dependence of selective area growth of InAs and GaAs constituents using trimethylindium and triethylgallium revealed that the origin of the compositional modulation is in the mask region. A larger escaping probability of desorbed Ga source materials from the mask surface to the fluid layer and a slightly larger sticking coefficient defined at the mask surface of Ga source materials enrich the In composition of InGaAs epilayers near the mask edge. The escape probability is determined by the length ratio of the vapor phase mean-free path of source materials to the stagnant layer thickness. The finite sticking coefficient at the mask surface corresponds to a chemical reaction which produces nonreactive species. With the larger escape probability of Ga source materials and the introduction of finite sticking coefficients at the mask surface, we successfully quantitatively predicted the experimental results.
An analytical approximation for the free electron density of the Hg1−xCdxTe alloy system for 0<x<1
Authors: Goren D. , Asa G. , Nemirovsky Y.
Published: July 2002

An analytical approximation for the electron density in the conduction band of the entire Hg1−xCdxTe (MCT) alloy system (0<x<1) as a function of the composition, temperature and Fermi energy location, is proposed. A hyperbolic expression for the conduction band is shown to yield an error which is practically not larger than Kane’s model in the entire composition range of MCT. The analytical approximation is compared with a numeric calculation of the Fermi-Dirac integral using this hyperbolic band approximation, and shows a deviation of a few percents for temperatures in the range 2<T<300[K], compositions in the entire range 0<x<1 and electron densities up to n=1020[cm−3]. This analytical approximation can be extremely useful for numerical band diagram and transport simulations of graded and abrupt MCT heterojunctions and devices.
Contactless microwave study of dispersive transport in thin film CdSe
Authors: Grabtchak Serguei Yu , Cocivera Michael
Published: July 2002

The contactless microwave technique was used to measure light-induced transients in the power absorbed by thin films of polycrystalline CdSe. Because the rise time of the microwave cavity was 60 ns, the analysis was limited to 100 ns or longer. Measurement of these transients at a number of fixed frequencies across the ‘‘dark’’ resonance frequency made reconstruction of the difference signal possible. This signal, which represents the difference between the ‘‘dark’’ and ‘‘light’’ Lorentz resonance curves, was determined at various times during the decay. Analysis of these signals provided the time dependence for the changes in the real and imaginary parts of the dielectric constant, which correspond to the densities of the trapped and free electrons. The decays of these parameters were characterized by three time domains. At the shortest times, the two parameters did not have the same time dependence. At intermediate times, the densities of both the trapped and free electrons had the same time dependence characterized by a power law decay, and a mechanism consistent with these results involves rapid equilibration between the free electrons and those in the shallow traps. Decay in this region was consistent with a dispersive transport mechanism. Intensity effects indicate saturation of the shallow traps. The third region occurred at the break in the power law dependence indicating a bimolecular recombination process. Measurements at higher temperatures indicate a change from a bimolecular to a monomolecular recombination mechanism.
X-ray scattering and absorption studies of MnAs/GaAs heterostructures
Authors: Huang S. , Ming Z. H. , Soo Y. L. , Kao Y. H. , Tanaka M. , Munekata H.
Published: July 2002

Ferromagnetic MnAs thin films grown on GaAs (001) substrates by molecular-beam epitaxy have been studied by the methods of grazing incidence x-ray scattering, x-ray diffraction, and extended x-ray-absorption fine structure. Microstructures in two films prepared with different first-layer growth conditions (template effects) are compared in terms of the interfacial roughness in the layer structure, lattice constants, epilayer thickness, local environment surrounding the Mn atoms, coordination number, and local disorder. Our results indicate that the template effects can cause significant differences in the local structures and crystallinity of the MnAs epitaxial layers.
A magnetic, neutron diffraction, and Mössbauer spectral study of the Ce2Fe17−xAlx solid solutions
Authors: Mishra S. R. , Long Gary J. , Pringle O. A. , Middleton D. P. , Hu Z. , Yelon W. B. , Grandjean F. , Buschow K. H. J.
Published: July 2002

