bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of gas viscosity on the photoacoustic signal in gas-microphone cells
Authors: Korpiun P. , Büchner B. , Tam A. C. , Wong Y. H.
Published: July 2002

We report the experimentally observed variation of the phase angle φ of the photoacoustic (PA) signal as a function of coupling ‘‘gas column thickness’’ for a one-dimensional PA cell. The PA signal is excited by a modulated continuous laser beam incident on an opaque solid surface, and the gas column thickness is the distance between the solid surface and the window for the incident laser beam. We show that the observed data can be explained in a PA model (called the extended Rosencwaig–Gersho model) which includes the consideration of a residual volume in the PA cell, together with the consideration of viscous damping of the PA signal at small gas column thickness. The effect of viscosity is visible already for gas column thickness of the order of magnitude of one thermal diffusion length.
On the existence of submillimeter-wave negative conductance in n-gallium arsenide diodes
Authors: Krowne Clifford M. , Blakey Peter A.
Published: July 2002

Earlier analyses which have predicted submillimeter-wave negative conductance in short GaAs n+nn+ structures are reviewed, and found to be deficient with respect to transport model and boundary conditions employed. The critical sensitivity of the results to boundary conditions is demonstrated analytically. Large-signal results for a specific GaAs n+nn+ structure are then presented. These results, obtained using a self-consistent particle-field Monte Carlo simulation, suggest that the structure may display negative conductance at very low temperatures, but that a room-temperature lattice condition precludes such negative conductance.
Characterization of the free-carrier concentrations in doped β-SiC crystals by Raman scattering
Authors: Yugami H. , Nakashima S. , Mitsuishi A. , Uemoto A. , Shigeta M. , Furukawa K. , Suzuki A. , Nakajima S.
Published: July 2002

LO phonon-overdamped plasmon coupled modes in n-type epitaxial films of β-SiC have been measured in the carrier concentration range from 6.9×1016 to 2×1018 cm−3. The carrier concentrations and damping constants are determined by line-shape fitting of the coupled modes and compared with the values derived from Hall measurements. The concentrations obtained from the two methods agree fairly well. The Faust–Henry coefficient determined from the fitting is 0.35. The line-shape analysis of the coupled mode has shown that the dominant scattering mechanisms in β-SiC are deformation-potential and electro-optic mechanisms.
Carbon clustering in detonations
Authors: Shaw M. S. , Johnson J. D.
Published: July 2002

The clustering of carbon in the detonation regime is studied with the assumption of a diffusion-limited clustering process. The diffusion constants are determined from modified Enskog theory and the Stokes–Einstein relation. With the size dependence of the cluster energy treated with a surface term, the energy difference between clusters and bulk carbon has an asymptotic time dependence of t−1/3. That is, for any given time it takes 1000 times as long to release the next 90% of the carbon energy. This leads to a very slow ‘‘reaction rate’’ which can couple to the reaction zone to produce nonideal time-dependent detonations on the scale of microseconds and centimeters. In addition, any ‘‘bottleneck’’ in the clustering process due to a sticking coefficient of less than one leads to a temporary delay in energy release that persists out to times several orders of magnitude larger than the time scale of the initial effect of the ‘‘bottleneck.’’
Formation and properties of In-doped high-conductivity CdS film
Authors: Hayashi Toshiya , Nishikura Takehiro , Suzuki Tatsuro , Ema Yoshinori
Published: July 2002

High dark-conductivity CdS films have been prepared by coevaporation of CdS and In and the physical properties of the films were investigated. The dark conductivity of the films prepared at room ranged from 10−1 to 103 S cm−1. Analyzing the film structure by x-ray analysis, it was found that the In atoms were doped substitutionally into the Cd site of a CdS crystallite at a low concentration In doping stage and then doped interstitially into the CdS crystallite at a high concentration In doping stage. In the carrier density versus the doped In concentration relation, the n-type characteristic was found. This was explained by the two In doping processes described above. Further, the negative temperature dependence of the carrier density was detected in the very high-concentration In-doped samples. We used an explanation similar to the one given by Hung and Gliessman [Phys. Rev. 96, 1226 (1954)], that is, by using a tentative model in which the substitutionally doped low concentration In atoms form a shallow discrete donor level and the interstitially doped high-concentration In atoms form an impurity band in the forbidden band.
Quantitative acoustic emission source characterization of fatigue cracks in a thin-plate of 7075-T6 aluminum
Authors: Chen Chi-Ping , Sachse Wolfgang
Published: July 2002

Acoustic emission (AE) signals generated by microcrack formation in front of a propagating fatigue crack in a thin-plate compact tension specimen of 7075-T6 aluminum alloy were analyzed to investigate the dynamics of microcrack formation. The dominant wave motion was in-plane so that the dynamic elastic wave field could be analyzed by generalized plane stress theory. The acoustic signals were detected by an array of four miniature piezoelectric transducers which were mounted on the edge of a specimen ahead of the advancing crack. A second-rank force moment tensor was used to model the microcrack formation and a time-domain double-iterative signal deconvolution method was used to process the detected acoustic signals to remove the effects of the structure and to recover the temporal and spatial characteristics of the AE source. It was found that a force moment tensor model can be used to represent the growth of the microcrack.
Electronic and optical properties of Ti-doped GaAs and InP; semi-insulating InP
Authors: Brandt C. D. , Hennel A. M. , Bryskiewicz T. , Ko K. Y. , Pawlosicz L. M. , Gatos H. C.
Published: July 2002

