bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of the electron beam evaporation rate of Pt and the semiconductor carrier density on the characteristics of Pt/n-GaAs Schottky contacts
Authors: Myburg G. , Auret F. D.
Published: July 2002

Schottky barrier diodes (SBDs) were fabricated on epitaxially grown n-GaAs materials, with different free carrier densities, by electron beam (e-beam) evaporation of Pt at various rates. The quality of the SBDs was evaluated by standard current-voltage (I-V) measurements, while the defects introduced during e-beam evaporation were characterized by deep level transient spectroscopy (DLTS). The results showed that if the GaAs was shielded during Pt deposition from stray electrons originating at the e-beam filament, high quality SBDs were formed. However, if the GaAs was not shielded during deposition, the quality of the diodes was poor and the degree to which their characteristics deviated from the ideal case increased as the total electron dose reaching the substrate increased (for slow evaporation rates) and as the free carrier density of the GaAs increased. DLTS revealed that several surface and subsurface defects were introduced during metallization without the electron shield and it is shown that these defects are responsible for the poor device quality. The nature of some of these defects depended on the free carrier density of the GaAs.
Dynamical study of polarization induced switching and bistability in a nonlinear prism coupler
Authors: Danckaert J. , Vitrant G.
Published: July 2002

Using a dynamic modal theory for nonlinear planar resonators, we study polarization induced switching and polarization bistability in a prism coupler with a diffusive third-order nonlinearity.
Microscopic surface structure of a globular molecule of ovalbumin in aqueous buffer solutions
Authors: Matsumoto Takayoshi , Inoue Hiroshi , Chiba Jiro
Published: July 2002

The surface roughness (graininess) of a globular molecule of ovalbumin in aqueous buffer solutions was investigated through data from small-angle x-ray scattering in the Porod region. The scattering intensity I changes according to I = K1q−4 or I = K1q−4 + K2. Here q is the wave vector, and K1 and K2 are constants. This means that the surface of the globular molecule does not have a fractal structure, but the electron density transition is relatively sharp at the interface. The specific inner surface Os of the native globular molecule is approximately 1.7 times that of a sphere with a smooth surface. The value of Os is almost independent of the ovalbumin concentration and pH values, but increases with heat denaturation of the system.
Efficiencies of single and graded gap solar cells at terrestrial solar spectra
Authors: Reuter Herbert , Schmitt Heinz
Published: July 2002

The maximum efficiencies of single and graded gap solar cells at 300 K are calculated by using a thermodynamic model. Direct and global AM1.5 and concentrated direct AM1.5 irradiance data sets are used for the calculations. The model is also applied on simulated global AM1.5 sunlight and on diluted blackbody radiation. The maximum efficiencies of single gap solar cells irradiated by natural and simulated global AM1.5 sunlight are 38% at Eg = 1.13 eV and 40% at Eg = 1.34 eV, respectively. The maximum efficiency for 1000-times concentrated natural direct AM1.5 irradiance is 42% at Eg = 0.96 eV. The maximum efficiencies are 34% at Eg =1.18 eV and 38% at Eg = 1.12 eV for 5800 K blackbody radiation of 102 and 105 mW/cm2 and 41% at Eg = 1.09 eV for undiluted 5800 K blackbody radiation of 6.4 × 106 mW/cm2. Graded gap solar cells can reach maximum efficiencies of up to 76% and 81% at natural and simulated global, 87% at 1000-times concentrated direct AM1.5 irradiance. The maximum efficiencies of graded gap solar cells for 5800 K blackbody radiation of 102, 105, and 6.4 × 106 mW/cm2 are 73%, 86%, and 93%, respectively.
Dependence of the cathodoluminescence of diamond films on deposition temperature
Authors: Shing Y. H. , Rich D. H. , Pool F. S.
Published: July 2002

The deposition temperature dependence of the cathodoluminescence (CL) of diamond thin films grown by microwave plasma-assisted chemical vapor deposition has been investigated. Depositions were made in the temperature range of 400 to 750 °C at a pressure of 10 Torr, with a gas mixture of 5% CH4 and 5% O2 in hydrogen. The intensity of the luminescent peak at 430 nm was used as a measure of diamond quality for the film. This peak was found to be a maximum above 600 °C. Examination of the intensities of CL emissions associated with nitrogen and silicon impurities at 530, 560, and 740 nm indicate incorporation of these impurities is more efficient at temperatures above 600 °C. Film quality was thus found to be an optimization of competing mechanisms, i.e., improvement of diamond quality as evidenced by the intensity of the 430 nm peak, with the apparent activation of impurities and vacancy defects at elevated temperatures.
Effects of Ca2+Zr4+ ion pairs on spectroscopic properties of Cr3+ multisites in Cr3+-doped Gd3Ga5O12 garnets
Authors: Brenier A. , Boulon G. , Pédrini C. , Madej C.
Published: July 2002

Some disorder exists in Gd3Ga5O12 (GGG) garnets because of a partial occupation of the octahedral sites by the Gd3+ ions. The luminescence of the Cr3+ ions introduced as dopant in the material is then affected. The situation is worse in (Ca,Zr)-substituted GGG due to a great deal of Ca-Zr defects introduced in large quantity in order to have a congruent composition. A simple model based on the local dilatation of the lattice around each defect is presented in order to describe the Cr3+ luminescence at any temperature. The influence of Ca-Zr ion pairs on the average Cr3+→Tm3+ energy transfer is also studied.
Nonequilibrium luminescence at the E+Δ gap in GaAs with Si-δ doping
Authors: Mestres N. , Cerdeira F. , Meseguer F. , Ruiz A. , Silveira J. P. , Briones F. , Ploog K.
Published: July 2002