The magnetic properties of a series of Ce2Fe17−xAlx solid solutions with x equal to 0.00, 0.88, 2.06, 2.81, 3.98, 5.15, 6.08, 7.21, 8.20, 9.08, 9.84, and 10.62 have been studied by magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. The compounds crystallize in the rhombohedral Th2Zn17-type structure. Magnetization studies indicate that the Curie temperature increases uniformly from 238 K for Ce2Fe17 to 384 K for Ce2Fe14Al3 and then decreases at higher aluminum content. Powder neutron diffraction results, obtained at 295 K, indicate that aluminum avoids the 9d site for all x values and preferentially occupies the 18h site at low aluminum content. Aluminum shows a marked preference for the 6c site for x≳6. The room-temperature iron magnetic moments increase from x=0 to 2 and then decrease for x≳2. The Mössbauer spectra have been fit with a binomial distribution of the near-neighbor environments in terms of a maximum hyperfine field, Hmax, for an iron with zero aluminum near neighbors, and a decremental field, ΔH, per aluminum near neighbor. The compositional dependence of the decremental field indicates the influence of aluminum on the long-range magnetic ordering in the compound.

The compositional dependence of the weighted average maximum hyperfine fields and the weighted average isomer shifts in Ce2Fe17−xAlx may be understood in terms of a mixing of the 3d conduction band electrons with the 3p valence band electrons of aluminum, a mixing which is more extensive than that associated with silicon in the Ce2Fe17−xSix solid solutions. We conclude that this mixing has a larger influence on the magnetic properties of these solid solutions than does the presence of a short iron–iron bond or the expansion or contraction of the lattice parameters and unit cell volume.
Surface roughness in Cu(100)/[Co/Cu]n systems grown by ion-beam sputtering
Authors: Minvielle Timothy J. , White Robert L. , Wilson Robert J.
Published: July 2002

The development of surface roughness in Co/Cu systems was investigated through the use of in situ scanning tunneling microscopy. Multilayers and single layers of cobalt and copper were sequentially grown on a Cu(100) substrate. We show that the growth mode of cobalt on copper is quite different from that of copper on cobalt. We characterize these differences by looking at the lateral variations and obtaining a mean measure of island length. The cobalt tends to nucleate in small (<5 nm) islands. An overlayer of copper broadens this length scale while maintaining approximately the same peak-to-peak roughness of 5 monolayers. These growth mechanisms are shown to affect the way in which roughness propagates through multilayers. The impact of deposition temperature is also examined and seen to alter the degree of roughness in these samples. We conclude by discussing the implications for the magnetoresistance of these structures.
Sputter deposited c-oriented LiNbO3 thin films on SiO2
Authors: Tan S. , Gilbert T. , Hung C.-Y. , Schlesinger T. E. , Migliuolo M.
Published: July 2002

We present the development of a magnetron sputtering process for the fabrication of exclusively c-axis oriented LiNbO3 thin films on silicon substrates. The heterostructure consists of LiNbO3/Si3N4/SiO2/Si. The deposition of the Si3N4 film is key to obtaining exclusively c-oriented LiNbO3 thin films. The deposition conditions for the LiNbO3 are discussed.
Exploration of magnetization reversal and coercivity of epitaxial NiO {111}/NiFe films
Authors: Lai Chih-Huang , Matsuyama Hideo , White Robert L. , Anthony Thomas C. , Bush Gary G.
Published: July 2002

We have grown epitaxial NiO {111} films of thicknesses ranging from 60 to 1200 Å, deposited 45 Å NiFe films on these NiO substrates, and made measurements of exchange field and coercivity, of the effective uniaxial anisotropy, of rotational hysteresis, and of the training effect on these films. We find that the large coercive fields, ∼500 Oe, observed in these epitaxial systems can be understood using a model in which the magnetization reversal process is by rotation, with the coercive field determined by the effective uniaxial anisotropy of the system. This effective anisotropy is in turn determined by the anisotropy of the NiO and depends on NiO thickness.
Sensitivity enhancement of Co/Pt superlattices through underlayer composition modification
Authors: Hatwar T. K. , Brucker C. F.
Published: July 2002