A systematic study of the effects of Ti doping on the electrical and optical properties of GaAs and InP has been carried out employing both melt and solution-grown crystals. Utilizing deep level transient spectroscopy, Hall effect measurements, photoconductivity, and optical absorption measurements, it was found that Ti introduces two deep levels in GaAs at Ec −0.23 eV and Ec −1.00 eV which were identified as the Ti3+/Ti2+ acceptor level and the Ti4+/Ti3+ donor level, respectively. In InP the Ti4+/Ti3+ donor level was found near midgap at Ec −0.63 eV, while the Ti3+/Ti2+ acceptor level was found to lie within the conduction band. As a consequence of the midgap position of this donor level, we developed a formulation for producing semi-insulating InP based on doping with Ti to compensate shallow acceptors. Resistivities in excess of 107 Ω cm can easily be obtained using this technique. This is the first semi-insulating III-V compound having a compensation mechanism based on a deep donor impurity. In view of the fact that Ti is expected to have a very low diffusivity in InP, Ti-doped semi-insulating InP should exhibit far greater thermal stability than Fe-doped InP and thus it should prove technologically significant.
Optimization of the Čerenkov sum-frequency generation in proton-exchanged Mg:LiNbO3 channel waveguides
Authors: Sanford N. A. , Connors J. M.
Published: July 2002

Optical sum-frequency generation has been investigated in channel waveguides produced by proton exchange in Mg-doped as well as standard LiNbO3. Pyrophosphoric acid was used as an exchange media and Ta mask were patterned on z-cut plates by reactive-ion etching to form diffusion apertures ranging in width from 1 to 3 μm running parallel to the y axis. Samples were made by direct immersion in acid held at a temperature of 240 °C for times ranging from 10 to 28 min. End-fire coupling at 1.06 μm revealed that the Čerenkov radiation output at the second harmonic was optimized for the narrowest channel with a diffusion time of 24 min. A coupled throughput of 0.57 mW would produce a Čerenkov second-harmonic radiation mode due to the TM00 fundamental of 0.031 μW for a sample 4 mm in length. Diffusion times differing by 1 min around the optimum time would result in a reduction in harmonic generation efficiency by approximately a factor of 5. Samples pumped at 0.833 μm showed peak efficiency for a correspondingly shorter diffusion time. Severe drift in both fundamental throughput as well as harmonic generation is observed if pump throughput exceeds 2 mW in specimens formed in either Mg-doped or undoped LiNbO3. In addition, diffusion times for optimized nonlinear generation varied from batch to batch. The Čerenkov generation efficiency has been modeled using a step-index approximation for channel waveguides generalized to the z-cut y-propagating orientation used. The nonlinear source polarization at the sum frequency is considered to be due to only d33. Using values for the increase in extraordinary index estimated from fitting mode indices to the step-index approximation, we find optimum second-harmonic generation

due to the TM00 takes place for the mode close to cutoff at either 1.06 or or 0.833 μm. The magnitude of the peak SHG efficiency is found to agree reasonably well with experiment. The variation in guide depth over which the optimum SHG is observed is roughly 0.05 μm. The model predicts this should be larger by a factor of 3. We attribute the discrepancy to the inability of the channel waveguide model to accurately calculate the propagation and decay constants of the pump mode near cutoff. Our effective index measurements further indicate the dispersion in the increase of the extraordinary index (Δne) under the ion-exchange process is essentially equivalent to results published for guides made using benzoic acid when the exchange is performed at 249 °C.
The interband absorption spectra of sputter-deposited boron nitride analyzed using the coherent potential approximation
Authors: Aita Carolyn Rubin
Published: July 2002

The visible–near-ultraviolet optical absorption spectra for a family of sputter-deposited boron nitride films are reported here. The spectra of films that are nominally stoichiometric (B/N=1.2–1.3±20%) with short-range order equivalent to hexagonal BN were analyzed using the coherent potential approximation with Gaussian site disorder in the bands, as formulated by Abe and Toyozawa. The relative amount of interlayer BN bond randomness determined from optical measurements is consistent with infrared spectrometry results.
Silicon nitride thin films prepared by the electron cyclotron resonance plasma chemical vapor deposition method
Authors: Manabe Y. , Mitsuyu T.
Published: July 2002

Silicon nitride thin films prepared by the electron cyclotron resonance plasma chemical vapor deposition method at low substrate temperature (<60 °C) exhibited excellent physical and electrical properties. The refractive indices of the films were 1.9∼2.0 at a wavelength of 633 nm. The etch rate in a buffered HF solution was extremely low (0.5 nm/min). The resistivity and breakdown voltage were >1017 Ω⋅cm and >10 MV/cm, respectively. These properties of silicon nitride thin films are close to those of stoichiometric silicon nitride (Si3N4) prepared by the thermal chemical vapor deposition method at high temperature (>600 °C). The interface trap state density between the silicon nitride film and silicon substrate was 4×1011 cm2/eV in the silicon band gap. An optical emission spectroscopy during the deposition indicated that the intensities of nitrogen molecular ions were much stronger than those of nitrogen molecules, and the silane was sufficiently decomposed into silicon and hydrogen atoms. It is considered that the excellent properties of the silicon nitride thin films are related to the existence of a large amount of nitrogen molecule ions.
Crosshatched surface morphology in strained III-V semiconductor films
Authors: Chang Kevin H. , Gilbala Ronald , Srolovitz David J. , Bhattacharya Pallab K. , Mansfield John F.
Published: July 2002

