ALBERT

Notes: This paper describes the application of an adaptive learning system comprising an associative memory to the characterization of ultrasonic phenomena. The mapping of source and waveform data and vice versa can be performed by utilizing the off-diagonal cross-correlation portions of the associative memory. It is suggested that a more general description of the ultrasonic phenomena can be obtained if the diagonal, autocorrelation portions of the memory are also utilized. In this case, the memory is applicable for the autoassociative optimal filtering of experimental data. Experiments are described which utilize such an adaptive system, running on a laboratory minicomputer, to process the signals from a transient ultrasonic source in a plate specimen. It is shown how the system learns from the experimental pattern vectors, formed from the ultrasonic waveforms and encoded information about the source. The source characteristics are recovered by the recall procedure from the detected ultrasonic signals and vice versa. Also, the changes in the wave phenomenon corresponding to changes in the boundary conditions of the specimen can be detected from the discrepancy between the presented and the learned signals.

Notes: A new type of solid-state sensor for the detection of minute concentrations of hydrogen gas has been developed. The sensor was made of thin, commercially available polyvinylidene fluoride (PVDF) pyroelectric film, sputter coated with Pd. An infrared laser beam served to produce alternating temperature gradients on the Pd-PVDF and on reference Al-Ni-PVDF films, which, in turn, generated ac voltages due to the photopyroelectric (P2E) effect. Exposure to hydrogen gas was shown to produce an increased differential signal between the Pd and reference electrodes; this was tentatively attributed to the adsorption and dissociation of hydrogen molecules on the Pd surface, which caused a shift on the Pd-PVDF pyroelectric coefficient, due to interactions at the Pd-PVDF interface. The differential signal was found to be proportional to the square root of the hydrogen partial pressure at very low concentrations (〈1000 ppm). A semiquantitative interpretation of the differential signal has been achieved using simple gas-solid interaction theory and the combination of the Langmuir isotherm with the photopyroelectric theory in the range of 4–200 Pa. For high pressures (〉200 Pa) the paper is limited only to a phenomenological description. The thickness of the palladium layer has been found to play an important role with respect to the signal response. Presently, hydrogen concentrations as small as 40 ppm, in a flowing H2+N2 mixture, have been detected. The influence of gas flow rate has also been studied. Other characteristics such as the response times, the reversibility, and the durability of the Pd-PVDF-P2E hydrogen detector will also be presented.

Notes: The quenching cross sections of Ne metastable levels by N2 and CO were measured to be (in A(ring)2 units): 8.3±0.4, 8.7±0.4 for 3s(3/2)2 level and 8.0±0.4, 9.1±0.4 for 3s(1/2)0 level, respectively, at 443 K. The metastables are produced by a pulsed electron beam and their population was monitored by fluorescence induced by a cw dye laser.

Notes: The generation of quasi-static uniform electric fields by a slotted conducting spherical shell is fully analyzed for an arbitrary number of slots pairs, and optimum surface potential and slots position are determined. Numerical results for spherical shells with up to 14 slots are discussed.

Notes: This paper discusses a theoretical as well as experimental study of the performance of electro-optic sampling using a noncontact electro-optic probe, i.e., external electro-optic sampling. Sensitivity, temporal resolution, and the capacitive loading effect of this system are calculated by analysis of a static electric field coupled to an electro-optic crystal placed in close proximity to transmission lines such as microstrip lines and coplanar strips. Full-wave analysis is also applied to investigate effect of the electro-optic crystal on the transient property of high-speed electrical signals. Based on these analytical considerations, we have developed an external electro-optic sampling system using precise probe-positioning technology, which improves the measurement reproducibility. A temporal resolution of 0.5 ps and a spatial resolution of 1 μm are confirmed with this system.

Notes: Experimental and theoretical studies of a fundamental mode large-orbit gyrotron designed to produce high-power rf radiation in the TE11 mode at about 1 GHz are reported. Theoretical calculations performed as part of the experimental design process are presented. Results of initial experiments are reported in which 500–1000 MW of rf radiation was observed at frequencies 700 and 1200 MHz, consistent with theoretical expectations. The estimated eletronic efficiency of the device is about 10%. Gas cell breakdown patterns produced by the rf pulse are consistent with operation in the desired TE11 mode.

Notes: As a second in a series of theoretical models for the electrical discharge machining (EDM) process, an erosion model for the anode material is presented. As with our point heat-source model in the previous article, the present model also accepts power rather than temperature as the boundary condition at the plasma/anode interface. A constant fraction of the total power supplied to the gap is transferred to the anode. The power supplied is assumed to produce a Gaussian-distributed heat flux on the surface of the anode material. Furthermore, the area upon which the flux is incident is assumed to grow with time. The model is capable of showing, via the determined migrating melt fronts, the rapid melting of the anodic material as well as the subsequent resolidification of the material foation from plasma dynamics modeling could improve substantially our results.

Notes: Designs are proposed, based on a series of one-dimensional calculations, for layered, hollow cylindrical targets to be placed on the axis of an imploding, hollow Z-pinch plasma that can create the approximate plasma conditions, as well as radiation spectrum, for a photoionization pumped, Ne-like recombination laser. The lasant must reach the Ne-like state and be at the appropriate density at the same time that the photoionizing pump radiation is present, placing severe constraints on designs for such targets. Target designs are further constrained by the fact that the 3s-2p resonance line, which depopulates the lower lasing state, must not be highly trapped and by the fact that the upper lasing state must not be collisionally depopulated. We find that hollow, cylindrical targets consisting of a few-micron-thick CH strongback, coated on the inside with a thin layer of Ni lasant and on the outside with an Al converter layer, can be optimized to achieve appropriate conditions for lasing and modest levels of gain.

Notes: The trigatron has been in widespread use as a demand-triggered, high-voltage switch for more than 40 years. In spite of the popularity and maturity of the technology, there persists an uncertainty over the basic physical mechanism(s) responsible for triggering breakdown in the devices. We present the results of an empirical study of trigatrons that directly demonstrates that breakdown is initiated by a streamer launched from the trigger pin, independent of the trigger spark. We compare our results with those of previous workers, and discuss the generality of our conclusions.