We studied the light scattering spectra of three molecular beam epitaxy GaAs samples with Si-δ doping. A broad feature appears in these spectra which is similar to that attributed by other authors to resonant Raman scattering by electronic intersubband transitions. By studying the dependence of this emission on exciting laser photon energy we believe that this line is really produced by nonequilibrium luminescence at the E0+Δ0 gap.
Efficient pulsed microwave excitation of a high-pressure excimer discharge
Authors: Rousseau V. , Pasquiers S. , Boisse-Laporte C. , Callède G. , Leprince P. , Marec J. , Puech V.
Published: July 2002

We report on a microstrip line structure able to sustain plasmas up to five bars of a He/Xe/HCl mixture, with good stability and reproducibility and without preionization of the mixture. The microwave power coupling has been characterized and the incoherent ultraviolet-308 nm emission has been studied by means of time-resolved spectroscopy.
Comments on the steady state photocarrier grating technique to measure diffusion lengths
Authors: Prabhu S. , Narasimhan K. L. , Sharma D. K.
Published: July 2002

The steady state photocarrier grating technique has emerged as an important technique for measurement of the diffusion length in amorphous silicon. In this communication we show that morphological inhomogeneities lead to an overestimation of the magnitude of the diffusion length. The magnitude of this error cannot be easily estimated.
Reply to ‘‘Comments on the steady state photocarrier grating technique to measure diffusion lengths’’
Authors: Weiser K.
Published: July 2002

It is pointed out that the experimental pitfalls in carrying out steady state photocarrier grating measurements, mentioned by Prabhu et al. are apparently very rare since the diffusion lengths in a-Si:H determined by this method in many laboratories yield very similar results.
Comment on ‘‘Rectification in heavily doped p-type GaAs/AlAs heterojunctions’’ [J. Appl. Phys. 70, 1081 (1991)]
Authors: Zeeb E. , Ebeling K. J.
Published: July 2002

In his recent paper ‘‘Rectification in heavily doped p-type GaAs/AlAs heterojunctions’’ [J. Appl. Phys. 70, 1081 (1991)] Yoffe presented calculations of barrier heights and current-voltage characteristics. Unfortunately, he adopted a typographical error in his basic equations. In this comment we show that significant differences in barrier heights and currents across heterojunctions arise, using the corrected expression for the calculations.
A simplified and improved model of ideal and almost ideal silicon p-n junctions: The role of oxygen
Authors: Pellegrini Bruno
Published: July 2002

Properly gettered and annealed silicon p-n junctions are either well described by Shockley’s diffusion theory (the ideal case) or they have (the almost ideal case) a reverse voltage–current characteristic constituted by three parts: a voltage-independent contribution Ir, smaller than the saturation diffusion current, and by two contributions, one, igr, of generation–recombination and the other, ioh, ohmic, which are both due to the same defect centers characterized by equal activation energy and by a null charge. In a recent work we have shown that such experimental findings may be ascribed to four-state traps (FST) which contain a ion dipole and which may be empty of carriers (the ground state), or filled by an electron or a hole, or both (the stimulated state), and to different Frenkel–Poole effects for electron and hole in their field-assisted thermal emission from trap states. Here, firstly, the model is simplified and improved (i) by considering the most likely emission (capture) of a carrier from (into) the only states containing (empty of) it, (ii) by taking into account that the transition between the ground and the stimulated states is thermally activated and assisted by both the electric field and the tunnel effect, and (iii) by considering the effects on the current of the dispersions Dλ and Dθ of the length and direction, respectively, of the ion dipole and of that DVm of the energy barrier height crossed by the electron in the above mentioned tunnel transition.

In this way, the FST kinetic equations are greatly simplified, the whole model is improved and generalized and, at the same time, the physical meaning of the analytical results becomes more simple and straightforward. As a matter of fact we find that (a) Ir is due to the field-assisted tunnel effect in the transition between the ground and the stimulated states and it depends on Dλ, Dθ, and DVm, (b) igr and ioh derive from the Frenkel–Poole effect on the carrier emission from the stimulated state and the (ioh, practically) are independent of Dλ, Dθ, and DVm and (c) the activation energies of igr and ioh are equal and the value is given by the smallest of the energies required to bring each of the carriers from the ground state, through the stimulated state, into the respective band. Finally, the physical origin of the FSTs is found in groups of localized states with different positions and energy levels, due to the oxygen, whose donor levels, related to SiyOx clusters of a few hundred atoms of oxygen, are destroyed and generated at the annealing temperature of 650 and 800 °C, respectively, at which the ideal and almost ideal p-n junctions are obtained, respectively.
Miniband Bloch conduction in semiconductor superlattices
Authors: Lei X. L. , da Cunha Lima I. C.
Published: July 2002