The correlation between the surface crosshatched morphology and the interfacial misfit dislocations in strained III-V semiconductor heteroepitaxy has been studied. The surface pattern is clearly seen on samples grown at high temperature (520 °C) and those with small lattice-mismatched (f<2%) systems. A poorly defined crosshatched morphology was found on layers grown at relatively low temperature (400 °C). As the lattice mismatch of the strained layer becomes larger than 2%, a roughly textured surface morphology is commonly observed in place of actual cross-hatching. Few threading dislocations are observed in the strained layer when the crosshatched pattern develops. It is also noted that the surface crosshatched pattern develops after the majority of the interfacial misfit dislocations are generated. The result suggests that the surface crosshatch pattern is directly related to the generation of interfacial misfit dislocations through glide processes.
Direct measurements of compressive and tensile strain during shock breakout by use of subnanosecond x-ray diffraction
Authors: Wark J. S. , Riley D. , Woolsey N. C. , Keihn G. , Whitlock R. R.
Published: July 2002

Shock waves of order 100 kbar were launched into 50-μm-thick single crystals of silicon (111) by irradiation with nanosecond pulses of 1.05-μm laser light at irradiances in the region of 2×1010 W cm−2. A separate laser beam, synchronous but delayed with respect to the shock-driving beam, and containing approximately 25 J of 0.53-μm laser light in a pulse of 1 ns (FWHM), was focused to a tight (<100 μm) spot on a separate titanium target to produce a plasma which was a prolific source of He-like Ti x rays. The x rays were Bragg diffracted from the rear surface of the shocked crystal and the spectrum recorded on an x-ray streak camera. Changes in interatomic spacings in a region within several microns of the surface were thus deduced from the resultant shift in Bragg angle with a temporal resolution of 50 ps. Shock waves with compressions of order 6% were observed. We observed the crystal in a state of dynamic tension as the two rarefaction waves met. The results are in good agreement with hydrocode simulations in conjunction with x-ray diffraction calculations.
Elimination of the Fabry–Perot effect in the 4×4 matrix method for inhomogeneous uniaxial media
Authors: Yang K. H.
Published: July 2002

Based on the 4×4 matrix method for inhomogeneous uniaxial media by Berreman [J. Opt. Soc. Am. 62, 502 (1972); 63, 1374 (1973)] and by Wöhler et al. [J. Opt. Soc. Am. A 5, 1554 (1988)], a simple method to eliminate the Fabry–Perot effect caused by optical discontinuities at the input and output interfaces has been developed. It facilitates an easy comparison between the results of the Jones matrix method and the 4×4 matrix method without the time-consuming spectral average for the latter. Modeling the case of liquid-crystal displays with incoherent back-light sources, this new method will either improve the accuracy with similar computation time or save computation time with similar accuracy, as compared to the ordinary ‘‘faster’’ 4×4 matrix method.
Fabrication and characterization of highly pure and homogeneous YBa2Cu3O7 superconductors from sol-gel derived powders
Authors: Kakihana Masato , Börjesson Lars , Eriksson Sten , Svedlindh Peter
Published: July 2002

The high Tc superconducting oxide YBa2Cu3O7−d has been fabricated by a simple colloidal sol-gel precursor technique based on citrate complexes uniformly dispersed into ethyleneglycol and water. The present method permits easier fabrication of highly pure and homogeneous superconducting materials when compared with other more elaborate preparation procedures based on solution techniques as it eliminates many steps such as centrifugation, filtration, and pH control. The purity of the final product was estimated to be >99.5% based on the combined results of x-ray diffraction and Raman scattering analyses. The composition of the sample was determined to be Y1.00Ba1.98Cu3.05O6.85 by inductively coupled plasma spectroscopy and idiometry. The sol-gel process produced homogeneous materials with particle sizes smaller than ∼3 μm. The superconducting transition determined with resistivity measurements was shown to be sharp with a transition width narrower than 1 K. The magnetic susceptibility measurements exhibited sharp superconducting transitions without any indication of two-phase character or impurity phases. The material fabricated by this method showed improved properties compared to those prepared by the usual dry powder-mix (so-called solid-state reaction) technique.
Misfit dislocations and critical thickness of heteroepitaxy
Authors: Hu S. M.
Published: July 2002

An expression is derived for the critical thickness of heteroepitaxial films by taking the energy minimization approach. It is exactly identical to the critical thickness obtained by Matthew and Blakeslee [J. Cryst. Growth 27, 118 (1974)] using a force balance approach, after allowing for a discrepancy judged to be an unmentioned approximation in the latter. The equivalence of the two formulations is explained conceptually. The reason is pointed out for the very different critical thickness obtained by People and Bean [Appl. Phys. Lett. 47, 322 (1985); 49, 229 (1986)] using the energy minimization approach. It is further shown that misfit dislocations do not appear en masse catastrophically when the critical thickness is reached. Rather, their density increases gradually with the epitaxial thickness, approaching only asymptotically a value required for a complete relief of film stress as the thickness tends to infinity. Any observed sudden mass appearance of misfit dislocations must be attributed to a substantial supercritical thickness actually existing, prior to nucleation of dislocations.
Influence of IIA metals (Mg, Ca, Sr, Ba) on the Co modification of γ-Fe2O3 particles
Authors: Fukumoto Yuji , Matsumoto Kazuyoshi , Matsui Yasushi
Published: July 2002