Co/Pt superlattice films for magneto-optical recording have a relatively high Curie temperature compared to rare earth-transition metal films, which has led to criticism of their recording sensitivity at higher disk velocities. We have prepared high-performance Co/Pt disk recording media using (In2O3)100–x(SnO2)x (ITO) as the underlayer. We have observed a two-fold increase in recording sensitivity as x is increased from 0 to 75 mol % SnO2. We have correlated measured sensitivity behavior with composition-induced changes in the microstructure and electrical resistivity in the ITO underlayer. ITO deposited with low SnO2 content exhibits crystalline microstructure and low electrical resistivity, whereas ITO deposited with high SnO2 content exhibits amorphous microstructure and high electrical resistivity. The reduction in crystallinity with increasing SnO2 content is expected to decrease the ITO thermal conductivity, which can explain the increase in recording sensitivity. Small changes in Co/Pt coercivity as a function of x can easily be compensated by adjusting the ITO sputtering pressure. The optimum combination of Co/Pt sensitivity and coercivity can be readily achieved by controlling the ITO underlayer composition and sputtering pressure.
Backscattering of an electromagnetic missile by a metal cylinder of degree higher than two
Authors: Houdzoumis Vassilios A. , Wu Tai T. , Myers John M.
Published: July 2002

The backscattering of an electromagnetic missile from a perfectly conducting curved obstacle is investigated. The obstacle is assumed to have zero curvature just at the point of reflection of the incident pulse. The asymptotic dependence of the backscattered energy is sought, as the distance separating the obstacle from the source of the incident pulse tends to infinity. The backscattered energy is found to depend on the rate at which the energy spectral density of the current pulse at the source decays with increasing frequency, as well as on the degree of flatness of the obstacle at the point of reflection.
Interface roughness effects on the surface anisotropy in Co/Pt multilayer films
Authors: Kim Jin-Hong , Shin Sung-Chul
Published: July 2002

We have investigated the effects of interfacial roughness on the surface anisotropy in Co/Pt multilayer films prepared by sputtering. Degree of interfacial roughness was varied by changing the Ar sputtering gas pressure from 2 to 20 mTorr in the sample preparation. The surface anisotropy was found to be increasing with decreasing interface roughness. We have observed that the surface anisotropy energy for the multilayer films prepared at various Ar pressures was logarithmically dependent on the integrated low-angle x-ray intensity.
Elementary and macroscopic two-level fluctuations in high-T
c superconductors
Authors: Jung G. , Savo B.
Published: July 2002

The relation between random telegraph noise and 1/f-type background voltage fluctuations in current biased high-Tc films has been investigated. The experiments have revealed that the telegraph noise in high-Tc superconducting films is due to a combined action of a macroscopic two-level fluctuator and a detector. The macroscopic telegraph signal does not constitute an elementary contribution to 1/f noise. However, the behavior of telegraph and 1/f-like voltage fluctuations have shown astonishing similarities. Telegraph voltage amplitudes and 1/f power spectrum magnitude scale linearly with changing current flow above almost identical threshold currents. The telegraph signal symmetry characteristics have been found to be linear also but with distinctively different offset currents. Analysis of the measured characteristics indicates that the telegraph and 1/f fluctuations are due to different flux fluctuators coupled to observable voltages by the same detector mechanism.
Correlation between the gettering efficiencies and the energies of interfaces in silicon bicrystals
Authors: Ihlal A. , Rizk R. , Hardouin Duparc O. B. M.
Published: July 2002