Linear and nonlinear characteristics of superlattice perpendicular conduction are examined from a balance-equation theory recently developed for miniband transport [Lei, Horing, and Cui, Phys. Rev. Lett. 66, 3277 (1991)]. Analytical expressions are given for linear mobilities due to impurity and due to optic-phonon scatterings. Attention is focused on the temperature dependence of low-field miniband mobility and the behavior of drift velocity versus electric field, derived from the balance-equation theory, as well as from Esaki–Tsu and Boltzmann theories. Good qualitative agreements are obtained in comparison with experiments, but more accurate measurements are needed to appreciate the differences between these two types of theories.
Magnetic field enhanced performance of a copper hollow anode cathode laser
Authors: Zhang Z. , Perry N. D. , Tobin R. C.
Published: July 2002

The output beam power and small-signal unsaturated gain on the He-Cu 780.8 nm transition in a hollow slotted-anode cathode laser are increased significantly by the application of a longitudinal magnetic field and each shows a marked dependence on the field in the range 10 to 40 mT. The laser output power and the gain increase with the discharge current, while the optimum magnetic field (∼22 mT) results in a further significant increase both in output power and gain. These effects have their origin in the transverse deflection of the beam electrons by the magnetic field.
Recrystallization behavior of silicon implanted with iron
Authors: de Souza J. P. , Amaral L. , Fichtner P. F. P.
Published: July 2002

The solid phase epitaxial growth (SPEG) of amorphized Si layers implanted with Fe (1×1015 cm−2, 100 keV) was investigated in the temperature range from 500 to 550 °C using Rutherford backscattering spectrometry. The push-out of Fe atoms by the moving amorphous-crystalline (a-c) interface was observed during annealing, and enhancement of the recrystallization rate was induced by the presence of Fe. These results are discussed in terms of a model that assumes that Fe atoms are trapped in the amorphous layer and released when they are reached by the moving a-c interface during the SPEG process.
Interwell enhancement of the photoluminescence efficiency in GaAs/AlGaAs quantum wells
Authors: Jaraíz M. , Bailón L. A. , Barbolla J. J.
Published: July 2002

We have investigated the effects of carriers generated in quantum wells adjacent to a single 100-Å-thick GaAs quantum well. The AlGaAs barrier thicknesses were such (200 Å) that the different quantum wells were not electronically coupled. At certain excitation power densities we observe an increase in luminescence in the excitation spectrum of the 100 Å quantum well corresponding to transitions in the quantum wells of different thicknesses. We attribute these interwell effects to the partial saturation of mutual nonradiative centers in the barrier material. At high excitation power densities we observe previously unreported negative interwell transitions which we attribute to an internally generated electric field.
A reflection high-energy electron diffraction study of the Si(111) surface during gas source molecular beam epitaxy
Authors: Liu W. K. , Mokler S. M. , Ohtani N. , Zhang J. , Joyce B. A.
Published: July 2002

Reflection high-energy electron diffraction was used to monitor Si(111) growth during gas source molecular beam epitaxy using disilane (Si2H6). Depending on the substrate temperature, the growing Si(111) surface was found to exhibit hydrogenated δ(7×7), (1×1), (5×5), and (7×7) reconstructions. Within the substrate temperature range of 490 to 560 °C where growth proceeded two-dimensionally, clear and well-defined intensity oscillations could be observed in the [21̄0] azimuths. Since consecutive atomic layers on the Si(111) surface are nonequivalent, the oscillation periods were found to correspond to bilayer growth. In the two-dimensional (2D) growth regime, the oscillation frequencies, and hence the growth rates deduced, were found to increase with increasing substrate temperature and flow rate of Si2H6. At higher temperatures, there was a change from 2D layer-by-layer growth to step propagation and consequently, the intensity oscillations were weak or absent. At low temperatures (<400 °C) where no dissociative adsorption of Si2H6 occurred, intensity oscillations were not observed.
Implosion-driven hypervelocity accelerator
Authors: Matsuo H. , Fujiwara K. , Hiroe T. , Komada M. , Tomikawa H.
Published: July 2002

A hypervelocity accelerator has been developed utilizing the extreme condition produced by the collapse of the cylindrical imploding shock wave. The cylindrical shell of the explosive is detonated by the explosion of rows of copper wires bound around it. The wire explosion is caused by the application of an impulsive electric current. The energy concentrated at the implosion center is imparted to the projectile accelerating it. The launch velocity of 1.2 km/s has been attained for a 2 g launched weight with a 60 g explosive (PETN: pentaerythritoltetranitrate). A further increase of the launch velocity is expected by increasing either the explosive thickness or the height of the cylindrical shell within practical limit.
Monte Carlo simulation of electron transport in mercury cadmium telluride
Authors: Gelmont Boris , Lund Bjornar , Kim Ki-Sang , Jensen Geir U. , Shur Michael , Fjeldly Tor A.
Published: July 2002

We derive expressions for electron scattering rates in mercury cadmium telluride accounting for correct wave functions in narrow-band-gap materials. These scattering rates differ slightly from the rates obtained from standard expressions for wide-band-gap materials. The difference is related to spin-flip processes and has a relatively small effect on the transport properties. However, it is very important for spin-orientation phenomena. Monte Carlo simulations have been performed to investigate the mobility and steady state velocity-field characteristics of electron transport in mercury cadmium telluride with x=0.205 at 77 K. The simulations include scattering by polar optical phonons, ionized impurities, and alloy scattering. The Pauli exclusion principle as well as the dependence of the screening length on the distribution function have been accounted for. The simulations show that the screening length increases with increasing electric field with this dependence being the strongest for low carrier concentrations. For n = 5.4 × 1015 cm−3, the inclusion of the Pauli exclusion principle and varying screening length reduces the electron velocity by 5% to 10% depending on the electric field. Velocity-field curves are in good agreement with experimental data up to the electric field where the impact ionization becomes important. We have also calculated the dependence of low-field mobility on the electron concentration, which shows reasonable agreement with experimental data.
Correlation of surface topography and flux pinning in superconducting YBaCuO films
Authors: McElfresh M. , Miller T. G. , Schaefer D. M. , Reifenberger R. , Muenchausen R. E. , Hawley M. , Foltyn S. R. , Wu X. D.
Published: July 2002