In the reaction of Co modification of γ-Fe2O3, we found a very interesting phenomenon. When cations of Ca2+, Sr2+, or Ba2+ belonging to IIA metals are adsorbed on the surface of γ-Fe2O3 prior to Co2+ ion adsorption, the coercivity of Co modified γ-Fe2O3 is signifcantly increased compared to γ-Fe2O3 modified only with Co2+. In contrast, when the Mg2+ ion is adsorbed prior to Co2+ ion adsorption, the coercivity is rarely increased. In this paper, we made the following hypotheses in order to interpret these phenomena: (1) Co2+ ions are adsorbed both on the tetrahedral (A) site and the octahedral (B) site in the spinel structure of γ-Fe2O3. When Co2+ ion occupies the B site and forms cobalt ferrite, the coercivity increases as a result of the surface magnetic anisotropy. (2) Because cations of alkaline earth metals (Ca,Sr,Ba) are easily adsorbed on the A site, adding alkaline earth metal before cobalt addition results in an increase in adsorption selectivity of Co2+ ion on the B site. (3) On the other hand, Mg2+ ion which is easily adsorbed on the B site prevents Co2+ ion occupancy of the B site and as a result the coercivity does not increase.
‘‘A transmission electron microscope study of twin structure in GaAs/GaAs (111)B grown via molecular-beam epitaxy’’
Authors: Rajkumar K. C. , Chen P. , Madhukar A.
Published: July 2002

A transmission electron microscope study of the twin structure in thin GaAs films grown by molecular-beam epitaxy on (111)B GaAs reveals the twinned region to be composed of three distinct crystallites which are single- and double-twin variations of the bulk crystal. A three-dimensional structure of the twin region involving variants of these crystallites, as required by the threefold symmetry of the (111)B plane, is proposed.
Vapor deposition processes for amorphous carbon films with sp3 fractions approaching diamond
Authors: Cuomo Jerome J. , Pappas David L. , Bruley John , Doyle James P. , Saenger Katherine L.
Published: July 2002

Trends in recently reported data on high sp3 fraction (up to 85%), nonhydrogenated amorphous diamond-like carbon films deposited by ion beam sputtering and laser vaporization are examined. The degree of diamondlike film character is found to depend upon the deposition technique as well as the substrate temperature and thermal diffusivity. The data suggest that the combination of incident particle kinetic energy and surface accommodation determine the physical properties of the resultant film. A model is proposed for the condensation of energetic carbon atoms into diamondlike films in which a quench-type surface accommodation mechanism is operative.
Strained quantum well valence-band structure and optimal parameters for AlGaAs-InGaAs-AlGaAs p-channel field-effect transistors
Authors: Laikhtman B. , Kiehl R. A. , Frank D. J.
Published: July 2002

Using the model of an infinite well we have performed detailed calculations of the valence-band structure and for the first time obtained analytic expressions for wave functions in a strained quantum well. In-plane effective masses and energy separations are calculated for different thicknesses of InGaAs wells and In mole fractions in the range of 0 to 0.50. Based on the calculations we estimate the optimal thickness of the well and In mole fraction for which the energy separation between the lowest two subbands has a maximum and the InGaAs layer is stable with respect to misfit dislocations. The results provide useful guidelines for the optimization of strained p-channel field-effect transistors.
Eddy-current probe impedance due to a volumetric flaw
Authors: Bowler J. R. , Jenkins S. A. , Sabbagh L. D. , Sabbagh H. A.
Published: July 2002

Eddy current induced in a metal by a coil carrying an alternating current may be perturbed by the presence of any macroscopic defects in the material, such as cracks, surface indentations, or inclusions. In eddy-current nondestructive evaluation, defects are commonly sensed by a change of the coil impedance resulting from perturbations in the electromagnetic field. This paper describes theoretical predictions of eddy-current probe responses for surface cracks with finite opening. The theory expresses the electromagnetic field scattered by a three-dimensional flaw as a volume integral with a dyadic kernel. Probe signals are found by first solving an integral equation for the field at the flaw. The field equation is approximated by a discrete form using the moment method and a numerical solution found using conjugate gradients. The change in probe impedance due to a flaw is calculated from the flaw field. Predictions of the theory are compared with experimental impedances due to eddy-current interaction with a rectangular surface breaking slot. Good agreement is found between predictions and the measurements.
Analysis of a magnetron electronegative rf discharge
Authors: Meyyappan M.
Published: July 2002

An analysis based on numerical solutions to the three moments of the Boltzmann equation is presented for magnetron discharges used in plasma processing of electronic materials. A chlorine discharge at 13.56 MHz is studied and the results are compared for discharges with and without the magnetic field. The charged particle densities increase by a factor of 5 and the ionization rate increases by an order of magnitude with the application of a 100 G magnetic field in a 50 mTorr discharge. The sheaths in the magnetron discharge are thin and the sheath electric field decreases with increasing magnetic field. The bulk ohmic heating is the dominant mechanism for power deposition under the conditions investigated here.
Tantalum nitride films as resistors on chemical vapor deposited diamond substrates
Authors: Katz A. , Pearton S. J. , Nakahara S. , Baiocchi F. A. , Lane E. , Kovalchick J.
Published: July 2002