A comparative study of the gettering efficiency of the twin grain boundaries Σ=25, Σ=13, and Σ=9 has been carried out by means of electron-beam-induced current measurements performed on quenched silicon bicrystals precontaminated by Cu or Ni. The extent of the denuded zone appearing on both sides of each interface type has been considered as the ‘‘rating’’ of its gettering efficiency. For both contaminants, the same scaling of the gettering efficiencies of the boundaries has been observed and was found to be in the order Σ=9≪Σ=13<Σ=25. To account for this ranking, we have correlated the gettering efficiency to the excess energy of the grain boundary with respect to the bulk energy, as theoretically calculated. The computational procedures have been performed by means of molecular-dynamics simulations using several potentials. On the basis of the specific disorder affecting the Σ=25 structure upon heat treatment, our calculations provided the same progression for the interfacial energies as that observed experimentally for the gettering efficiencies of the corresponding grain boundaries according to the extents of the appropriate denuded zones. Calculations of the strain fields developed by the three grain boundaries also support this view.
Imaging of the strain field around precipitate particles using transmission ion channeling
Authors: King P. J. C. , Breese M. B. H. , Meekeson D. , Smulders P. J. M. , Wilshaw P. R. , Grime G. W.
Published: July 2002

This paper shows ion channeling images of the strain field produced by precipitate particles in a crystal matrix. Images have been produced by mapping the energy of 3 MeV protons transmitted through a thinned silicon crystal containing colonies of copper silicide particles, with the incident beam at or close to planar channeling directions of the lattice. Features of the precipitate contrast observed as a function of beam tilt angle away from channeling alignment are qualitatively explained using a model based on symmetrical plane rotation of the crystal lattice around the colonies.
Deep level defects in GaAs on Si substrates grown by atomic hydrogen-assisted molecular beam epitaxy
Authors: Okada Yoshitaka , Harris James S. , Götz Werner
Published: July 2002

The electrical activity of defects in GaAs p+n diodes grown on Si and GaAs substrates by both conventional molecular beam epitaxy (MBE) and atomic hydrogen-assisted MBE (H-MBE) were characterized by deep level transient spectroscopy. The well-known electron traps typical of MBE-grown GaAs were detected without the presence of any new levels in the upper half of the band gap. The trap densities and diode reverse saturation currents are significantly reduced in the homoepitaxial GaAs grown by H-MBE compared to that grown by MBE. The trap densities for the heteroepitaxial GaAs-on-Si grown by H-MBE have values higher than those of homoepitaxial GaAs grown by H-MBE at 330 °C, which are possibly affected by the residual dislocation density and stress. The reduction of trap density is attributed to in situ passivation of these defects by atomic H during the growth.
Mechanisms of a remanent optical alignment of dyes in a gel matrix
Authors: Bentivegna Florian , Canva Michael , Brun Alain , Chaput Frédéric , Boilot Jean-Pierre
Published: July 2002

We present the mechanisms of a remanent alignment of polarizable molecules dispersed in a solid silica gel matrix, when submitted to powerful, femtosecond, linearly polarized, optical pulses emitted from a dye laser. This alignment can be related to third-order nonlinearities of the dopants through an orientational optical Kerr effect. Its remanence results from the strong interactions the dopants share with their host matrix, particularly through hydrogen bonds. We expose here the detailed mechanism of this phenomenon, considering that such a rotation of large chromophores may not be compatible at first sight with subpicosecond pulse durations. Through several experiments, we show that this apparent contradiction is solved once analyzed, by the structure of the excitation pulses, which cannot be simply described as ‘‘femtosecond pulses’’, but consist of two temporal components—a femtosecond one and a nanosecond one—playing different but essential roles in the process.
A nonendoreversible model for wind energy as a solar-driven heat engine
Authors: Barranco-Jiménez M. A. , Angulo-Brown F.
Published: July 2002

We study the Gordon and Zarmi model [Am. J. Phys. 57, 995 (1989)] for dealing with the earth’s wind energy as a solar-driven endoreversible heat engine under a nonendoreversible approach. We take into account the departure from an endoreversible regime through a parameter arising from the Clausius inequality. We use this approach for the Gordon–Zarmi maximum power criterion with two possible cold reservoirs: the 3 K surrounding universe and the 200 K isothermal tropopause shell. Our numerical results for the average extreme temperatures of the earth’s atmosphere and for the annual average energy of the earth’s winds are reasonable. We also use an ecological optimization criterion for the Gordon–Zarmi model. This criterion consists of an austere compromise between power and entropy production. With this model, we also obtain reasonable numerical results for extreme temperatures and the wind’s power.
Molecular beam epitaxy of InSb on Si substrates using fluoride buffer layers
Authors: Liu W. K. , Winesett J. , Ma Weiluan , Zhang Xuemei , Santos M. B. , Fang X. M. , McCann P. J.
Published: July 2002