The surface topography of YBa2Cu3O7 thin films prepared by pulsed laser deposition is studied by atomic force microscopy and scanning tunneling microscopy. The possible contributions from observed structural features to the critical current density Jc are discussed. It is shown that the contribution of the screw dislocations associated with some structures can be either repulsive or attractive, depending on the dominant pinning mechanism. At low magnetic fields (to about 0.25 kOe) these dislocations can account for the observed critical current density values (Jc≳1011 A/m2) while for larger fields the higher-density steplike growth structures can account for significant values of Jc. It is shown that the pinning strength associated with pinning at steps varies inversely with thickness.
Selective area, synchrotron radiation induced, delta doping of silicon
Authors: Rosenberg R. A. , Frigo S. P. , Lee Sunwoo , Dowben P. A.
Published: July 2002

We have used broadband synchrotron radiation to induce selective area surface doping of boron into silicon. The source of the boron was nido-decaborane (B10H14) adsorbed on Si(111) at 100 K. Irradiation caused decomposition of the adsorbed molecule which lead to an enhanced concentration of free boron in the irradiated area. Using Si 2p core level photoelectron spectroscopy, the surface chemical composition and Fermi level position in both the irradiated and unirradiated regions were determined. The downward movement of the Fermi level was greater in the irradiated region than in the unirradiated region, and greater for n-type than for p-type Si.
The role of dislocation-dislocation interactions in the relaxation of pseudomorphically strained semiconductors. II. Experiment−The high-temperature relaxation of ultrahigh-vacuum chemical-vapor-deposited SiGe thin films
Authors: Stiffler S. R. , Stanis C. L. , Goorsky M. , Chan K. K. , de Frésart E.
Published: July 2002

The thermal relaxation of SiGe films deposited by ultrahigh-vacuum chemical vapor deposition was studied by annealing the films for times up to 21/2 h at a temperature of 950 °C. Strain relaxation was determined by misfit dislocation density obtained by planar-view transmission electron microscopy and by double-crystal x-ray diffractometry. When the relaxation process requires relatively few dislocations (≲2 μm), the films relax to a remnant strain which is in agreement with previous experimental measurements; however, when higher densities of misfit dislocations were generated, substantially larger remnant strains were observed. This is interpreted as resulting from energetic interactions among the dislocations and analyzed in terms of the theory developed previously. It is found that the cutoff distance for dislocation interactions is substantially greater than the film thickness and a value of 1.4±0.5 μm is determined for 75–150-nm-thick films. Limited data from the literature also indicate a cutoff distance that is substantially in excess of the film thickness. In addition, as the annealing time is increased, a marked propensity for dislocation banding is observed, attesting to the mixed nature (attractive and repulsive) of the interactions.
Interface effects and elastic constants of Ag/Ni superlattices studied by Brillouin scattering
Authors: Carlotti G. , Fioretto D. , Socino G. , Rodmacq B. , Pelosin V.
Published: July 2002

The elastic properties of sputter-deposited Ag/Ni superlattices have been studied by means of the Brillouin light scattering technique. Measurements have been taken on both free-standing and glass-supported superlattices, the former deposited at 100 K, the latter at 300 K. The two effective elastic constants c11 and c44 have been determined on specimens 5 μm thick for different superlattice periods; c44 decreases by almost 40% as the bilayer periodicity decreases from 19 to 1.3 nm, while c11 shows no appreciable variation. Measurements performed on supported specimens, 0.5 μm thick, enabled us to get information also on the constant c33, which exhibits a monotonic increase with period. The elastic anomalies found can be related to the appreciable lattice distortion observed by x-ray diffraction experiments. Both Brillouin scattering and x-ray analysis have shown that the interface quality improves appreciably when the temperature of deposition is lowered from 300 to 100 K.
The growth kinetics of rf sputtered Ba2Si2TiO8 thin films
Authors: Li Yi , Chao Benjamin S. , Yamauchi Hisao
Published: July 2002

Radio-frequency sputtered barium titanium silicate (BST), Ba2Si2TiO8, thin films were grown on crystalline Si(100) substrates at substrate temperatures ranging from 750 to 955 °C and were characterized using x-ray diffraction, optical microscopy, and scanning electron microscopy. The result of x-ray diffraction analysis indicates that the BST films deposited at an optimum substrate temperature of 845 °C were strongly c-axis oriented. The corresponding film growth rate in the direction normal to the film surface and lateral grain growth rate were 1.95 nm/min and 0.77 μm/min, respectively, at the initial stage of deposition. The former decreased with sputtering time and the latter increased with grain size. The fast lateral grain growth rate indicates a strong interaction between the overgrown BST film and the Si substrate. The increase in lateral grain growth rate suggests a surface diffusion controlled nucleation and growth mechanism in the initial stage of the deposition, and a coalescence mechanism dominating in the later stage. The activation energy for lateral grain growth was 359±30 kJ/mol for 0.01 μm size grains, and decreased to 148±20 kJ/mol for 1 μm size grains, which is in good agreement with the proposed growth model.
Thermal effects in Hg-diffused long-wave infrared HgCdTe photodiodes
Authors: Reisinger A. R. , Weaver F. J. , Rader M. A. , Voelker J. J. , Caputi S. J. , Myers T. H. , Shanley J. F.
Published: July 2002