Tantalum nitride films were reactive sputter deposited onto chemical vapor deposited (CVD)-diamond self-standing thick layers, to be used as resistors for microelectronic applications. The TaN films had excellent morphology and were very stable through heating cycles at temperatures up to 400 °C for a few hours. Post-deposition sintering of the films at temperatures up to 300 °C stabilized the film resistance at values in the range of 75–85 Ω. The deposited film was later patterned with photoresist and dry etched, at rates of up to 70 nm min−1 and the resulting features served as masks for further self-aligned etching processes of the underlying CVD-diamond layer.
Increases in photovoltage of ‘‘indium tin oxide/silicon oxide/mat-textured n-silicon’’ junction solar cells by silicon preoxidation and annealing processes
Authors: Kobayashi H. , Kogetsu Y. , Ishida T. , Nakato Y.
Published: July 2002

Indium-tin-oxide (ITO)/silicon oxide/mat-textured n-Si junction solar cells having an energy conversion efficiency of 15% are fabricated by the spray pyrolysis method. Their characteristics and the junction properties are compared with the same junction solar cells having a flat Si surface. In cases where the ITO film is deposited on a hydrofluoric acid-etched mat-textured Si surface, the open circuit photovoltage (Voc) is low (405 mV). Scanning electron microscopy observation shows that high-density dislocations are formed near the Si surface, and the temperature dependence of the current-voltage characteristics suggests that the trap-assisted multistep tunneling through the Si depletion layer is a dominant current flow mechanism. In cases where the ITO film is deposited on a thermal silicon oxide-covered mat-textured Si surface, the formation of the dislocations is suppressed, and consequently Voc is increased to 485 mV. For this solar cell, a surface recombination current takes the dominant part of the dark current in the bias region below ∼250 mV, and a thermionic-assisted tunneling current is dominant in the higher bias region. For a cell where the thermal silicon oxide-covered mat-textured Si surface is annealed at 800 °C under nitrogen before the deposition of the ITO film, Voc is further increased to 540 mV, and the energy conversion efficiency of 15% is achieved. In this case, the thermionic-assisted tunneling current density is decreased by an increase in the barrier height due probably to a reduction in the density of the positive charge in the silicon oxide layer. The surface recombination current density is also reduced by the removal of interface states, leading to the improvement of the fill factor.
Power and efficiency limits for internal combustion engines via methods of finite-time thermodynamics
Authors: Orlov Vladimir N. , Berry R. Stephen
Published: July 2002

Analytical expressions for the upper bounds of power and efficiency of an internal combustion engine are obtained taking into account finite rate of heat exchange with the environment and nonzero entropy production due to chemical reactions. Recommendations of theoretically possible ways of improving internal combustion engines are presented.
Mass-selected ion angular impact energy distributions at the powered electrode in CF4 reactive-ion etching
Authors: Janes Joachim
Published: July 2002

The ion energy distributions and ion angular distributions of CF+, CF+2, and CF+3 currents were measured at the powered electrode of a capacitively coupled 13.56 MHz discharge parallel-plate reactor running on CF4. The pressure range is varied between 5 and 50 mTorr, and by adjusting the rf power dissipated in the plasma the dc bias potential at the cathode is varied between −150 and −450 V. Ions are sampled through a small orifice in an aluminum wafer bonded to the cathode. Downstream the orifice ions are selected in a quadrupole mass spectrometer equipped with an energy filter. Tilting this detection system with the vertex lying in the orifice ion angular distributions are measured. The ion energy distributions were found to exhibit structures which appear as continua. The structures result from collision processes of CF+x (x=1–3) with the CF4 parent gas in the sheath. Charge-exchange collisions could be identified neither in the ion energy distributions nor in the ion angular distributions. The ion angular distributions of CF+3 at the powered electrode are characterized by angular widths between 3° and 4° for the entire energy range. Angular distributions of CF+2 have angular widths of about 5° for high-energy ions while low-energy ions show angular widths of about 15°. Similar observations are made for CF+ ions with angular widths of about 6° for high-energy ions and about 16° for low-energy ions. In general, experiments indicate a slight increase in angular width for increasing CF4 pressure. The existence of continua in the energy distributions and the broad angular distributions are interpreted as being caused by the combined effect of elastic scattering and dissociative collisions in the sheath. Additionally, for increasing CF4 pressure multiple scattering becomes important giving rise to further broadening of impact angles and intensifying low-energy continua. Some consequences for the interpretation of profile developments in submicrometer dry etching are discussed.
Effect of scattering centers on the optical loss of As2S3 glass fibers in the infrared
Authors: Sanghera J. S. , Busse L. E. , Aggarwal I. D.
Published: July 2002

Scattering theory has been used to determine the effect of various defects on the scattering loss of As2S3 glass fibers. The scattering is related to the particle shape, size, relative refractive index, and wavelength of light. Bubbles possess the highest relative refractive index and therefore the highest scattering factor. For a given particle size and wavelength, the scattering decreases in the order bubbles≳carbon≳SiO2≳Al2O3≳As2S3. The calculations have been used to predict the maximum tolerable number of defects to obtain low attenuation equal to 10 dB/km at 5.0 μm.
Investigation of carrier transport through silicon wafers by photocurrent measurements
Authors: Bousse Luc , Mostarshed Shahriar , Hafeman Dean , Sartore Marco , Adami Manuela , Nicolini Claudio
Published: July 2002