Notes: Reaction kinetics for the direct synthesis process of compound GaSb from Ga and Sb metals was studied by observing viscosity changes of the melt during the reaction process. The mole fraction of Ga (cGa) was obtained during synthesis using the equation; log η=cGa(log ηGa +log ηSb)+(1−2cGa)log ηGaSb, where η is the viscosity of the total system, and ηGa,ηSb, and ηGaSb are the viscosities of molten Ga metal, molten Sb metal, and molten GaSb, respectively. The reaction rate was obtained from the variation of the mole fraction of reactant cGa. Assuming a second-order reaction, the activation energy was estimated to be 15.5 kcal/mol.

Notes: A PbTe flux has been used for n-type (Te) doping of GaAs, GaSb, and AlGaSb. The effects of surface accumulation and Te desorption were noticeable in secondary-ion mass spectroscopy profiles of GaAs layers grown at temperatures in excess of 540 °C. Te accumulation was not apparent in GaSb layers grown at temperatures up to 630 °C, but Te desorption occurred from GaSb at temperatures above 540 °C. The donor ionization energy of Te in AlxGa1−xSb is 44 meV for 0.4〈X〈0.5, i.e., significantly lower than the ionization energies of S or Se in similar material.

Notes: The low-frequency excess electrical noise has been measured on carbon fibers with a wide range of crystalline perfection and corresponding electrical and mechanical properties. Fibers include those prepared from ex-PAN and ex-pitch polymers, and a catalytic-chemical vapor deposited filament. The extensional (Young's) moduli of these fibers varied from about 220 to 890 GPa (35–130 Msi), while the electrical resistivities varied from about 19 to 1 μΩ m. The low-frequency electrical noise of each fiber was found to be proportional to I2 and to vary as 1/f α, where f is the frequency and α is about 1.15. The most striking feature of the results was the strong dependence of the normalized noise power on the degree of crystalline perfection.

Notes: The densities of states of the conduction and valence bands of silicon and GaAs have been calculated at 300 K for the case of an electron-hole plasma, which can occur at high-injection levels in bipolar devices or in bulk material under intense optical excitation. The results show considerable narrowing of the band gap, which needs to be included in the analysis of device measurements or the interpretation of photoluminescence data. Furthermore, the band-gap narrowing that results from dopant ions is reduced by excess carriers because of the reduced free-carrier screening radius.

Notes: We investigate the transient response of photogenerated carriers to an external electric field in bulk GaAs. The results of our Monte Carlo simulations indicate that the initial velocity rise times are a strong function of the carrier density. This is caused by a combination of the hot-phonon effect and the enhanced electron-hole scattering within the plasma. Contrary to some previous suggestions, the hot-phonon effect alone is insufficient to explain the initial velocity behavior seen experimentally. The steady-state velocity is limited by the electron-hole scattering.

Notes: Data obtained on a set of GaAs/AlGaAs double-barrier quantum-well resonant tunneling structures are compared with model calculations of the ideal case where scattering is negligible and tunneling is coherent throughout the entire structure. The comparison points to interface roughness in the well as the most likely cause for the observed large valley currents. The currents at low biases, before resonance sets in, are also studied. Their magnitude is found to be consistent also with the sequential tunneling picture.

Notes: The microstructures of three ZnO varistor materials with different Bi2O3 contents have been evaluated by analytical electron microscopy in combination with x-ray diffractometry. The results have been correlated to microelectrode measurements, where breakdown voltages of individual ZnO junctions were measured, and also to current/voltage characteristics of bulk specimens. The volume fraction of the continuous intergranular network of Bi-rich phases, which lies along the triple junctions of the ZnO grains, increases with increasing Bi2O3 content, The conductivity of this network is strongly influenced by its internal microstructure. It was found that increased volume fractions of δ-Bi2O3 and less interpenetration between α-Bi2O and δ-Bi2O3 increases the conductivity of the network. Individual ZnO/ZnO grain boundaries exhibited breakdowns at 3.2 and 3.6 V, depending upon whether they contained segregated Bi atoms or thin Bi-rich amorphous films. The current/voltage characteristics of heterojunctions between ZnO and intergranular Bi2O3 were asymmetrical with respect to the polarity of the applied voltage. It was found that α-Bi2O3 and δ-Bi2O3 give rise to different breakdowns for electrons traveling from the Bi2O3 into an adjacent ZnO grain.

Notes: Isothermal anneal experiments have been conducted over a wide temperature range from 300 to 900 °C for polycrystalline YBa2 Cu3 O7−x samples in air. The phase transformation and oxygen contents of the annealed samples were studied and determined by x-ray diffraction and iodine titration. Oxygen diffusion coefficients at different temperatures were determined by a novel technique based on direct observation of the oxygen diffusion fronts by polarized light microscopy. The activation energy for the diffusion of oxygen in YBa2 Cu3 O7−x was obtained by an Arrhenius plot of the diffusion coefficients at different temperatures. The tetragonal-to-orthorhombic phase transformation controlled by oxygen diffusion is discussed.

Notes: In the limit of negligible exchange and anisotropy (except uniaxial), certain shapes of magnetic samples in certain subsaturating field configurations can be analyzed analytically to obtain magnetization and domain wall patterns. Here we consider two cases: (1) generalized cylindrical objects of arbitrary cross section (with several specific examples given) and (2) a general ellipsoid. We also discuss a "tipping instability'' which might occur in an ellipsoid with uniaxial anisotropy, resulting in a sudden transition from a "curling'' state to a "transverse vortex'' state.

Notes: Fe multilayered films with various intermediate layers are formed. Their magnetic properties and film structures are examined to understand the mechanism originating soft magnetic properties. The soft magnetic properties change with the lattice mismatches between Fe and intermediate layers, showing low coercive force and high relative permeability at lattice mismatches from 0.4 to 1.2%. It is thought that the lattice mismatches above 0.4% decrease Fe crystallite size and improve soft magnetic properties. Although Fe crystallite size is small at the lattice mismatches above 1.2%, the soft magnetic properties are poor. This is because the large lattice mismatch increases internal stress and magnetic anisotropy energy. This report indicates that the good soft magnetic properties are obtained when both Fe crystallite size and internal stress are small.