The molecular beam epitaxy of InSb/Si structures was accomplished using group IIa fluoride buffer layers. InSb growth was initiated by opening the In and Sb shutters simultaneously at substrate temperatures between 300 °C and 400 °C, producing In-terminated InSb(111)-A surfaces on CaF2/Si(111) substrates. Reflection high-energy electron diffraction, electron channeling, and high resolution x-ray diffraction measurements indicated that the InSb layers were of good crystalline quality. Electron mobilities at room temperature were as high as 65 000 cm2/V s for an 8-μm-thick InSb layer grown on CaF2/Si(111). On CaF2/Si(001) substrates, the InSb layers grew in the (111) orientation with two domains 90° apart. These InSb layers and ones grown on BaF2/CaF2/Si(111) substrates exhibited inferior electrical and structural properties compared to structures grown on CaF2/Si(111) substrates.
Effects of native defects on carrier concentrations in heavily Si-doped and adjoining lightly doped GaAs layers
Authors: Fushimi Hiroshi , Shinohara Masanori , Wada Kazumi
Published: July 2002

Generation of native defects during growth of heavily Si-doped GaAs and their effects on carrier concentrations in heavily doped and adjoining lightly Si-doped GaAs layers are investigated. The mechanism of their intrusion from the heavily doped layer into the lightly doped layers is discussed. The behavior of the native defects during annealing after growth is also studied. As Si doping concentration increases, the concentration of triply ionized gallium vacancy (VGa3−), generated by a Frenkel-pair defect formation process, increases. The limit of free-carrier concentration in the heavily doped layers is caused by this VGa and not by electron occupation of a highly localized state of the donor-related DX center. VGa also causes carrier compensation in the adjoining underlayers. However, the carrier concentration in the adjoining overlayer grown on the heavily doped layer is not affected. We infer that drift is the predominant process for VGa flow into the lightly doped layers. This drift is caused by the electric field induced by a surface–Fermi-level pinning, mainly in an early growth stage of the heavily doped layer. On the other hand, the diffusion process of VGa from the heavily doped layer during growth is negligible. Therefore, the carrier concentration in the layers grown on the heavily doped layer is not affected. During annealing after growth the VGa, which is supersaturated in the lightly doped underlayer, disappears as the result of a first-order reaction, so the carrier concentration is recovered. These results not only suggest a carrier compensation mechanism, but are also useful in improving the characteristics of devices consisting of structures with heavily Si-doped GaAs layers.
An x-ray photoelectron spectroscopy investigation of O impurity chemistry in CdS thin films grown by chemical bath deposition
Authors: Niles David W. , Herdt Gregory , Al-Jassim Mowafak
Published: July 2002

We used x-ray photoelectron spectroscopy to investigate the chemistry of O impurity atoms in CdS thin films grown for photovoltaic purposes by chemical-bath deposition (CBD). We compared the Cd 3d photoline, O 1s photoline, Cd MNN Auger line, and O KLL Auger line taken from a CBD CdS thin film, CdS single-crystal reference, Cd metal reference, CdO reference, and Cd(OH)2 reference. This comparison showed that the O present in thin-film CBD CdS is a manifestation of H2O incorporated into the film during the CBD growth. Ar+ ion sputtering, a technique frequently used in thin-film analyses, preferentially removed S from the CBD CdS thin film and created CdS1−xOx (x∼0.04) in the surface region from the incorporated O impurity.
Determination of complex modulus by atomic force microscopy
Authors: Fretigny C. , Basire C. , Granier V.
Published: July 2002