Measurement of the ac photocurrent in metal/insulator/semiconductor capacitors can be used as a tool to measure minority-carrier diffusion and lifetime. The amplitude of the ac photocurrent generated at a silicon surface biased into inversion depends on the number of excess minority carriers present at that surface. By comparing this amplitude when intensity-modulated light is directed to each side of the same device, minority-carrier diffusion from the back to the front of the device can be characterized. An analytical model of this transport process predicts the dependence of the ac photocurrent on frequency and wafer thickness, and allows the determination of a value of the bulk lifetime free of the influence of surface recombination. Measurements under low-light intensity levels are presented on n-type silicon wafers with lifetimes in the 10–100 μs range. Lifetimes are found about a factor of 2 lower than those measured with noncontact photoconductive decay, at high-light intensity levels. This is expected due to the difference between high- and low-level minority-carrier injection. Fitting the data to the model also yields a value of 115 μm for the average depth at which carriers are generated and diffuse to the front with backside illumination at 940 nm.
Lattice instability in β-SiC and simulation of brittle fracture
Authors: Tang Meijie , Yip Sidney
Published: July 2002

Brittle fracture of β-SiC (polytype 3C) under hydrostatic tension has been modeled by molecular dynamics simulation using an interatomic potential function that treats the solid as fully covalent. The critical stress at which the lattice becomes structurally unstable is shown to agree quantitatively with that predicted by stability analysis based on elastic stiffness coefficients. The instability mode is the spinodal (vanishing of bulk modulus), and decohesion occurs as spontaneous nucleation of cracking on {111} shuffle planes. Atomic relaxation on the newly generated cracked surfaces appears to take place immediately following crack opening.
Subband structure and ionized impurity scattering of the two dimensional electron gas in δ-doped field effect transistor
Authors: Fu Y. , Willander M.
Published: July 2002

We have investigated theoretically the electronic subband structures and the scattering probability of electrons by ionized impurities in δ-doped field-effect transistors (δ-FETs). The self-consistent calculation of the Poisson and Schrödinger equations shows that the electrons are quite extended, even though the impurities are very much confined (δ doped). It is shown that the impurity scattering probability decreases if the carriers and impurities are well separated spatially, or if the kinetic energies of the carriers are large. For both the singly and doubly δ-FET, the averaged kinetic energies of the carriers are increased when the sample temperature is increased; the carriers are pushed away from the δ-doped impurity layers when the gate bias is increased. The combination of the two effects result in an enhanced electron mobility, as demonstrated by experiments. Our detailed numerical calculation thus provides us with some basic guidelines to improve the performance of δ-FETs: Multiple δ-doped layers and a high sheet density are desirable.
Propagation of surface acoustic waves across the randomly rough surface of an anisotropic elastic medium
Authors: Shchegrov A. V.
Published: July 2002

The propagation of surface acoustic waves across the randomly rough surface of an anisotropic solid is studied. In the small roughness limit the dispersion relation for such waves is derived. The frequency dependences of the dispersion and attenuation of surface acoustic waves are obtained for the case when the wavelength λ is long compared to the transverse correlation length a of the roughness. In this limit the numerical curves for the relative change of the phase velocity of surface acoustic waves versus the angle of propagation are calculated for the various cuts of anisotropic media. Crystals of cubic, tetragonal, trigonal, hexagonal, and orthorhombic symmetry, adopted in applications, are considered. The effects of piezoelectricity are taken into account through the approach of ‘‘partially stiffened elastic constants.’’
Fluorinated amorphous carbon thin films grown by plasma enhanced chemical vapor deposition for low dielectric constant interlayer dielectrics
Authors: Endo Kazuhiko , Tatsumi Toru
Published: July 2002

Fluorinated amorphous carbon films were proposed as low dielectric constant interlayer dielectrics for ultralarge scale integration circuits. The films were deposited by plasma enhanced chemical vapor deposition with CH4 and CF4 in a parallel plate rf (13.56 MHz) reactor. The dielectric constant of the amorphous carbon films deposited with CH4 was increased with increases in rf power. The addition of CF4 to CH4 raised the deposition rate and reduced the dielectric constant. At an rf power of 200 W, and at a flow rate of 47 sccm for CF4 and 3 sccm for CH4, the dielectric constant of the fluorinated amorphous carbon films was 2.1.
Line-of-sight measurements of the radiation-enhanced sublimation of graphite
Authors: Franzen P. , Davis J. W. , Haasz A. A.
Published: July 2002

The radiation-enhanced sublimation of different types of graphite was measured during H+ or D+ irradiation, by line-of-sight quadrupole mass spectroscopy; the quadrupole mass spectrometer was modified to block the transmission of reflected beam particles. The radiation-enhanced sublimation yield depends on both the type and orientation of the graphite; the highest yields (∼0.2 C/D at 1800 K for 1 keV D+) are associated with the most dense and most ordered graphite (pyrolytic graphite, HPG99). The yield at 400 K temperature is of the order of 10−3 C/D for 1 keV D+, more than one order of magnitude lower than the physical sputtering yield. The measured radiation-enhanced sublimation yields are in good agreement with model calculations.
Atomic force microscopy imaging of viscoelastic properties in toughened polypropylene resins
Authors: Nysten Bernard , Legras Roger , Costa Jean-Louis
Published: July 2002