Notes: The advantage of a film with high saturation magnetic flux density for perpendicular magnetic recording is investigated through computer simulations and read/write measurements. A single-pole head using Fe-C/Ni-Fe multilayered film with a saturation magnetic flux density of 2.0 T is fabricated. Read/write tests are performed with the Co-Cr double-layer media. It is shown that the recording ability of this head is very high and does not change with increasing Co-Cr layer coercivity. Computer simulations show that the recording ability also increases when using film with high saturation magnetic flux density for the back layer of a double-layer medium. The upper limit of saturation magnetization of the recording layer can be extended by increasing coercivity. Thus, the higher saturation magnetization medium, which offers higher output voltage and resolution, can be used by applying film with high saturation magnetic flux density for the main pole and the back layer.

Notes: In this paper we study the dielectric response of materials which display a dc conductivity at low frequencies. A framework based on the generalized Master equation and time dependent transition rates is argued to be applicable to disordered materials and exhibits an interesting fractal dynamics. The relationship of this framework to the "Distribution of transition rates'' theory and the models put forward by Ngai [Comments Solid State Phys. 9, 141 (1980] and by Dissado and Hill [J. Chem. Soc. Faraday Trans. 2, 80, 291 (1984)] is discussed. Model calculations of the dielectric response by means of the above-mentioned three theories are carried out and are compared to experimental results on Al2O3 and SiOxNy thin films. The theories can be distinguished only when the exponent of the power law at high frequencies is close to unity. The Dissado–Hill model is in better agreement with experiments than the other theories, but important discrepancies can easily be seen.

Notes: Band-gap narrowing of GaAs as a function of doping concentration has been measured using photoluminescence spectroscopy on samples grown by molecular beam epitaxy. Both n- (Si) and p- (Be) doped samples with concentrations varying from 3×1017 to 3×1018 cm−3 have been measured. The experimental results obtained from a line-shape analysis of the spectra taking tailing effects into account are in good agreement with recent theoretical calculations. A simple expression for the band-gap narrowing as a function of concentration for both n-and p-doped GaAs is given.

Notes: We propose that short-period doping superlattices are suitable for the enhancement of a third-order susceptibility arising from free carriers in nonparabolic energy subbands. The inherent advantage lies in the ability to simply engineer the superlattice potential profile, yielding control of miniband energy dispersion. We consider short-period GaAs doping superlattices composed of uniformly doped donor and acceptor layers, and planar-doped n- and p-type monolayers separated by intrinsic regions. Calculations of the electronic structure of compensated and n-type noncompensated n-i-p-i superlattices incorporating miniband dispersion at nonzero temperature are reported. We show that small modulations of the superlattice potential lead to large subband nonparabolicities and we calculate a twentyfold improvement in the third-order susceptibility over bulk GaAs at room temperature, comparable to that predicted for GaAs/AlGaAs compositional superlattices.

Notes: The physical mechanism of photostimulated luminescence (PSL) of RbBr:Tl, resulting from x-ray irradiation, has been investigated. Spectroscopic methods have been utilized to identify the bromine F center as the occupied electron trap. By means of absorption studies the charge state of the dopant Tl was found to be monovalent. The spectral emission showed the characteristic Jahn–Teller splitting of the 3P1 state of the Tl+ and its known temperature dependence. PSL lifetime and efficiency experiments have been conducted in the temperature regime from liquid helium to 500 K to understand the charge transport from the F center to the activator and have been compared with corresponding data obtained from direct optical excitation into the A band of the Tl+. The observed temperature dependencies of both lifetimes could be explained by the properties of the s2 ion Tl+. As a resulting model for the PSL process a tunneling from the relaxed excited state of the F center to an excited state of the Tl2+ ion following photostimulation is suggested. The temporal, thermal, and spectral behavior of the subsequent radiative fraction of the deexcitation process is solely determined by the properties of the Tl+ ion in the RbBr matrix.

Notes: Experiments were conducted in which a high-power CO2 TEA laser interacted with metallic cathode in a high-vacuum (10−8 Torr) diode. For power densities lower than 5×107 W/cm2, no current was detected. For power densities in the range of 5×107–5×108 W/cm2, the Cu cathode emitted a maximum current of 40 mA. At a higher power density level, a circuit-limited current of 8 A was detected. The jump of a few orders of magnitude in the current is attributed to breakdown of the diode gap. The experimental results are similar to those of a triggered vacuum gap, and a thorough comparison is presented in this paper. The influence of the pressure in the vacuum chamber on the current magnitude shows the active role that adsorbed gas molecules have in the initial breakdown. When the cathode material was changed from metal to metal oxide, much lower laser power densities were required to reach the breakdown current region.

Notes: The intrinsic magneto-optical readout performance in reflection is calculated for bismuth and cobalt-substituted iron-garnet films on a multilayer interference mirror at 800-, 633-, 488-, and 420-nm wavelengths and is compared with that of a trilayer medium composed of an antireflection layer, a rare-earth transition-metal film, and a metallic mirror. It is found, when disregarding inhomogeneities, like irregular domain shape, ripple of the magnetic anisotropy, and surface roughness, that iron garnets are superior to rare-earth transition-metal films at blue to near-ultraviolet wavelengths if operated at thicknesses where optical interference occurs in the magnetic layer. Optical transmittance at these thicknesses is sufficiently high so that multilevel recording media can be conceived. In contrast, the optical absorption of rare-earth transition-metal alloys is much higher so that only thicknesses much above interference conditions are feasible, thus precluding them from multilevel recording.This comparative study is supplemented by calculating the magneto-optical performance in reflection of a recently reported multilayer medium composed of an antireflection coating and a periodically repeated sandwich of 4-A(ring) Co and 9-A(ring) Pt layers. In contrast to conventional rare-earth transition-metal films, the magneto-optical Kerr effects of this material do not degrade when decreasing the wavelength from 800 to 400 nm, but still do not reach the performance of bismuth-iron garnets in the green to ultraviolet spectrum. For the garnet system Y3−xBixFe5O12 the spectra of the real and imaginary parts of the diagonal and off-diagonal component of the dielectric tensor εij are reported in the range of photon energies between 1 and 5 eV, i.e., 1240- and 248-nm wavelengths and a bismuth concentration up x=1.4 Bi3+ atoms per garnet formula. In addition, the off-diagonal components ε'12 and ε(large-closed-square)12 are parametrized in terms of paramagnetic optical transitions, taking the spectra for x=1.25 as a typical example. Furthermore, optical and magneto-optical spectra are presented for Co2+- and Co3+- substituted iron garnets and barium hexaferrite BaFe12O19.Finally, the spectral dependence of the magneto-optical figure of merit 2aitch-thetaF/α of (Y,Bi)3Fe5O12 and amorphous TbFe is compared. Furthermore, high-resolution transmission electron micrographs and x-ray double-crystal diffractograms are presented that elucidate the perfect epitaxial alignment of single-crystalline iron-garnet films and the columnar morphology of polycrystalline iron-garnet films prepared by rf magnetron sputtering. The initial nucleation period of polycrystalline garnet films can be influenced by low-energy ion bombardment for improving the film texture. Under favorable sputtering conditions single- and polycrystalline bismuth-iron garnet films develop a perpendicular magnetic anisotopy. It is not yet clear whether sputtered iron-garnet films can meet the critical requirements on magnetic wall coercivity and magnetic remanence.