Experimental current–versus–bias-voltage data obtained on Hg-diffused, p-n homojunction, infrared photodiodes are critically examined against a theoretical model. A satisfactory agreement between measured and predicted data requires that the thermal impedance of the device be included into the model. Thermal calculations confirm the magnitude of the thermal impedance suggested by the model. Values of several thousand K/W are possible for front-side-illuminated detectors.
Analytic calculation of electron transmission probability for planar-doped potential barrier devices
Authors: Young Mary R. , Demas Nicholas G. , Ventrice Carl A. , Kanousis Demetrios P.
Published: July 2002

The electron barrier transmission probability, T, is calculated for planar-doped potential barrier devices. The calculations are based on the analytic expression derived by Christodoulides et al. obtained from the exact solution to the Schrödinger time-independent equation. The original Airy function solution was recast in Bessel function form for ease of computer evaluations. The barrier is assumed to be triangular with the forward slope S1, much larger than the reverse slope S2. The quantity T is calculated for values of electron energy, E, above and below the barrier peak, Um. It is found that T is a sensitive function of S1 for all values of E. The reverse slope, S2, is observed to have very little effect on T for values of (E−Um)≥10 meV, but have a large effect on T for values of (E−Um)≤0. The quantity T is observed to increase monotonically with increasing E, for practical values of device parameters. These results are in qualitative agreement with those found from the numerical solution of Chandra and Eastman. However, their results give values of T that deviate significantly from those obtained in the present work.
Large third-order optical nonlinearities of nickel-dithiolene-doped polymethylmethacrylate
Authors: Winter C. S. , Oliver S. N. , Rush J. D. , Hill C. A. S. , Underhill A. E.
Published: July 2002

The third-order optical nonlinearity χ(3) of two novel guest-host polymers has been measured. The polymers consisted of nickel dithiolene guest molecules in a polymethylmethacrylate host and were prepared as cast or spun films. The optical nonlinearity was measured by degenerate four-wave-mixing experiments at a wavelength of 1.06 μm using 100-ps laser pulses. χ(3) coefficients of up to 10−17 m2/V2 were observed, corresponding to nonlinear refractive index n2 of up to 10−8 cm2/kW with a damage threshold ≳1 GW/cm2.
Polishing of YBa2Cu3O7−y by He-ion etching
Authors: Inoue Noriyuki , Takahashi Yasuo , Sudo Takashi , Sakamoto Kageyoshi , Shima Toshio , Nishi Yoshitake
Published: July 2002

Clean polishing is performed by helium-ion etching on a surface of high-Tc YBa2Cu3O7−y. Based on a knock-on cascade model, the decrease of surface roughness is discussed. A rate process of etching is applied for the surface roughness of the high-Tc YBa2Cu3O7−y.
Cell thickness and surface pretilt angle measurements of a planar liquid-crystal cell with obliquely incident light
Authors: Ong Hiap Liew
Published: July 2002

Two simple and fast optical methods are presented for the cell thickness and the surface pretilt angle measurements of a planar-oriented liquid-crystal (LC) cell, in which the LC director always lies in a plane of a Cartesian coordinate system. The methods employ an obliquely incident light propagating in the same plane containing the liquid-crystal molecules. Simple analytic formulas for the angular-dependent optical transmission are obtained. The phase retardation between the ordinary and extraordinary waves is used to measure the cell thickness with the use of a compensator. Independent data can be obtained for the cell thickness measurement by varying the angle of incidence of the incident light. The phase retardation-extremum method is used to measure the pretilt angle. Approximate formulas relating the pretilt angle and the angle of incidence corresponding to the transmission minimum are also obtained for the pretilt angle measurement for vertically and parallel-oriented LC cells with a small pretilt angle. For the parallel cell, the approximate formula agrees with that obtained by Birecki and Kahn [The Physics and Chemistry of Liquid Crystal Devices, edited by G. J. Sprokel (Plenum, New York, 1980), p. 115]. The methods are applicable to a planar LC cell, including the planar parallel nematic LC, homeotropic nematic LC cell, obliquely oriented nematic LC cell, and the planar-oriented ferroelectric smectic LC cell.
Solute pairing in solution-hardened Cu-Ni, Cu-Pd binary, and Cu-Ni-Pd ternary fcc alloys
Authors: Wong Joe , Nixon W. E. , Mitchell J. W. , Laderman S. S.
Published: July 2002