The bulk morphology of two toughened polypropylene/(ethylene propylene)copolymer resins (PP/EP) presenting different impact resistances has been studied by means of different atomic force microscopy techniques: contact atomic force microscopy, lateral force microscopy (LFM), and force modulation microscopy (FMM). The three techniques reveal two different morphologies as observed in transmission electronic microscopy. In LFM, a higher friction force is observed on the rubbery phase which has the lower Young’s modulus confirming the relationship between friction force and elastic properties. In force modulation, the elastic moduli is found to be much lower on the EP nodules in both resins. FMM also reveals that the difference of viscous response between the PP matrix and the EP nodules is much lower in the resin which is less impact resistant.
Magnetic and crystal phase transitions in KNiCl3
Authors: Petrenko O. A. , Collins M. F. , Stager C. V. , Collier B. F. , Tun Z.
Published: July 2002

Neutron scattering measurements revealed that even a seemingly perfect crystal of KNiCl3 at room temperature may segregate into two phases with different lattice distortions in the basal ab plane at temperatures below T≊270 K. In phase A, distortion leads to an increase of the unit cell size by a factor of 3, giving rise to nuclear Bragg peaks of type (h/3,h/3,l). In phase B the distortion gives rise to Bragg peaks at (3h/4,0,l), with integer h and l. Two different magnetic structures have been observed with TN=12.5 and 8.6 K in phases A and B, respectively. Magnetization measurements confirm the values of TN found by neutron scattering for the two magnetic structures.
Magnetization reversal in patterned Co(0001) ultrathin films with perpendicular magnetic anisotropy
Authors: Bardou N. , Bartenlian B. , Chappert C. , Mégy R. , Veillet P. , Renard J. P. , Rousseaux F. , Ravet M. F. , Jamet J. P. , Meyer P.
Published: July 2002

Using x-ray lithography we have patterned dot arrays in Au/Co/Au(111) sandwiches based on ultrathin Co layers with perpendicular anisotropy. Large area arrays of dots with diameters of 1 and 2 μm have been obtained, keeping mostly undamaged the ultrathin Co layer. Hysteresis loops of the arrays depend drastically on the dot diameter. Magneto-optical domain visualization experiments confirm a magnetization reversal mechanism based on a large distribution of nucleation fields in the film, with complete reversal of the magnetization of one dot through the domain wall propagation after a local nucleation process. This could give information on the magnetization reversal processes in Au/Co/Au(111) continuous films.
Effects of Pr, Tb, and Zn doping into YBa2Cu3O7 on magnetoresistivity and magnetic phase boundaries
Authors: Freibert F. , Cao G. , McCall S. , Shepard M. , Crow J. E.
Published: July 2002

Transverse AB-plane magnetoresistivity ρAB (C axis ∥H) of (Y1−XPrX)Ba2Cu3O7−δ and YBa2(Cu1−XZnX)3O7−δ thin films and (Y1−XTbX)Ba2Cu3O7−δ thin films and single crystals has been measured as a function of 0.0<X<XC, 2 K<T<300 K, and 0 T<H<20 T. Mean-field upper critical field HC2 and irreversibility field HI phase boundaries have been determined and are discussed in terms of proposed theories. Tb doping of YBa2Cu3O7 reflects an enhanced pinning or ‘‘stiffening’’ of the vortex system without apparent alteration to TC or the slope of the mean-field normal phase-mixed phase boundary line, dHC2/dT‖T=Tc. However, Pr and Zn doping of YBa2Cu3O7 tends to depress both TC and dHC2/dT‖T=Tc and shows a ‘‘softening’’ of the vortex system. This behavior may be understood in terms of the magnitude of the superconducting pair coherence length, which is controlled by doping, and the divergence of the vortex-glass correlation length as it relates to flux line coupling of vortices in the CuO2 planes. These systems are compared with other high TC superconducting systems which possess similar phase boundary properties.
Preparation and thermoelectric properties of the skutterudite-related phase Ru0.5Pd0.5Sb3
Authors: Caillat T. , Kulleck J. , Borshchevsky A. , Fleurial J.-P.
Published: July 2002

A new skutterudite phase Ru0.5Pd0.5Sb3 was prepared. This new phase adds to a large number of already known materials with the skutterudite structure which have shown good potential for thermoelectric applications. Single phase, polycrystalline samples were prepared and characterized by x-ray analysis, electron probe microanalysis, density, sound velocity, thermal-expansion coefficient, and differential thermal analysis measurements. Ru0.5Pd0.5Sb3 has a cubic lattice, space group Im3 (T5h), with a=9.298 Å and decomposes at about 920 K. The Seebeck coefficient, the electrical resistivity, the Hall effect, and the thermal conductivity were measured on hot-pressed samples over a wide range of temperatures. Preliminary results show that Ru0.5Pd0.5Sb3 behaves as a heavily doped semiconductor with an estimated band gap of about 0.6 eV. The lattice thermal conductivity of Ru0.5Pd0.5Sb3 is substantially lower than that of the binary isostructural compounds CoSb3 and IrSb3. The unusually low thermal conductivity might be explained by additional hole and charge transfer phonon scattering in this material. The potential of this material for thermoelectric applications is discussed.
Residual stresses and microstructure in tungsten thin films analyzed by x-ray diffraction-evolution under ion irradiation
Authors: Durand N. , Badawi K. F. , Goudeau Ph.
Published: July 2002