Notes: Chemical analysis of the surface of an oxidized silicon-germanium mixture by Rutherford backscattering reported recently showed that the silicon is oxidized, but not the germanium. This result provides evidence that the effective oxygen concentration at the silica-silicon interface is low, which is contrary to a slow interface reaction. Strain in the oxide film can result in linear-parabolic oxidation kinetics without a slow interface reaction.

Notes: Low energy inelastic electron tunneling spectra are presented for thermally formed oxides on copper films. Thinner oxide tunnel barriers are observed to contain interfacial gradients as evidenced by spectral intensity asymmetries between forward and reverse bias, whereas thicker oxides appear to be more uniform in composition. Spectral results are in good qualitative agreement with reported studies of bulk cuprous oxides, and the semiconducting nature of the oxide layers is reflected in a negative temperature coefficient of resistivity of the tunnel junctions.

Notes: Optical waveguide profiles of He+ ion implanted polymethylmethacrylate have been analyzed. The surface index increases by up to 3% and the depth profile of the index is consistent with a diffusion limited decomposition of the target. This was confirmed by 77- or 300-K implants with a capping layer to inhibit decomposition.

Notes: Infrared absorption spectroscopy was used to study the oxidation of hydrogenated amorphous silicon carbide (a-Si:C:H) films prepared by the glow-discharge decomposition of gaseous mixtures of silane and methane. It has been found that carbon-rich samples incorporate oxygen when exposed to air, as detected by an increased absorption of the Si-O-Si stretching vibration band. The analysis of the infrared spectra of samples annealed in air at room temperature and at 200 °C indicates that, except for their oxidation rate, no appreciable difference exists in the mechanisms of oxygen incorporation in the films at the two temperatures. The oxidation kinetics suggests an open porous structure for these carbon-rich films. On the contrary, samples having a low carbon content appear to oxidize on the surface only, in a way similar to amorphous silicon.

Notes: Thin-film transistors have been made using nickel phthalocyanine (PcNi) as active component. The characteristics of these transistors are studied with positive and negative gate-source and drain-source polarizations. The channel conductivity was p type. Very different thicknesses of PcNi layer are used, showing that in the two polarizations the effects are interfacial.

Notes: A technique for measuring the field-effect mobility of a gateless In0.53Ga047As/In0.52Al0.48As heterojunction has been developed by using the nonpersistent photoconductive effect. Subband depopulation in a parallel magnetic field is consistent with the "universal'' depopulation observed in InAs, InSb, and HgCdTe metal-oxide-semiconductor field-effect transistors.

Notes: Transmission versus voltage operating characteristics of a GaAs/AlGaAs multiple-quantum-well light modulator have been measured at temperatures between 4.3 and 290 K. At 4.3 K, the linewidth of the heavy-hole exciton absorption is 1.3 meV, and the modulation ratio is greater than 1000 at a 9-V bias. At room temperature, the exciton resonance is broadened to 5.3 meV by phonon scattering and the maximum modulation ratio is 4.93 at 12 V. The measured temperature and voltage dependence of the exciton absorption energies, linewidths, and intensities have been used with a simple model for the modulator's optical transmission to calculate the operating characteristics as a function of material quality. The upper limit on the room-temperature transmission modulation ratio is estimated to be 5.4 for a device fabricated from 100 quantum wells of width 100 A(ring).

Notes: Adhesion-induced deformations of a polyurethane substrate in contact with cross-linked polystyrene spheres, having diameters ranging from less than 2 μm to approximately 12.5 μm were observed using scanning electron microscopy. The diameters of the contact areas were measured from the micrographs. It was found that the contact radius varied as the particle radius raised to the 0.75±0.05 power. Experimental results are compared to the predictions of various adhesion models. The results are also discussed in terms of the Dupré work of adhesion.

Notes: We have observed very rapid migration of silicon atoms along the grain boundaries of electron-beam-annealed polycrystalline silicon film in a silicon-on-insulator (SOI) island structure with a capped layer consisting of Si3N4 and SiO2 films. A considerable amount of mass transport was observed in the SOI structure at a temperature high enough to melt grain boundaries but not high enough to melt the entire polycrystalline silicon film. Our results support the notion that electromigration of Si+4 ions in molten silicon at the grain boundaries is responsible for the rapid migration of silicon atoms.

Notes: Traditionally, the chemical composition of surfaces prepared by various chemical, thermal, and plasma oxidation are analyzed by x-ray photoelectron spectroscopy. In this work, both x-ray wide-angle and glancing angle diffraction techniques were used, not only to investigate chemical composition, but also to characterize the structural features of the chemical oxide grown on an InP (100) surface in both HCl and HNO3 etches. These oxides all showed different degrees of preferred orientation with respect to the substrate. In 1M–8M HCl etches, the gradient composition, identified as crystalline hydrous InPO4, was the predominant phase and the rest was mainly composed of hydrous InPO4-based oxides. Coherent hydrated layers, also identified as hydrous InPO4, were obtained in HNO3 etches. In addition, the observation that InI -InIII mixed valence and InI chloro compounds are produced helped clarify the understanding of the dissolution mechanism of InP.