The pairing of solute atoms in solution-hardened binary and ternary face-centered cubic (fcc) binary and ternary Cu alloys has been investigated with the EXAFS (extended x-ray-absorption fine structure) technique using synchrotron radiation. Two binary Cu alloys, one containing 6 at. % Ni and the other 6 at. % Pd and a ternary Cu alloy containing 3 at. % Ni and 3 at. % Pd alloy were studied. The solute concentration in each system was chosen below that (8.33 at. %) required for finding one solute-solute pair in the first coordination sphere in the fcc structure. Detailed simulations of the experimental EXAFS signal arising from the first coordination shell of the Ni and Pd solute atoms in these alloys give the following results: (i) In both binary and ternary alloys, Ni is coordinated by 12 Cu host atoms at a distance equal to sum of the Goldschmidt radii. There is little evidence for Ni-Ni pairing. (ii) On the other hand, Pd-Pd pairing is found in both the binary and ternary systems. In addition, chemical interaction with the Cu matrix is evident from the Pd-Cu separation of 2.60 Å which is ∼0.05 Å shorter than the sum of their Goldschmidt radii. (iii) Finally, there is no pairing of Ni-Pd solute atoms in the ternary alloy.
Scanning force microscopy observation of GaAs and AlGaAs surfaces grown by molecular beam epitaxy
Authors: Fatt Yoon Soon
Published: July 2002

Scanning force microscopy is used to analyze GaAs and AlGaAs surfaces grown using molecular beam epitaxy (MBE). This relatively new technique, which is an offshoot of scanning probe microscopy allows atomic resolution imaging of surface topography of conductors as well as nonconductors. The roughness of the GaAs and AlGaAs surfaces are analyzed as their surface reconstructions are varied from (2×4) to (4×2) patterns by deliberately reducing the As partial pressure during MBE growth. It is found that the average surface roughness increases dramatically by as much as 245 Å as the surface reconstruction of the GaAs layer approaches the Ga-stabilized (4×2) pattern. Also, there is evidence to indicate that the incorporation of Al into GaAs to form AlGaAs degrades the surface smoothness of the GaAs layer beyond that of its As-stabilized (2×4) pattern.
Numerical study of single-vapor-bubble dynamics in insulating liquids initiated by electrical current pulses
Authors: Oliveri S. , Kattan R. , Denat A.
Published: July 2002

In this paper we present a study of the dynamics (growth, collapse, and rebound) of a single bubble (typical radius 1–10 μm) as a function of various parameters: injected energy, hydrostatic pressure (for a wide range 0.1–12 MPa), and several liquid hydrocarbons of different physical properties. It appears that between two rebounds the dominant role is played by liquid inertia. However, rapid damping occurs, and we examine here the influence of both liquid viscosity and acoustic radiation on this mechanism, discarding heat and mass transfers. Two simple models are proposed: the first one, based on a Herring modified model, and the second one, based on the Rayleigh–Plesset equation. The discrepancy with experimental results suggests that further insight must be achieved, taking into account the neglected transfers between vapor and liquid.
Time-resolved reflectance studies of surface melting during laser-assisted deposition with a modulated laser source
Authors: Comita Paul B. , Price Peter E. , Kodas Toivo T.
Published: July 2002

Time-resolved reflectance measurements have been used to study the time evolution of the surface reflectance during deposition of gold from dimethylgold hexafluoroacetylacetonate with both modulated and unmodulated laser light. The observed change in reflectivity found during deposition with a modulated laser source is due to surface melting or sintering of the gold deposit. The change in reflectance observed upon undergoing a transition from a rough to a smooth surface is theoretically calculated for a surface with a random distribution of facet shapes and sizes. The observed changes in reflectance are consistent with the rough polycrystalline surface layer becoming smooth in a transient molten state. An impulsive temperature rise due to a rapid exothermic reaction taking place at the leading edge of the laser pulse has been estimated theoretically and is consistent with the observed time-dependent data.
Velocity modulation in GaAs/AlxGa1−xAs impact avalanche transit-time diodes
Authors: Kearney M. J. , Couch N. R. , Smith R. S. , Stephens J. S.
Published: July 2002

Velocity modulation using AlxGa1−xAs layers is, in principle, an attractive way of improving the efficiency of conventional GaAs impact avalanche transit-time (IMPATT) devices. Critical to the concept, however, is a detailed understanding of the nature of high-field electron transport in AlxGa1−xAs. We explore this via Monte Carlo simulations, with particular reference to the behavior found under nonuniform field conditions. Preliminary experimental results are presented for a simple, single-drift IMPATT diode in which the drift region is made entirely from Al0.3Ga0.7As.
Rapid thermal annealing of sol-gel derived lead zirconate titanate thin films
Authors: Chen J. , Udayakumar K. R. , Brooks K. G. , Cross L. E.
Published: July 2002

Sol-gel derived ferroelectric thin films of lead zirconate titanate have been annealed through the rapid thermal annealing (RTA) technique to investigate the effect of various annealing temperature-time combinations. Crystallization of the film into the perovskite phase required 10 s at 600 °C and a mere 1 s at 700 °C. Rapid thermally annealed films recorded weak-field permittivities greater than 1000, dissipation losses of 0.02–0.05, maximum remanent polarization of 29 μC/cm2, and coercive field around 40 kV/cm. RTA films are distinguished by superior breakdown strengths, and morphologically smoother surfaces. The frequency dependent dielectric constants have been discussed in terms of a lumped circuit model.
Multi-shot evolution of laser-induced periodic structure on the surface of polycrystalline silicon-on-insulator
Authors: Lee T. D. , Lee H. W. , Nam C. H. , Kim J. K. , Park C. O.
Published: July 2002