Microstructure and residual stresses have been studied in 100 nm tungsten thin films deposited by ion beam sputtering on silicon substrates. Residual stresses, stress-free lattice parameter, crystal microdistortions, and average length of the coherently diffracting domains have been deduced from x-ray diffraction measurements. The as-deposited film is strongly compressed (−5.2 GPa) and its microstructure is very far from the bulk tungsten one: the coherently diffracting domain size is nanometric (about 5 nm), the stress-free lattice parameter is larger than the bulk one (about 0.6%), and microdistortions are considerable (0.6%). The ‘‘atomic peening’’ model is proposed to explain the mechanical state of these films. Diffraction analysis, correlated with impurity concentration measurement, evidences the main role played by backscattered Ar ions in stress genesis. Nevertheless, the contribution of the most energetic W particles to the stress generation process cannot be neglected. We have equally studied Ar+ ion (340 KeV) irradiation effects. We have found that irradiation induces a total stress relaxation, a return of the stress-free lattice parameter to the bulk one, a strong decrease of the microdistortions, and an increase of the coherently diffracting domain sizes. A thermal irradiation effect seems appropriate to explain residual stresses and microstructure modifications induced by ion irradiation. These features are in agreement with the interpretation proposed in the case of as-deposited films.
Electron paramagnetic resonance of porous silicon: Observation and identification of conduction-band electrons
Authors: Young C. F. , Poindexter E. H. , Gerardi G. J.
Published: July 2002

New features in electron paramagnetic resonance (EPR) of porous silicon have been examined here. A new isotropic EPR center was observed at g=1.9995(1) at T=4.2 K, in both p-type and n-type porous silicon. By comparing its g value with those of shallow donors in bulk silicon, the center was identified due to the conduction-band (CB) electrons in silicon microcrystals. The CB signal, present in freshly prepared p-type and n-type samples, can be dramatically and surprisingly enhanced by the presence of a polar solvent on the n-type porous silicon surface. Even though it was shown that most of the donor electrons in an n-type sample can be pulled into the porous layer from the substrate by solvent exposure of the porous layer, the possible electrochemical effects are not yet completely understood; to establish a reasonable model for them would require appropriately controlled experiments.
Hall mobility minimum of temperature dependence in polycrystalline silicon
Authors: Nussbaumer H. , Baumgartner F. P. , Willeke G. , Bucher E.
Published: July 2002

Molten zone recrystallized as well as sheet grown polycrystalline silicon has shown a minimum in the temperature dependence of the Hall mobility. In order to explain this experimental finding a new model is proposed, which is based on negatively charged grain boundaries for the p-type silicon material under study. This results in a potential well at the grain boundaries instead of the more generally observed potential barrier. A key feature in the model is that the space charge density at the grain boundary depends on the Fermi level position and therefore on temperature. In addition, the change in the measured Hall mobility before and after hydrogen passivation of the grain boundaries is discussed.
Dielectric relaxation in Ba0.96Bi0.04Ti0.96Fe0.04O3
Authors: Kumar M. Mahesh , Suresh M. B. , Suryanarayana S. V. , Kumar G. S. , Bhimasankaram T.
Published: July 2002

Samples of Ba0.96Bi0.04Ti0.96Fe0.04O3 were prepared by a solid state reaction method. Data on the dielectric constant as a function of temperature and frequency were obtained in the temperature range 35–180 °C at 10 and 100 kHz. The material shows a diffuse phase transition. With increasing frequency, the transition temperature shifts towards lower temperature, resembling that of a relaxor ferroelectric with a negative shift in Tc. Impedance analysis confirms the relaxor behavior.
Low temperature solid phase crystallization of amorphous silicon at 380 °C
Authors: Yoon Soo Young , Oh Jae Young , Kim Chae Ok , Jang Jin
Published: July 2002

Amorphous silicon (a-Si) was crystallized by metal induced crystallization (MIC) in an electric field. A 2 nm Ni layer on a-Si was used for the crystallization. The MIC temperature can be reduced to 380 °C in an electric field of 360 V/cm. The Ni-MIC poly-Si has a crystalline volume fraction of 92% with an average grain size of ∼150 nm. The fact that the crystallization temperature can be reduced appears to be due to the enhancement of NiSi2 migration through a-Si in an electric field.
Thermoelectric properties of the skutterudite Co1−xFexSb3 system
Authors: Katsuyama S. , Shichijo Y. , Ito M. , Majima K. , Nagai H.
Published: July 2002

We have examined the phase equilibrium and thermoelectric properties of Co1−xFexSb3 ternary system up to high iron context x=0.40. Traces of Sb were observed in the hot-pressed samples with x⩾0.06, and FeSb2 (Fe0.73Co0.27Sb2) compound with marcasite structure was also observed in the samples with x⩾0.25 by x-ray diffraction. The lattice parameter of Co1−xFexSb3 is slightly larger than that of the binary compound CoSb3. The Seebeck coefficient and the electrical resistivity are generally reduced by the substitution for Co by Fe. The thermal conductivity is also reduced by the substitution especially at high iron content region. These behaviors of the thermoelectric properties in the samples with low iron content are ascribed to the substituted Fe, while those in the samples with high iron content are ascribed to the precipitated FeSb2 (Fe0.73Co0.27Sb2) compound. For x⩽0.04, the figure of merit for Co1−xFexSb3 decreases with increasing x. However, above x=0.06 the figure of merit increases with x and as a result, Co0.75Fe0.25Sb3 has a maximum value of figure of merit of 4.8×10−4 K−1 at 773 K. This value is much larger than the maximum value for CoSb3 3.2×10−4 K−1 at 482 K.