Notes: To understand the behavior of a YBa2Cu3O7−x weak link (constriction), a simple model of two Josephson junctions in series with each other is proposed. Electronic simulation of this two-junction array under various conditions generates dc and ac IV curves. Some of these curves duplicate very well what has been observed on a real YBa2Cu3O7−x weak link, while others predict phenomena that have not yet been observed. These results suggest the possible applicability of this model to explain the complicated experimental features of a YBa2Cu3O7−x weak link.

Notes: The superconductive properties in the Bi(Pb)-Sr-Ca-Cu-O system with nominal composition of Bi2−x Pbx Sr2 Ca2 Cu3 Oy (x=0, 0.2, 0.4, and 0.6) and annealed at 860 °C for 240 h were studied using dc resistivity and ac susceptibility measurements. For x=0.2 and 0.4, the bulk zero resistivity temperatures were found to be ∼103 K. For x=0 and 0.6, these temperatures were found to be ∼73–75 K. The ac susceptibility data show enrichment of the volume fraction of the high-Tc phase in compositions with x=0.2 and 0.4. Tc of this phase is constant at 107 K for all values of x. The enhancement of the bulk zero resistivity temperature for x=0.2 and 0.4 is due to the improved intergranular coupling of the high-Tc phase, although the resulting weak-link behavior reflects a low current carrying capacity in these materials. The presence of the low-Tc phase (∼65 K) is still visible in the susceptibility data. For x=0.6, the intergranular coupling of the high-Tc phase is almost destroyed; however, the coupling of the low-Tc phase is better than when x=0. Powder x-ray diffraction and scanning electron microscopy data show the formation of large grains of the calcium plumbate, Ca2PbO4 phase with increasing Pb up to x=0.6. The large grains of the Ca2 PbO4 phase may incorporate other impurity phases within it when x=0.6, cleaning up the remaining grain boundaries, thereby improving the intergranular coupling within this material. Energy dispersive x-ray analysis indicates that some Pb is incorporated into the structure of both superconducting phases.

Notes: A possible pairing mechanism involving the collective oscillations ("dipolons'') of the crystal-field-generated dipoles of the oxygen ions in the CuO2 planes in the superconductor YBa2Cu3O7−δ has been used to estimate, in the framework of the BCS theory, the variation of Tc as a function of δ, which compares very well with the experimental observations.

Notes: The demagnetizing energy of thin magnetic films can be written in terms of the Fourier components of the magnetization distribution. This formalism is used to investigate the structure of jagged domain walls in both perpendicular and in-plane media. Also introduced is a set of correlation functions and their relationship to demagnetizing energy density in media with random magnetization distribution.

Notes: The resonance spectrum of composite plates made of alternating piezoelectric ceramic and epoxy elements is studied both theoretically and experimentally in the frequency range below the thickness resonance. The transmissivity of the finite structure to plate modes is calculated, taking into account the effective plate velocities of the two constituent materials and the finite number of elements in the structure. The results are in excellent agreement with the electrically excited acoustical resonances of samples with different geometry and number of elements. In particular, the effects of the finite size on the number and frequency of modes, as well as their selection rules in the given experimental setup, are successfully interpreted. New "surface'' resonances, attributed to the end elements, are observed in the stopbands when the surface elements have lower impedance.

Notes: Thirty-seven of 97 AlxGa1−xAs laser diodes from three tests conducted in vacuum (10−6 Torr) at a −20 °C heat sink temperature (Tj∼0 °C) failed catastrophically. Causes of failure varied from testing error to defects in the metallization. In all cases, p-side anomalies allowed indium solder (and probably gold in addition to indium in one case) to migrate into the GaAs bulk, causing the observed failures. The results of this research show that the integrity of the p-side metallization is crucial for reliable operation. Stringent controls have been implemented in the design and processing of the p-side metallization and catastrophic failures due to p-side anomalies have not been observed in subsequent life tests. A correlation was made between shifts in the forward voltage at 1 mA (Vf at 1 mA) and the specific failure mechanisms found in this research. In all cases Vf at 1 mA was found to decrease. The amount of decrease depended on the failure mechanism. A quantitative model is presented that explains the shift in Vf as a function of the type of defect in the active region.

Notes: To accurately evaluate the integrity of the interfaces of a multilayered structure using ultrasonic pulse echo methods, we must understand the wave propagation in the structure and identify the appropriate interface signal. However, the signal is often complicated by the material and defect characteristics of the preceding interfaces. We have derived generalized mathematical formulas for the reflected mode time and the pulse amplitude response of an arbitrary n-layered structure for use in modeling the wave propagations in multilayered structures and identifying the critical structural interfaces. The time and echographic amplitude responses are calculated by computer using routines programmed for a given specimen. A comparison of the results of laboratory measurements of various glass and aluminum specimens with the results from computer-simulated experiments have verified the validity of the formulas.

Notes: Acceleration instability occurs when a body is accelerated by surface tractions. This situation resembles classic Rayleigh–Taylor instability, but differs due to the temporal and spatial variation of the stress field in the accelerated body caused by wave propagation and the time dependence of the accelerating forces. These factors produce phenomena in acceleration instability which are without precedent in classical Rayleigh–Taylor analyses. An extensive numerical study of acceleration instability using a Lagrangian finite-difference wavecode has determined the influence of various parameters including amplitude and wavelength of initial surface perturbations, material yield strength, and time dependence of the driving force. The nature of the criteria determining stability or instability is established, and the fundamental physical quantity controlling perturbation growth at an interface is shown to be the local stress gradient.