The static friction regime of an atomic force microscope tip on a soft material is analyzed. A mechanical model shows that the experimental response is characteristic of the complex modulus of the sample. Moduli deduced from experiments on styrene-butadiene films compare favorably with macroscopically determined ones. Small discrepancies can be attributed to the imprecise knowledge of the mechanical properties of the cantilever. Relative measurements should, however, be accurately made. Spatial resolution depends on mechanical and adhesive properties of the material. For the experiments shown, the resolution is a few microns. Contrary to others, this method is little dependent on the geometrical properties of the tip end since in this regime the tip is deeply intruded in the material. It applies on thin films and heterogeneous materials and provides a new method for determining the mechanical properties at a local scale.
Organic light emitting devices based on vapor deposited films of end-capped sexithiophene: Evidence for Schottky barriers and transport limitations
Authors: Väterlein C. , Neureiter H. , Gebauer W. , Ziegler B. , Sokolowski M. , Bäuerle P. , Umbach E.
Published: July 2002

Light emitting devices with a sandwich structure: metal/EC6T/indium-tin-oxide, using end-capped sexithiophene (EC6T) as active organic material were fabricated by vapor deposition. Current and intensity of electroluminescence (EL) of the EC6T layers were measured as a function of voltage for various metals (Ca, Mg, Al, In, Ag) in a wide range of temperatures (4–300 K) and thicknesses of the EC6T layers (40–350 nm). External quantum efficiencies (10−6–10−3) and rectification ratios significantly depend on the metal contact which is compatible with a Schottky barrier. Electron injection from the metal at higher voltages correlates with the onset of significant EL. Current–voltage (I–V) curves exhibit a strong temperature and thickness dependence, mainly due to the charge transport across the EC6T layer. At low temperatures I–V curves show space charge limited currents. Modeling including double injection and trap states is performed. Results are discussed under the aspect of further device optimization.
Electron emission from lead–zirconate–titanate ceramics
Authors: Flechtner D. , Golkowski C. , Ivers J. D. , Kerslick G. S. , Nation J. A. , Schächter L.
Published: July 2002

We report extensive experimental data on electron emission from lead–zirconate–titanate ferroelectric ceramics. A 1–2 MV/m pulse is applied to a gridded ferroelectric cathode and diode currents of up to 120 A/cm2 are measured across an A–K gap of 5×10−2 m, with the anode at 35 kV. Both the current and the anode voltage pulse duration may extend to several microseconds. The measurements extend previously reported data by nearly two orders of magnitude in the diode voltage and by a factor of more than 3 in the diode spacing. Two major regimes of operation were identified. In the first ∼1 μs the ferroelectric cathode controls the electron flow through the diode. Beyond this time plasma effects dominate the current flow. The results are of importance to the development of novel cathodes for high current electron beam generation.
Molecular-beam epitaxy of gallium nitride on (0001) sapphire substrates using ammonia
Authors: Grandjean N. , Massies J. , Vennéguès P. , Leroux M. , Demangeot F. , Renucci M. , Frandon J.
Published: July 2002

Ammonia is used for growing undoped GaN layers by gas source molecular-beam epitaxy on c-plane sapphire substrates. The growth mode is layer by layer as shown by the observation of reflection high-energy electron diffraction intensity oscillations. The structural quality is studied by x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Low-temperature photoluminescence (PL) and reflectivity demonstrate intrinsic excitonic emission. Room-temperature PL exhibits a strong band-edge intensity and a weak deep-level emission, the so-called yellow band. Finally, secondary ion mass spectroscopy is carried out to check the residual impurity levels of Si, C, and O.
Physical properties of osmium doped tin oxide thin films
Authors: Rella R. , Siciliano P. , Vasanelli L. , Gerardi C. , Licciulli A.
Published: July 2002