The multi-shot evolution of laser-induced periodic structure was studied by the time-resolved optical diffraction and reflection from the surface of polycrystalline silicon-on-insulator during the accumulation of 20 ns ruby laser pulses. In contrast to the results previously reported for the bulk sample, it was observed that the channel of the periodic structure development changed during the multiple irradiations even with a fixed fluence. The observed change occurs because the energy absorption is enhanced by the presence of the periodic structure preformed by the preceding pulses, and the absorbed laser energy at the polycrystalline layer is insulated by the underlying SiO2 layer. A previously unappreciated dip structure was observed in the time-resolved diffraction and a possible explanation is suggested postulating the preferential energy deposition in the valleys of the periodic structure.
A kinetic model for silicide formation through thin-film metal-silicon reactions
Authors: Zhang Lin , Ivey Douglas G.
Published: July 2002

A simple method, which uses numerical calculations to estimate diffusion fluxes into reactive interfaces during silicide formation processes, has been developed. Based on analysis of calculated diffusion fluxes from experiments from five different published papers, a model has been proposed to explain the unique kinetic behavior associated with thin-film metal-silicon diffusion couples. The reactive interface in a metal-silicon diffusion couple is considered to be a reaction region, and the reaction process is divided into three steps. Several physical quantities have been defined to describe each of these steps, i.e., the diffusion flux of moving reactant to the reaction region Jm, the release rate of nonmoving reactant rn, and the formation rate of the growing phase F. The relationship between these quantities has been demonstrated by means of a reaction process plot, which is also developed in this study.
Kinetics of light induced metastable defect creation in amorphous silicon: A dispersive excitonic model for the weak bond-dangling bond conversion
Authors: Dhariwal S. R. , Deoraj B. M.
Published: July 2002

The Stutzmann, Jackson, and Tsai model [Phys. Rev. B 32, 23 (1985)] for the creation of metastable defects in illuminated a-Si:H by tail-to-tail recombination of electrons and holes and conversion of weak bonding and antibonding orbitals into dangling bonds has been extended to include the exponential nature of tail states and saturation of dangling bond density. A first-order approximation of the general result gives a stretched exponential formula similar to that of Redfield and Bube [Appl. Phys. Lett. 54, 1037 (1989)] removing an apparent contradiction between the two models. The theory is also extended to include the thermal annealing process. This gives an apparent saturation of dangling bond density at a steady state equilibrium value Nm which tends to the actual saturation value Ns as the intensity of illumination is increased.
Method for determining the electrode overpotential on circular samples
Authors: Riess I.
Published: July 2002

Current-voltage relations on circular samples with four equally spaced semicircle electrodes are investigated both theoretically and experimentally. An expression is given for the dependence of the two-point resistance on the electrode size, sample radius, and thickness. Combining this with a four-point van der Pauw conductivity measurement enables the determination of the electrode overpotential. Experimental results of the I-V relations in a thin carbon layer are presented.
Preparation and characterization of Tl2Ba2CaCu2O8 thin films by laser ablation and thallium diffusion
Authors: Liu R. S. , Shih W. C. , Scott K. , Edwards P. P. , Phillips W. A. , Greer A. L.
Published: July 2002

We report here the growth of Tl2Ba2CaCu2O8 thin films by a combination of laser ablation and thallium diffusion on [001] MgO substrates. We find that the process configuration and sintering temperatures for thallium diffusion are of paramount importance in achieving single phase thin films. From x-ray diffraction analysis, the films have a highly preferred orientation with the c crystal axis perpendicular to the surface of the substrate. A typical film has a Tc(onset) = 120 K, Tc(midpoint) = 115 K, and Tc(zero) = 109 K. The critical current density for a film, estimated from hysteresis loop and the Bean Model, is greater than 105 A/cm2 at 5 K in zero applied field.
Fourier series method applied to different shaped quantum well laser potentials
Authors: Wahlstrand K. J. , Yee J. H.
Published: July 2002

Here we present a method using Fourier series representation of the potential to obtain the eigenvalues and eigenfunctions for a general isolated quantum well potential. We compare the method with analytical results in the case of a square well and get excellent agreement. Finally, we apply the method to two different graded-index, separate confinement heterostructure potentials of interest and compute the corresponding gain with k selection. We find the gain to be much narrower in energy range for multiple quantum wells as compared with a single quantum well.
The role of an ultrathin silicon interlayer at the SiO2-Ge interface
Authors: Hattangady S. V. , Mantini M. J. , Fountain G. G. , Rudder R. A. , Markunas R. J.
Published: July 2002

Recent studies [Hattangady et al., Appl. Phys. Lett. 57, 581 (1990)] have shown greatly reduced interface state densities (5×1010 cm−2 eV−1) in Ge-based, metal-insulator-semiconductor structures with the use of an ultrathin, pseudomorphic Si interlayer between the gate dielectric, SiO2, and the Ge semiconductor substrate. The Si and the SiO2 layers are deposited in situ and sequentially at low temperature (300 °C) in a remote-plasma-enhanced chemical-vapor-deposition system. This report presents an analysis of the Si-Ge heterostructure before and after the SiO2 deposition. Low-energy He ion scattering spectroscopy shows that the silicon layer (28 Å) provides complete coverage of the Ge surface prior to the deposition of the SiO2 film. The existence of the silicon interlayer after the remote-plasma-enhanced deposition of 150 Å of the SiO2 film is established by x-ray photoelectron spectroscopy (XPS). Throughout a cumulative series of thin (∼10 Å) oxide depositions, XPS showed no evidence of Ge oxidation states other than Ge0+ (elemental Ge) at the interface. Quantitative XPS has been used to evaluate the extent of subcutaneous oxidation which could determine the amount of Si remaining at the interface and thereby influence the electrical properties of the semiconductor-oxide interface. For the conditions studied, it is observed that oxidation consumes only 4 Å of the initial 28 Å of silicon.