Notes: We present a new model (including both temperature and electron-hole effects) of two-beam coupling in photorefractive semiconductors under an external dc field E0. This model predicts that the exponential gain Γ can exhibit an intensity-dependent resonant behavior, yielding a π/2 phase shift of the space-charge field with respect to the incident fringe pattern. This optimum intensity strongly depends on crystal temperature but it is practically independent of the grating period. As an illustration this model is applied to InP:Fe. In this case the resonance occurs when the hole photoionization rate and the electron thermal emission rate are equal. Values of Γ as high as 20 cm−1 at 1.06 μm, for a fringe spacing of 15 μm and E0 =10 kV/cm, are predicted. The comparison between theory and experimental data requires taking into account the variation of the pump intensity throughout the sample (due to optical absorption), which reduces the volume where the resonance condition is fulfilled; in this way a satisfactory fit of the experimental gain versus intensity curve has been obtained. The characteristics of this new resonance mechanism suggest several gain enhancement techniques which are briefly discussed. In particular, an increase of Γ by using an additional incoherent backward propagating beam is predicted and experimentally demonstrated.

Notes: Despite their increased safety, alkylarsine compounds have not generally replaced arsine (AsH3) in the metalorganic vapor phase epitaxy (MOVPE) of GaAs because of carbon incorporation and high background doping levels. We have studied the thermal decomposition of AsH3 and its alkyl derivatives (methyl, ethyl, and butyl compounds) to determine the impact of the thermochemistry on growth processes. The thermal stability of the As-precursor compounds was found to decrease in the order AsH3〉MenAsH3−n 〉Et3As〉t-BuAsH2. We report the first evidence for production of diarsine (As2H4) from t-BuAsH2 and for formation of lower substituted methylarsine homologs from Me3As and Me2AsH. The presence of these species is strong evidence that decomposition of the alkylarsines occurs via a free-radical mechanism. Formation of carbon-free arsenic products appears to be the key difference between t-BuAsH2 and the more highly substituted alkylarsines in attaining high quality films by MOVPE.

Notes: A new type of application of porous silicon formation is proposed, which does not deal with the material properties, but with the characteristics of the involved electrochemical silicon dissolution reaction. The anodization potential of p-type silicon in concentrated hydrofluoric acid under galvanostatic conditions has been shown to be characteristic of the silicon doping concentration at the interface and independent of the porous silicon-layer thickness. This close correlation between potential and doping level is used to determine concentration profiles of p-type implanted dopants. During anodization of silicon presenting dopant concentration variations, the anodization potential varies according to silicon doping concentration. The potential values can be converted to dopant concentration, and the electrolysis time scale to a depth scale, leading then to the doping impurity profile. This method, which is used as well for deep and shallow diffused impurities, exhibits a very good agreement with spreading resistance measurement and secondary-ion mass spectrometry analysis. Simultaneous anodization of regions of different doping concentration has also been investigated. It allows the characterization of the porous silicon formation selectivity by a selectivity coefficient, which is measured on a large range of silicon doping concentration. The selectivity is found to be strongly dependent on the doping level: it is nearly infinite for samples presenting highly doped regions (greater than 3×1018 cm−3), but decreases sharply when the doping concentration is below 1018 cm−3.

Notes: The complete reduction of oxides on the chemically polished surface of (100)InP has been obtained by exposure at 300 °C to low-pressure ammonia. The whole process was monitored by various surface techniques. In a first step, exposure to ammonia removes the weakly bonded oxygen atoms and stabilizes the one-monolayer oxide, i.e., turns the initial wet oxide to a thinner, well-defined, and more stable oxide, which should prove to be a better base for further processing. Excitation of the ammonia gas by an ion gauge, i.e., use of highly active radicals, is necessary for the second step, where the oxygen atoms strongly bonded to InP are finally completely removed. The final surface is free of all contaminants and of nitrogen, its crystal structure displays a 4×1 reconstruction, and its Fermi level is pinned at 0.8 eV above the top of the valence band, i.e., 0.2 eV lower than on the clean InP surface. It is mostly InP covered by adsorbed hydrogen, with a small coverage of indium atoms liberated by the reduction of the native oxides and grouped as metallic clusters.

Notes: GaAs layers grown by organometallic chemical vapor deposition (OMCVD) on Si substrates oriented 2° off (100) toward [011] have been studied by 10-K filtered cathodoluminescence, deep-level transient spectroscopy (DLTS), electron-beam-induced current (EBIC), and secondary ion mass spectrometry (SIMS) measurements. The mapping of the near-band-edge emission intensity shows the existence of zones of lower defect density, whose relative volume increases with the layer thickness. Four deep-level energy peaks have been characterized at 1.33, 1.20, 1.15, and 1.02 eV. The 1.33 and 1.20 eV energy peaks are attributed to Si in GaAs. Strong inhomogeneities in the intensity of these two emissions have been observed with depth of the layers, and this behavior has been related to the silicon distribution. From measurements on structures grown with different buffer layers, we show that a GaAs/AlAs supperlattice located at 1 μm from the Si substrate is efficient in decreasing the width of a highly Si-doped interface region. DLTS measurements confirm the decrease in the deep-level concentration with increasing thickness. Furthermore, by EBIC characterization, we measure a minority-carrier diffusion length of 2.7–3 μm on a 5-μm-thick layer. This corresponds to a dislocation density of 8×106 cm−2.

Notes: Polycrystalline barium titanate that has been doped to give a positive temperature coefficient of resistance (PTCR) effect is an inhomogeneous material electrically. Analysis of ac impedance data using the complex impedance plane representation gives the dc resistance of PTCR ceramics. By additional use of the complex electric modulus formalism to analyze the same data, the inhomogeneous nature of the ceramics may be probed. This reveals the presence of two, sometimes three elements in the equivalent circuit. Grain-boundary and bulk effects may be distinguished from capacitance data: grain-boundary effects have temperature-independent capacitances, whereas bulk effects show a capacitance maximum at the Curie point and Curie–Weiss behavior above the Curie point. Both grain-boundary and bulk effects appear to contribute to the PTCR effect. These results reveal limitations in current theories of the PTCR effect.