The sol-gel process has been used to prepare osmium doped tin oxide thin films. The energy gap and refractive index were evaluated from optical measurements. Electrical characterization has been performed in a controlled atmosphere in order to investigate the influence of methane on the electrical conductance. New results have been obtained that make tin oxide doped with osmium a very interesting material for fabrication of low power consumption sensors for methane detection.
Correlation between emitted and polarization current in ferroelectric lead lanthanum zirconate titanate ceramics
Authors: Benedek G. , Boscolo I. , Moscatelli A. , Scurati A. , Handerek J.
Published: July 2002

The emission of electrons from lead lanthanum zirconate titanate ceramics, showing ferroelectric or/and antiferroelectric properties at room temperature, after excitation by a rectangular high voltage electric pulse, is explained by the switching and relax-back of the spontaneous polarization. The reported simultaneous measurements of emitted and polarization switching pulses show clearly the expected correlation. From the shape of the switching current it is possible to foresee if the sample can emit or not.
Output characteristics and optical efficiency of SrS:Ce and ZnS:Mn thin-film electroluminescent devices
Authors: Richter S. , Mauch R. H.
Published: July 2002

The output characteristics and the optical efficiency of SrS:Ce and ZnS:Mn thin-film electroluminescent devices are studied by measuring and evaluating light either directly emitted from the active area or indirectly emitted from its surroundings, including substrate edge. A special preparation of the devices allows access to edge emission and emission between pixel area and sample edge caused by scattering. The measuring method is optimized for registration of the entire output into the far field, exploiting the azimuthal symmetry of the pixel emission. In this study the optical efficiency is defined as the ratio of the directly emitted luminous flux to the total flux emitted from the segment within the sandwiched phosphor layer, which is activated. Optical efficiencies ranging from 0.16 for a smooth ZnS:Mn to 0.26 for a rougher SrS:Ce specimen are found. Theoretical limitations of the measuring method are discussed. A new quantity called scattering gain is introduced for characterizing the coupling of the output into the front hemisphere. Differential scattering gains ranging from a few percent to nearly 20% are observed. The optical characterization of SrS:Ce and ZnS:Mn samples also allows for an estimate of the optical efficiency of future inverted electroluminescent structures.
Simulations of charge transport across a grain boundary in n-silicon bicrystals
Authors: Joshi R. P. , Wood R. F.
Published: July 2002

Electron transport in Si low-angle bicrystals is analyzed by a novel Monte Carlo–molecular dynamics simulation scheme. The effect of discrete charges at the grain boundaries are studied and compared to results from one-dimensional treatments. The average grain boundary charge density strongly influences transport, and a field-dependent threshold effect is predicted. Details of the internal charge arrangement are shown to be relatively important at low fields and/or high grain boundary charge densities.
Ion mass influence on transient enhanced diffusion and boron clustering in silicon: Deviation from the “+1” model
Authors: Herner S. B. , Gossmann H.-J. , Pelaz L. P. , Gilmer G. H. , Jaraı́z M. , Jacobson D. C. , Eaglesham D. J.
Published: July 2002

Boron in silicon doping superlattices is used to trace native point defect behavior during a 790 °C, 15 min anneal following a 200 keV, 1×1013/cm2 Pb+ or 40 keV 1×1013/cm2 Si+ implant. These nonamorphizing implants lead to transient enhanced diffusion and clustering of the boron doping spikes. The enhancement in B diffusion scales sublinearly with mass of the ion implant. Clustering of the boron occurs deeper and more extensively in the Pb+-implanted sample due to greater mass of the ion. Measurement of the number of interstitials bound by extended defects after an 800 °C/10 s rapid thermal anneal confirm that the Pb+ implant has “+4.5” of the implant dose bound by extended defects, compared to “+0.6” in the Si+ implant for the same anneal. Both of these results indicate that the “+1” model is not valid for heavy mass ion implants.
Asymmetric nonlinear magneto-optic effects in PtMnSb thin films
Authors: Carey R. , Newman D. M. , Wears M. L.
Published: July 2002