Furthermore, this is apparently due to the plasma oxidation of the silicon at the initiation of the remote oxygen plasma discharge. Subcutaneous oxidation is limited thereafter by the oxide film that forms a barrier to oxygen diffusion. In addition, the XPS analysis reveals several important characteristics of SiO2-Si interface formation with these plasma-deposited SiO2 dielectric films. Inversion-mode, p-channel Ge field-effect transistors fabricated with this composite SiO2-Si gate dielectric structure show a maximum room-temperature transconductance of 52 mS mm−1 at a gate length of 2 μm and a peak effective channel hole mobility of 430 cm2 V−1 s−1. These devices exhibit negligible charge-induced threshold shifts.
The properties of resonant transmission in triple-barrier/double-quantum-well heterostructures
Authors: Xu D. X. , Shen G. D. , Willander M. , Hansson G. V.
Published: July 2002

To understand the nature of resonant tunneling in triple-barrier/double-quantum-well structures, we have investigated the properties of transmission probability TT of such structures with use of the transfer-matrix method. The transfer matrix of the middle barrier is calculated to analyze the amplitude decay, the phase shift, and the coupling effects that are induced between the two quantum wells. The dependencies of TT and the splitting of the resonant peaks on the middle-barrier thickness, the well width, and the symmetry of the structures are calculated. The origin of the transmission resonance and the resonant condition for symmetric (equal well widths) and asymmetric structures are discussed. The variations of TT with the applied voltage are also analyzed. The calculation results lead to some suggestions on how to design triple-barrier structures with improved performance.
Effects of magnetic field orientation on a liquid-metal free surface in a sliding electrical contact
Authors: Walker J. S. , Audet D. M. , Talmage G. , Brown S. H. , Sondergaard N. A.
Published: July 2002

This paper treats a free surface between a liquid metal and an inert gas in the presence of a magnetic field with arbitrary orientation relative to the free surface. The free surface intersects a perfectly conducting surface at rest and an insulated surface rotating about an axis which is perpendicular to both surfaces and which is far from the liquid-metal region. This problem models free surfaces in liquid-metal sliding electric contacts for motors and generators. There is a primary azimuthal liquid-metal velocity which is driven by the rotation of the insulated surface, and there is a secondary flow which involves radial and axial velocities and which is driven by the centrifugal force due to the primary velocity. The free-surface positions, pressures, and velocities are presented as functions of the magnetic-field orientation and strength.
Effect of annealing on carbon concentration in edge-defined film-fed grown polycrystalline silicon
Authors: Pivac B. , Amiotti M. , Borghesi A. , Sassella A. , Kalejs J.
Published: July 2002

Carbon complex formation during annealing in the temperature range from 450 to 1150 °C is studied in a carbon-rich polycrystalline edge-defined film-fed grown (EFG) silicon sheet. The result is compared with that obtained in Czochralski (CZ) single-crystal silicon. Infrared (IR) spectroscopy reveals that carbon removal from substitutional sites above 600 °C is greatly inhibited in EFG silicon with respect to that seen in the CZ material. A broad IR peak attributed to C-O complexes appears only after annealing at highest temperatures, while there is no evidence for appearance of the sharp band at 794 cm−1 usually assigned to SiC precipitation. The suppression of carbon complex formation in EFG material is attributed to decreased availability of silicon self-interstitials necessary to promote removal of carbon from substitutional sites and to enhance its diffusivity.
Tangentially phase matched second-harmonic generation in various crystals
Authors: Bhar G. C. , Datta P. K. , Das S.
Published: July 2002

Tangential phase matching is studied for second-harmonic generation. It occurs at any wavelength for certain values of noncollinearity in phase matching when it provides an increase in angular tolerance between the interacting beams. It is verified experimentally for barium borate and deuterated potassium dihydrogen phosphate crystals.
Admittance properties of thin/thick metal-oxide-semiconductor structures. I. Experiment and equivalent network
Authors: Fahrner W. R. , Bräunig D. , Ostrop P. , Ruhl S.
Published: July 2002

A metal-oxide-semiconductor structure with a stepped oxide under a common gate metallization is investigated. The G(V) and C(V) curves strongly deviate from those of wafers with uniform oxide thicknesses: Three sharp G(V) maxima and a well structured C(V) curve are seen. A general equivalent network has been established for the description of the behavior of the structure. By choosing special bias conditions, the network can be reduced to simpler configurations. In this case all the network elements are accessible from independent measurements (secondary electron microscopy, control samples), so that the C(V) curve can be reconstructed solely from geometrical and technological data for its essential portions. The network is exploited for a separate determination of the surface-state density underneath the thin and the thick oxide regions. This has been done for a series of thin/thick oxide combinations. A lateral field induced p-i junction is found underneath the thick/thin oxide boundary for appropriate biases, V. The G (ω;V=const.) and C(ω;V=const.) analysis delivers the (forward)conductance and capacitance of the junction.