Notes: The etch rate of GaAs during reactive ion etching (RIE) in a CCl2F2:O2 discharge (4 mTorr, 0.56 W cm−2) shows a strong temperature dependence, increasing from ∼500 A(ring) min−1 at 50 °C to 2800 A(ring) min−1 at 400 °C. Arrhenius plots of the etch rate show two activation energies (0.17 eV from 50 to 150 °C and 0.11 eV from 150 to 400 °C). There is no significant plasma power density dependence of the etch rate at elevated temperatures (≥100 °C) in contrast to the strong dependence at 50 °C. The surface morphology undergoes smooth-to-rough-to-smooth-to-rough transitions at ∼150, 250, and 400 °C, respectively, although TiPtAu Schottky diodes exhibit near-ideal behavior on GaAs etched at 150 °C. The As-to-Ga ratio in the first 100 A(ring) from the surface increases with increasing RIE temperature, with chloride residues absent above 150 °C. Fluorocarbon residues were present on all samples, but were limited to the first 10–15 A(ring). As determined by x-ray photoelectron spectroscopy, fluorine was present almost exclusively as metallic gallium fluorides, while oxygen was present as both Ga and As oxides. Ion channeling detected lattice disorder to depths of ∼200 A(ring) for etch temperatures from 50 to 250 °C, while there was less damage for etching at 400 °C. No new RIE-induced deep levels were observed by capacitance transient spectroscopy in any of our samples, although the concentration of the midgap donor EL2 increased with increasing temperature during the RIE process.

Notes: Thin films of III-V nitride semiconductors (AlN, GaN, InN), mixed-crystalline films (AlxIn1−xN), and multilayered films (GaN/InN)n were grown by rf magnetron sputtering at low substrate temperatures below 500 °C. These films were characterized by x-ray diffraction, Raman scattering, optical absorption, and electrical measurements; it was proved that they have high crystal quality comparable to the previously reported data obtained by other growth methods. Dependence of the band-gap energy of AlxIn1−xN on composition x was determined. Multilayered films of (GaN/InN) were prepared for the first time; these films showed the characteristic diffusion property to make an ordered-alloy superlattice.

Notes: Hg1−xZnxTe-CdTe strained layer superlattices were grown by molecular-beam epitaxy. Their structural properties and interplay with a CdTe buffer layer were investigated with transmission electron microscopy. There is an order of magnitude reduction in the density of threading dislocations in the superlattice compared with the CdTe buffer layer. The reduction is accomplished by using the lattice-mismatch-induced strain to bend threading dislocations over into the superlattice-buffer layer interface. The magnitude of the reduction agrees well with predicted values for this system.

Notes: Concentration depth profiles of carriers (or of electrically active defects) and/or deuterium in GaAs following exposure to a hydrogen plasma (or, in one case, to molecular hydrogen) are fit using a simple diffusion model with second-order reactions. We find that (1) the activation energy for hydrogen diffusion and the dissociation energies of hydrogen-defect complexes are dependent on the concentration of hydrogen, (2) there is no molecular hydrogen formation and no fast-diffusing species of hydrogen away from the near-surface region, and (3) atomic hydrogen can in-diffuse and passivate EL2 defects when semi-insulating GaAs is annealed at a high temperature in a molecular hydrogen ambient.

Notes: Reaction between Cu and PtSi is studied over 200 to 700 °C. Using structures of Cu on a preformed PtSi, and of Cu on Pt/Si, Cu-PtSi reaction is observed below 300 °C for both structures, resulting in increasing sheet resistances above this temperature. The low thermal stability of the Cu/PtSi metallurgy, compared with that of Al/PtSi, is attributed to the ready formation of Cu silicides at low temperatures. Cu3 Si and Cu4 Si are observed for both Cu/PtSi and Cu/Pt/Si structures heated to 300–400 °C. Little improvement is obtained using an 1000 A(ring) barrier layer of Ti, W, or Cr.

Notes: High-power microwaves (20–50 MW, 8.8 GHz) have been produced in repetitive pulses at a rate of 10 Hz in a joint experiment by Harry Diamond Laboratories and Sandia National Laboratories. These microwaves were produced with Harry Diamond Laboratories' backward-wave oscillator (BWO), driven by a 600 to 800-kV, 2-kA electron beam, which was guided by a magnetic field of variable strength from 0.75 to 1.05 T. The beam was produced with Sandia National Laboratories' repetitive high-voltage pulser, RAVEN. Microwave diagnostic instrumentation included a directional coupler, heterodyne receiver, dispersive waveguide delay line, and calorimeter. In a train of 10 pulses, the shape, duration, and amplitude of the injection voltage and beam current remained unchanged for each individual pulse. The intensity of the final microwave pulse was 40% of the first pulse, and the final pulse was 80% as long as the first. The basic pulse shape remained unchanged. More intense microwave radiation should be obtained with a higher magnetic field. Computer simulations using the code magic predicted the characteristic dip in the magnetic field strength versus microwave power output curve. Code predictions of microwave power output with a warm beam agree very well with experimental results. Finally, cyclotron resonance is presented in a descriptive fashion to account for the power output dip. This is the first experiment with a repetitively pulsed high-power BWO driven by a relativistic electron beam performed outside the USSR.

Notes: The behavior of trap-related transfer loss in acoustic charge transport devices is characterized experimentally through measurements of pulse degradation and frequency response rolloff. A theoretical loss analysis which incorporates a physical model of the charge packets in the device and the characteristics of electron traps in the GaAs epitaxial layers is presented and correlated with experimental results. It is found that the trap-related loss cannot be described by a proportional loss model. Good qualitative agreement is found between theoretical predictions and experimental results; however, the time constants observed experimentally in pulse measurements are 50–100 times faster than the theoretical predictions. Additional loss due to traps with long time constants is predicted theoretically but not observed experimentally.

Notes: We present a theoretical study of semiconductor T-structures which may exhibit transistor action based on quantum interference. The electron transmission through a semiconductor quantum wire can be controlled by an external gate voltage that modifies the penetration of the electron wavefunction in a lateral stub, affecting in this way its interference pattern. The structures are modeled as ideal two-dimensional electron waveguides and a tight-binding Green's function technique is used to compute the electron transmission and reflection coefficients. The calculations show that relatively small changes in the stub length can induce strong variations in the electron transmission across the structure. Operation in the fundamental transverse mode appears to be important for applications. We also show that a bound state of purely geometrical origin nucleates at the intersection between waveguide and stub. The performance of the device can be improved by inserting additional stubs of slightly different lengths. Taking into account the applicable scaling rules, we give estimates of the experimental parameters that optimize the transmission characteristics and speed of operation of the proposed transistor.