Nonlinear magneto-optic measurements on thin films of PtMnSb exhibit asymmetric behavior in both the transverse and longitudinal Kerr configurations. Second harmonic generation under magnetization reversal is asymmetric about the state of zero magnetization with the degree of asymmetry dependent on the angle of incidence. The origin of the asymmetry arises from simultaneous detection of even (∝M2) and odd (∝M) magneto-optic effects. In principle such effects exist in both the optical linear and nonlinear regimes but in most materials even effects are very small and only observed in specific experimental arrangements where the linear effects in M vanish (such as the Voigt configuration). This is not the case with PtMnSb in which the band structure produces total polarization of the majority and minority spin carriers giving rise to unusually strong magneto-optic interactions.
Influence of the permanent magnet overhang on the performance of the brushless dc motor
Authors: Wang J. P. , Lieu D. K. , Lorimer W. L. , Hartman A.
Published: July 2002

Axial overhang of the permanent magnets has been used to enhance the performance of radial flux brushless dc motors, but its precise contribution to performance is not well known. This article aims at the investigation of the overhang effects by finite element and lumped parameter modeling. An empirical formula which allows two-dimensional analysis to account for overhang effects is proposed. A three-dimensional equivalent magnetic circuit model is developed and its ability to accurately predict overhang effects is assessed. Results of finite element and lumped parameter models are compared and a design methodology is forwarded.
Magnetic properties of NdFeB thin films synthesized via laser ablation processing
Authors: Yang ChoongJin , Kim SangWon , Kang Jong Seog
Published: July 2002

High energy product Nd2Fe14B films have been grown onto Si(100) substrate by KrF pulsed laser ablation using the targets of NdxFe90.98−xB9.02 (x=17.51–27.51). The films exhibit no preferred texture, however, good hard magnetic properties were produced from as-deposited condition: 4πMs≃7 kG, 4πMr≃4 kG, and Hc=300–1000 Oe. The beam density of 3 J/cm2 gave the optimal condition to have the highest 4πMr and Hc for a substrate T of 650 °C. The higher content of Nd induces a higher coercivity and 4πMr at the same time without prominent change in 4πMs. The films grown at elevated temperature (620–700 °C) are understood to suggest a magnetically coupled interaction between Nd2Fe14B and α-Fe grains by taking into account the monotonously decreasing 4πMr values accompanying with a broad peak regime in Hc at around 550–625 °C.
Improvement of CdTe solar cell performance with discharge control during film deposition by magnetron sputtering
Authors: Wendt R. , Fischer A. , Grecu D. , Compaan A. D.
Published: July 2002

We have used a combination of rf and dc excitation to control the plasma density during planar magnetron sputter deposition of CdTe films and CdTe-based solar cells. While adding dc current we have adjusted the rf power to maintain a constant deposition rate. We find that the lower plasma density obtained with negative dc currents yields films with a more highly faceted surface, more columnar growth structure, and stronger photoluminescence. The solar cells prepared with rf sputtered CdS and with CdTe deposited at reduced rf power and −70 mA of dc current produce substantially higher performance. The results indicate advantages to reducing the ion bombardment energy and flux to the growing film by the addition of dc control during rf sputtering from highly insulating semiconductor targets.
Temperature dependence of the resistivity and tunneling magnetoresistance of sputtered FeHf(Si)O cermet films
Authors: Strijkers G. J. , Swagten H. J. M. , Rulkens B. , Bitter R. H. J. N. , de Jonge W. J. M. , Bloemen P. J. H. , Schep K. M.
Published: July 2002

We have studied the tunneling resistivity and magnetoresistance of reactive sputter deposited FeHfO and FeHfSiO thin granular films. Maximum magnetoresistance ratios at room temperature of 2% and 3.2% were observed for films with compositions of Fe47Hf10O43 and Fe40Hf6Si6O48, respectively. The magnetoresistance shows a decrease with temperature, which cannot be explained by spin-dependent tunneling only. We propose that spin-flip scattering in the amorphous FeHf(Si)O matrix causes this decrease as function of temperature. A two current model for the tunnel magnetoresistance, taking into account spin-flip scattering, is presented which can describe the observed temperature dependence of the magnetoresistance.