Notes: We have developed a new fabrication process of as-grown Y-Ba-Cu-O thin films using a reactive coevaporation method specially with the rf-plasma cooling in the low oxygen pressure of 0.4 mTorr. By this O2 plasma cooling process, the transition temperature Tc is improved from 40 to 81 K for the film with a thickness of 1000 A(ring). The x-ray diffraction analysis shows that the activated oxygen species generated by the rf plasma make Y-Ba-Cu-O films oxidize sufficiently even in the low pressure of oxygen. In addition, we have studied the thickness dependence of Tc for as-grown films with various thicknesses of 60–2000 A(ring).

Notes: Optical gain measurements on the donor-acceptor (D-A) photoluminescence band of CdIn2S4 revealed maximum gain coefficients varying from 1.5 to 3.5 cm−1 at 300 K, in an optical excitation energy range of 2.016–2.101 eV. The threshold pumping intensity required for the onset of gain was found to be dependent on the excitation energy. Such characteristics were used to calculate the energy distribution of the acceptor levels in that energy range, which turned out to be exponential.

Notes: The radiation hardness of fluorinated SiO2/Si interface in metal-oxide-semiconductor field-effect transistors has been found to depend strongly on the amount of fluorine introduced. In this study, the fluorine was introduced by low-energy F implantation onto the surface of the polycrystalline silicon gate electrode, followed by annealing at 950 °C to diffuse F into the gate SiO2 toward the SiO2/Si interface. The improved radiation hardness is attributed to the strain relaxation near the SiO2/Si interface by fluorine incorporation.

Notes: Temporal laser pulse distortion during amplification in the presence of nonsaturable absorbers is analyzed. The laser pulse duration ranges between the inverse of the laser bandwidth and the lasing state lifetime. It is found that nonsaturable absorption in the amplifying medium has two effects: (i) it limits the maximum pulse fluence and (ii) it reduces the pulse-width growth.

Notes: The indentation-induced cracks in brittle coating/substrate systems are examined. The crack paths induced by indentation with different applied loads are exploited by a post-bending test. Sandwiched bending bars are designed to conduct this post-bending test. The influence of the mechanical properties of each component and the interface and the thickness of coating on the paths of the indentation cracks are addressed.

Notes: Photoluminescence (PL) spectra have been investigated at liquid-He temperature in the ZnTe layers grown by metalorganic vapor-phase epitaxy. The epitaxial layer exhibits the spectra characterized by a neutral acceptor bound-exciton line in the near-band edge and the donor-acceptor pair emission in the wavelength range of 550–580 nm. At the low substrate temperature or low transport rate ratio of diethyltelluride to dimethylzinc, the spectrum of layer becomes dominated by this donor-acceptor pair emission. From the results of secondary-ion mass spectrometry analysis and PL property, it is concluded that Na and Cl are important impurities in the layer.

Notes: 4H and 6H silicon carbide (SiC) crystals are piezoelectric and can be excited to resonant vibrations with quality factors up to Q≈100 000 and an electromechanical coupling factor of k31≈0.03 for 6H material. The resonance frequencies are used for the determination of elastic constants from 4 to 320 K. One finds a Poisson's ratio σ=0.212 and low-temperature values s11=2.035×10−12 m2/N and c33=55.12×1010 N/m2 for 6H, s11=2.114×10−12 m2/N, and c33=60.52×1010 N/m2 for 4H crystals. All elastic constants show a nonmonotonic dependence from the temperature at about 50 K.

Notes: The magnetic field induced by a current which flows in a semiconductor during charge redistribution when photoeffect relaxation takes place is measured in a contactless way. The experimental results presented verify the above explanation of the contactless measurement and illustrate how to utilize it.

Notes: We have found that at least four kinds of oxygen-related thermal donors are distinguishable by their deep-level transient spectroscopy peaks and generation kinetics around 520 °C for 10–106 min in phosphorus-doped Czochralski silicon. Each kind of thermal donors successively appeared with its preceding kind decaying. The first appearing kind is identified as a family of thermal donors that have been most studied by many investigators. It is suggested that several kinds of thermal donors arise from various oxygen clusters with different sizes and thermal stabilities.

Notes: Transparent ultrathin films (〈100 nm) of lead lanthanum zirconate titanate (PLZT) with good dielectric properties were successfully grown on a platinum plate by rf magnetron sputtering and following an anneal in PbO atmosphere. The sputtering conditions were controlled by monitoring the plasma emission from the sputtering species. The PLZT thin films were deposited at an extremely low growth rate of 36 nm/h at 400 °C and annealed at 800 °C for 3 h in PbO atmosphere. The obtained thin films are oriented to [110] of the perovskite-type structure and have relatively low dc conductivity (∼10−7 Ω−1 cm−1). The evaluated dielectric constants are anomalously large around 5000 at room temperature and 1 kHz.

Notes: The optical properties of AlxGa1−xP alloys of various compositions have been investigated by electrolyte electroreflectance. E0 and E1 optical transitions have been observed for the first time in the spectral region between 3000 and 4500 A(ring). Both empirical relationships, E0=2.78+1.05x and E1=3.77+0.26x, between the transition energies and the alloy composition were found. The extrapolated values for x=1 were the first semiempirical values for the E0 and E1 transition of AlP at room temperature. The indirect energy gap of the sample with x=0.30 was obtained from photoelectrochemical measurements to be 2.43 eV, a value slightly higher than that reported in the literature for the same composition.

Notes: This work is the first which clearly demonstrates, by deep-level transient spectroscopy analysis in a Schottky structure, the presence of excimer laser-related defects in both n- and p-type virgin monocrystalline silicon. The detected point defects are correlated to those observed after solid-state laser (ruby, YAG) annealing in silicon. The main part of these traps could be attributed to the melt-related defects produced by the fast quenching rate in pulsed laser treatment.